10.1  Purpose and Scope

Part 3 of the report presents a general geologic and geotechnical evaluation of the original three specific study areas previously mentioned in the Preface of this report.  The emphasis is on evaluating factors that influence soil stability, and presenting general remedial measures for the types of slope instability found in the West Seattle, Magnolia/Queen Anne, and Madrona study areas. 

The purpose for our studies and recommendations regarding stability improvements in these study areas is to provide the City of Seattle (City) with information that can be used to prioritize remedial efforts and to develop order-of-magnitude budgets based on the cost data given in Part 2, Section 8.0 of this report.  The remedial measures presented are intended to be preliminary, with final scopes of work and corresponding cost estimates based on additional engineering studies and subsurface explorations.

The purpose described above has been accomplished in accordance with the following scope of services:

    We field checked the location of the reported landslides in the original three study areas.  During this effort and an additional field visit, we evaluated the alternatives for stability improvements in the areas based upon the conditions observed (slide type, groundwater and surface water conditions, soil stratigraphy, etc.).
    For each study area, we prepared a description of the topography, geologic and groundwater conditions, slide types, timing, and slide locations.
    We divided each study area into smaller Stability Improvement Areas where landslide activity has been prevalent.  For each smaller area, we evaluated the conditions contributing to current instability and/or potential future instability.
    Based on the above, we formulated stability improvements for consideration in the Stability Improvement Areas.  The types of improvements recommended are described in Part 2, which also presents unit costs relative to the various types of improvements.
    The above scope of work is presented in this part of the report and is summarized in Table 3-1.  The table provides preliminary estimates of quantities (length, square footage, etc.) related to improvements in the various areas.

In general, two site visits were made to each Stability Improvement Area, as indicated above.  The first site visit, actually made prior to formulating the improvement areas, was primarily to field check the database locations and make appropriate changes in the database.  The second site visit was for the purpose of formulating general types of measures that could be considered by the City and/or private property owners to improve stability and reduce landslide risk.  Specific sites were not evaluated.  The stability improvements listed on Table 3-1 include homeowner education; existing storm drainage facilities maintenance; storm drainage facilities improvement, as may be indicated by future observations or studies; subdrainage systems; fill stabilization; and retaining wall construction.  The number, length, square footage, etc., listed on the table are rough estimates presented only to formulate order-of-magnitude budgets.  Upon further studies needed to prioritize improvements, such studies may conclude that the extent or type of recommended improvements may or may not be needed, or that changes and/or additions may be advisable. 

It should be mentioned here that some landslides have occurred outside the designated Stability Improvement Areas.  These are usually isolated cases and the improvement areas were selected for locations where instability was prevalent.  For landslides outside the designated areas, the stability improvement methods described in Part 2 of this report would apply, including homeowner education and drainage control.

The stability measures recommended do not consider the location of property lines and relate to improvements made on City property, private properties, or both.  Since landslides and areas of potential instability do not obey property boundaries, improvements are sometimes necessary on both public and private land to suitably improve stability in an area.  Therefore, the improvements recommended in Part 3 are those that could be made by the City to protect utilities, drainage features, streets, and other City facilities; and also those measures or actions to be taken by the City and/or adjacent property owners to improve stability of an unstable slope.  In the latter case, the City and private property owners should coordinate efforts to improve stability and/or provide protection (such as catchment walls) should instability take place.  It is anticipated that some improvements will be made by the City, while other improvements or protection will be the responsibility of private property owners. 

It should be noted here that there are always risks of damage to property and structures involving landslides, for property located on or adjacent to a slope.  Property owners need to accept those risks.  Although the recommended improvements and homeowner education can lead to immediate or eventual improved slope stability conditions, private property owners should also obtain professional geotechnical advice to reduce current risks for their properties.

The analyses and recommendations presented in Part 3 of this report must be considered only in conjunction with the Limitations Section 1.5 presented in the Preface of this report.

10.2  Actions by City

In the succeeding sections of Part 3, various improvement measures and other actions are presented that we recommend be considered by the City.  These actions include:

        Providing homeowner education materials regarding actions private property owners can take to reduce instability.
        Maintaining and/or improving storm drainage facilities.
       Conducting further detailed engineering studies in areas of prevalent landslides, including subsurface explorations.
       Implementing stability improvements.
        Coordinating stability improvements with private property owners.

Homeowner education is important so that the public is made aware of the factors that cause landslides and the steps homeowners should take to improve stability.  Information should be provided to homeowners relative to prudent construction practices and obtaining professional advice for improving stability for existing homes, additions, or new construction.  It is particularly important that homeowners learn that filling on a slope (especially at the top of a slope), or cutting into a slope (especially at the toe), can lead to instability and should only be undertaken with proper advice and consultation with competent geotechnical engineers or engineering geologists.  Even the placement of yard waste on a slope decreases stability and, therefore, should be properly composted on flat ground or taken off-site.  Homeowners should also be required to properly maintain and control their on-site drainage systems and to discharge drainage in accordance with applicable regulations, since improperly channeled water decreases slope stability, particularly when concentrated.

In addition to the above, we recommend that the City continue to conduct neighborhood informational meetings to facilitate two-way discussion regarding stability matters.  Valid concerns of homeowners should be taken into account in planning and implementing improvements.  We also recommend that the general public be made aware of a telephone "hot line" that can be readily reached to report locations of poor drainage, landslides, or potential instability.

In areas of potential landsliding, it is important that existing storm drainage facilities be maintained.  In addition, storm drainage improvements could be considered when indicated by subsequent observations and studies.  In this regard, the City has retained a consulting engineering firm (Black & Veatch) to evaluate surface drainage systems throughout the city.  The scope of this "Needs Assessment" included visual observation of the roadway runoff where it had potential to impact landslide-prone slopes.  Their studies are to be coordinated with the landslide studies presented herein, with the goal of improving stability conditions.  In the succeeding sections of this report, recommendations regarding maintaining and/or improving storm drainage facilitates are subject to the evaluations and recommendations to be made by Black & Veatch.  Therefore, prioritizing and budgeting relative to surface drainage improvements are beyond this current landslide study.

As stated previously, the stability improvements presented in Part 3 are preliminary and for the purpose of providing the City with information they can use to prioritize remedial efforts and develop "ballpark" budgets.  Further detailed studies, including subsurface explorations, should be undertaken by the City to determine final scopes and design of remedial measures, and more accurate cost estimates.  Geotechnical and other consultants should be used as appropriate.  Implementing stability improvements by the City would consist of preparing plans and specifications using the data presented in Part 2 of this report, and observing actual construction to verify suitable conformance with project requirements. 

Since landslides and potential instability cut across property boundaries, a cooperative effort between property owners is advisable in obtaining the greatest benefits of stability improvements.  In addition to homeowner education, previously discussed, the City should facilitate the processing of permits submitted by private property owners so remedial work can take place expeditiously to improve stability.  Variances to code requirements should be allowed where needed to improve stability for private and/or public properties.  Temporary and/or permanent easements on or across City property could be granted, where allowed by ordinance, such as when needed to construct protective structures or to allow gravity flow, in lieu of pumped drainage, for suitably designed drainage facilities on private properties.  Coordination between the City and private property owners may also include shared costs, such as by Challenge Grants or Local Improvement Districts (LIDs). 

10.3  Actions by Private Property Owners

Improvement of stability involves actions not only by the City, but actions by private property owners.  Such actions by private property owners should include accepting existing conditions and the risks of slope instability.  Measures should accordingly be implemented on private properties as may be needed to protect and improve stability for existing property, structures, additions, or new construction.  Those measures to be taken by private property owners are the same types of improvements presented in Part 2 of this report, and professional advice should be obtained from geotechnical and other appropriate consultants regarding the improvements.  Such advice should also be obtained by prospective buyers of property in slide potential areas.

Stability improvements would include proper drainage of surface water, including suitable discharge of roof gutter downspouts.  Surface water should not be improperly channeled to or concentrated on slopes and particularly not onto adjacent property.  Other remedial measures would consist of properly designed subdrains, site grading, soil retention systems (walls, soil reinforcement, tieback anchors, etc.), drilled drains, or other measures as conditions may dictate.

Of particular concern are structures located above or at the bottom of a potentially unstable slope.  Private property owners should seek professional advice regarding such measures as underpinning walls and/or tieback anchors near the top, or catchment/retaining walls at the bottom of a slope.

Private property owners should take advantage of the homeowner education materials prepared by the City or other entities.  Cooperation with the City and with adjacent property owners is also important so that remedial measures can be coordinated to achieve the greatest benefits of stability improvement.  Private property owners should also notify the City regarding areas observed with poor drainage, landsliding, or potentially unstable ground, so that drainage and stability improvements can be coordinated between City and private property owners as appropriate.  

10.4  Additional Considerations

The contributing factors to instability, as described for the Stability Improvements section of this report, include terms such as surface drainage, runoff, storm water runoff, surface water runoff, etc.  Such drainage or runoff includes that from pavement areas as well as from soil or vegetated areas.  The more pervious the soil, such as sand and/or gravel, the more that rainfall will infiltrate the ground, which reduces the amount of runoff.  Conversely, for more impervious soils like silt or clay, runoff will be greater.  Runoff also takes place from vegetated slopes, being greater for areas of sparse vegetation than for slopes with heavy vegetation. 

Cuts at or near the toe of a slope, or fills on or near the top, are also contributing factors to instability.  Such factors, particularly where cuts or fills took place years ago, may still have some influence on the stability of an area; however, such a factor may or may not be the predominant cause of recent or future instability.  For example, a road cut area may remain stable for years, yet experience instability as the direct result of such things as a leaking or broken pipe, improper drainage from adjacent property, new filling or excavation on a slope, or other unwitting actions by owners or adjacent property owners.  Each occurrence of instability requires evaluation to assess the predominant factor or factors leading to slope failure.

In describing some of the Stability Improvement Areas, we noted remedial measures of landslides that had recently been completed or were taking place.  However, there are probably other remedial measures being planned, in progress, or completed by the City or private property owners that are not mentioned.  Furthermore, we have not mentioned specific locations where surface drainage improvements have recently been undertaken or are being planned in conjunction with the "Needs Assessment" portion of the surface drainage studies by Black & Veatch. 

11.0  WEST SEATTLE

The West Seattle area contains the most documented landslide events of the three study areas and of the whole city, as well as one of the two specific areas with the highest density of landslides, i.e., the Alki Avenue S.W. area.  (The other area with the highest density of landslides is the Perkins Lane W. area in Magnolia.)  In the early part of this century, West Seattle consisted primarily of summer homes that Seattle residents used only seasonally.  Initially, Alki Avenue Southwest was constructed on piles around the Duwamish Head to provide access to the summer beach houses at the base of the Duwamish Head bluff.  Later, Alki Avenue Southwest was filled to create a permanent roadway, which eliminated shoreline erosion at the base of Duwamish Head.  The City of Seattle annexed the Arroyo Heights and Seola Beach areas, south of Lincoln Park, in the 1950s; therefore, instability south of Lincoln Park prior to the 1950s is not recorded in the City files. 

11.1  Site Description

West Seattle is comprised of two linear ridges separated by Longfellow Creek (refer to Figure B-1).  These north-south ridges and parallel depressions were shaped by the last glacial ice to occupy this area.  West Seattle is bounded on the east by the Duwamish Waterway and on the west by Puget Sound.  The eastern longitudinal ridge (Puget Ridge) is bounded on the east by West Marginal Way aligned between the base of the slope and the Duwamish Waterway.  West of Puget Ridge is Longfellow Creek, which is one of Seattle s longest and lowest gradient streams.  Pigeon Point represents the northern-most extension of this lineal ridge.  West of Longfellow Creek the ground surface rises to a maximum elevation of 425 feet (High Point) atop a broad plateau representing the second longitudinal ridge.  The margins of this broad ridge are steep and drained by several short and steep streams including Fairmount Gulch, Schmitz Creek, Fauntleroy Creek, and Seola Creek.  This west ridge is bounded on the west by Puget Sound.  Both longitudinal ridges extend farther south, beyond the city limit. 

11.2  Soil Stratigraphy

Soils deposited during the most recent glaciation of the central Puget Lowland dominate the surface geologic conditions in West Seattle.  Because West Seattle is south of the Seattle Fault (an east-west-trending reverse fault, dipping to the south), Tertiary bedrock is shallower in depth south of the fault, relative to those areas north of the fault.  Tertiary bedrock outcrops sporadically near Alki Point along the beach and just east of the Alki Point lighthouse.  For the most part, the bedrock is not landslide prone.  One shallow colluvial landslide occurred on the west slope of one of these topographic bedrock highs.

The primary geologic units in West Seattle are the Vashon glacial deposits, although older, glacially deposited and nonglacial soils are present in stream cuts and at lower elevations.  The glacially transported soils consist of all ranges in particle size, from clay to boulders.  They can be divided into six broad categories based on the environment in which they were deposited:  pre-Vashon glacial deposits, pre-Vashon nonglacial deposits, glaciolacustrine deposits (Lawton Clay), advance outwash (Esperance Sand), lodgement till (Vashon Till), and recessional outwash.

A seventh geologic unit in West Seattle is colluvium, which is a by-product of the weathering, erosion, and movement of the previously deposited soils.  Colluvium is an accumulation of eroded soils and landslide debris on moderate or steep slopes.  At some locations, it exists as a thin rind of soft or loose soil on very steep slopes such as the Duwamish Head bluff area.  When direct precipitation and/or groundwater seepage saturate colluvium (generally soft or loose), it can lose strength and fail.  Resulting failures occur typically as either a shallow or deep-seated colluvial slide.  Colluvium mudflows (debris flows) commonly travel for a significant distance (greater than 50 feet) beyond the toe of the steep slope and are common throughout the landslide history of West Seattle.  Colluvial landslides also occur where colluvium on a bench becomes unstable due to water pressures and moves over the top and down the face of steep bluffs.

11.3  Groundwater

Groundwater plays an important role in slope instability in West Seattle.  There are three general types of groundwater present in this study area:

    Groundwater perched atop the lodgement till after percolating down through the relatively permeable recessional outwash near the highest elevations of West Seattle.  (This source of groundwater has not contributed to instability in West Seattle to the same extent as the other two types of groundwater identified below.)
    Groundwater perched atop glaciolacustrine deposits after percolating through "windows" or cracks in the overlying lodgement till, and through the relatively permeable advance outwash sand.
    Groundwater perched on slopes at the contact between the overlying loose or soft colluvial soils and the glacially overridden soils.

As mentioned earlier, a key stratigraphic marker for landslide location is the contact between the advance outwash sand (Esperance Sand) and the underlying glaciolacustrine silt and clay (Lawton Clay), i.e., "The Contact" (Tubbs, 1974).  The contact includes interbedded layers of silt, clay, and sand, which transition between the two geologic units.  West Seattle has the longest trace of this sand-clay contract of any neighborhood within the City of Seattle (refer to Figure B-2).  Although this contact is pronounced and well exposed in the Alki area, it extends continuously southward along the west-facing slope to the Arroyo Heights area.  This contact is also present on the east-facing slope west of West Marginal Way and on the slope west of Longfellow Creek.  This hydrologic discontinuity produces springs on the flanks of all of the West Seattle hills.

11.4  Landslide Types

11.4.1   High Bluff Peeloff Landslides

Please refer to Section 4.1.1 for a detailed description of high bluff peeloff type landslides.

There are no documented high bluff peeloff type landslides in West Seattle.  The main reason for the absence of high bluff peeloff landslides in this area is the presence of Harbor Avenue Southwest, Alki Avenue Southwest, and Beach Drive Southwest along the shoreline of Puget Sound.  These roadways protect the base of the steep slopes against shoreline erosion along West Seattle, thereby eliminating undercutting of the bluffs. 

South of Lincoln Park, no high bluff peeloff landslides are documented in this area; however, the absence of documented landslides may be a reflection of the relatively recent (1950s) annexation of this area by the City of Seattle.

11.4.2   Groundwater Blowout Landslides

As previously described, a key stratigraphic marker for landslide location is the contact between advance outwash sand and an underlying glaciolacustrine silt and clay.  Groundwater blowout landslides occur at this contact or other locations where pervious soil zones with high groundwater pressure influence the ground displacement.  Therefore, the initiation point of earth movement, also referred to as the headscarp, generally lies on or near the contact between the pervious soil and the underlying less permeable soil.  Because colluvium is usually involved in groundwater blowout landslides, it is common to classify them merely as shallow colluvial landslides.  For this reason, there is an anomalously low incidence of reported groundwater blowout landslides throughout West Seattle.  Figure B-2 illustrates the locations of documented groundwater blowout type landslides in West Seattle.  For reference, the sand-clay contact (Tubbs, 1974) is also shown on the map.  Nearly all of the groundwater blowout landslides were initiated at the sand-clay contact.  A high percentage of shallow colluvial landslides were also initiated at or near this contact, and some may be improperly classified in the historical records.

11.4.3   Deep-Seated Landslides

Deep-seated landslides were identified in the database as ground displacement deeper than about 6 to 10 feet.  The plane of movement may be arcuate or relatively planer and may involve glacially overridden soils as well as the surficial colluvial soils. 

A map illustrating the distribution of deep-seated landslides in West Seattle is presented on Figure B-3.  The highest densities of deep-seated landslides in West Seattle occur along Alki Avenue S.W., Delridge Way S.W. (23rd Avenue S.W.), S.W. Jacobsen Road, 5900-block of Beach Drive S.W., S.W. Admiral Way, and the intersection of Chilberg Avenue S.W. and Boyd Place S.W.  With the exception of Delridge Way, all of the densest concentrations of deep-seated landslides occur at or near the sand-clay contact, similar to the groundwater blowout and shallow colluvial landslides.  Grading of roadways by either cutting material from the toe of a slope or placing fill at the top of a slope may be one of the influences of the deep-seated failures. 

The deep-seated landslides shown on Figure B-3 in the Alki Avenue area occurred on a topographic bench formed at the contact between the Esperance Sand and the underlying Lawton Clay; refer to Figure 3-1.  The bench was formed by the erosion, sliding, and gradual regression of the upper portion of the bluff composed of Esperance Sand.  The mechanism for this type of landslide is as follows:

Landslides from the upper slope deposit thick colluvium on the bench. 

As colluvium accumulates on the bench, it becomes unstable due to groundwater pressure at the contact between the colluvium and the clay/silt (Lawton Clay), decreasing the frictional resistance to sliding.

The thick wedge of unstable material then translates along the lower portion of the bench, depositing debris (trees, vegetation, and colluvium) over the top of the bluff and onto the lower slope.  Deep-seated, rotational sliding predominates on the bench with the slide planes reaching depths as much as 50 to 60 feet into the thick wedge of colluvium and slide debris on the bench.

The added material on the lower slope becomes unstable because of several factors, including: abundant groundwater emerging along the sand-clay contact, the steep slope angle, and the relative lack of vegetation on the lower slope.  Shallow colluvial landslides predominate along the steep, lower slope, and trees from the bench move downslope with the colluvium.

11.4.4   Shallow Colluvial Landslides

Shallow colluvial landslides occur when loose, mostly heterogeneous soil on a moderate to steep slope becomes saturated.  Commonly, these landslides result in rapidly moving saturated soil acting as a viscous fluid that can travel significant distances.  In cases where the travel distance of the slide mass is greater than 50 feet, it was termed a debris flow for purposes of this study (refer to Section 4.1.4).

A map showing the distribution of historical shallow colluvial landslides in West Seattle is presented in Figure B-4.  Shallow colluvial landslides make up 74 percent of the total reviewed landslides in West Seattle.  Although they typically result from short duration heavy precipitation, regional groundwater also can be a factor.  This is illustrated by the frequent occurrence of shallow colluvial landslides in West Seattle close to the sand-clay contact.

The highest concentrations of shallow colluvial landslides occur along 47th Avenue S.W., Atlas Place S.W., S.W. Jacobsen Road, and along the Alki area of West Seattle.  The conspicuous double row of landslides on the northwest-facing slope of the Alki Avenue area represents shallow colluvial landslides occurring on two distinct topographic levels.  The southeastern-most row of landslides is located along the upper bluff, which is composed primarily of overridden Esperance Sand (advance outwash) (refer to Figure 3-1).  The lower or northwesternmost row of shallow colluvial landslides is located along the lower slope below the bench.  Between the two distinct slopes is the bench created by ongoing deep-seated landsliding in the thick accumulations of reworked Esperance Sand (colluvium).  Few of these landslides are reported because they are on forested, undeveloped property.

11.5  Landslides with Debris Flows

Debris flows are shallow colluvial landslides and generally consist of rapid movements of saturated soils that act as a fluid and travel considerable distances.  As mentioned previously, landslides that have runout distances of greater than 50 feet are considered debris flows in this study.

Figure B-5 presents a map showing the distribution of debris flows in West Seattle.  The Alki Avenue area of West Seattle has the highest density of debris flows in the City of Seattle.  A debris flow typically begins as either a groundwater blowout or shallow colluvial landslide on a steep slope.  These slides may also initiate as an earth fall of saturated colluvial debris from a bench onto the lower slope.  Debris flows include not only mud but wood debris and other objects that can act as projectiles that may cause structural damage to structures in their path.  Structures situated at the toe of the slope along Alki Avenue are susceptible to this type of landslide because of their close proximity to the steep slope.

In the vicinity of the 1300 block of Alki Avenue S.W., an area with several debris flow landslides (1956, 1983, 1997), colluvial landslides on the upper slope near Sunset Avenue S.W. flowed into a confined, short, steep gully and down to Alki Avenue S.W. (refer to Figure B-5).  Once in the gully, the slide debris mixed with additional water in the intermittent stream channel, decreasing the viscosity and increasing the volume and runout distance of the debris flow.

11.6  Timing of Landslides

A map showing the historical distribution of landslides by decade is presented in Figure B-6.  Areas where the landslides are chronologically dispersed through time in West Seattle include Beach Drive S.W., Alki Avenue S.W., and Delridge Way (23rd Avenue S.W.).  More recent (post-1960) landslides dominate the 47th Avenue S.W. and Seola Beach Drive S.W. areas.  As previously discussed, these two areas were not significantly developed until after 1960, so it is likely that older landslides occurred in these two areas but were not reported.

11.7  Severe Storm-Related Landslides

A map illustrating the distribution of landslides in West Seattle during the four most notable landslide winters (1933/34, 1971/72, 1985/86, 1996/97) is presented in Figure B-7.  The most notable trend in the quantity and distribution of the severe storm-related landslides in this area is the high proportion of 1996/97 landslides in West Seattle.  The scarcity of 1933/34 and 1971/72 reported landslides may be a function of lesser urban development during those time frames rather than the relative magnitude of the earlier severe storms.  

11.8  Potential Slide Areas

The City of Seattle presently regulates development in steep slope and potential slide areas.  Historical landslide locations and the location of the sand-clay contact were used by the Department of Design Construction and Land Use (DCLU) to define the Potential Slide Areas, as described in Section 20.0 of this report.  Figure B-8 illustrates the location of the potential slide areas with all of the landslides in the database for West Seattle.  About 63 percent of the reviewed landslides in West Seattle fall within the existing Potential Slide Areas.  The obvious areas where landslides occur outside of the potential slide areas are the 47th Avenue S.W. area, Seola Beach Drive S.W. area, and the upper slope along Alki Avenue S.W.

11.9  Stability Improvements

This section presents possible stability improvements that could be made by the City to protect utilities, drainage features, streets, and other City facilities.  It also presents measures that could be made by the City and adjacent property owners to improve the stability of an unstable slope.  We present further comments regarding educating private property owners on steps they may take to improve stability.

The West Seattle area has been divided into ten smaller Stability Improvement Areas, where landslide activity has been prevalent.  As shown on Figure B-9 (Appendix B, Map Folio), the ten areas are as follows:

23rd Avenue S.W.

Admiral Way

Fairmount Gulch

Harbor Avenue

Alki Avenue

Boyd Place/Chilberg Place

Jacobsen Road

Beach Drive/Atlas Place

47th Avenue S.W.

Seola Beach

For each area, we summarize the general subsurface conditions, landslide types and causes, and present actions that could be considered for improving slope stability.  Table 3-1, located following the text in Part 3 of this report, presents a summary of this information.

11.9.1   23rd Avenue S.W.

In the 23rd Avenue S.W. Stability Improvement Area, as designated on Figure B-9, 24 landslides were recorded.  Both deep-seated and shallow colluviual landslides occurred, and a number of landslides were not identified as to the type.  The landslides in this area have taken place along the west-facing slope generally between 21st Avenue S.W. and Delridge Way S.W. at the toe of the slope.  Instability in this area was reported as early as 1914.  The most recent landsliding took place in January 1997, which damaged 23rd Avenue S.W. (one block east and uphill of Delridge Way) at S.W. Dakota Street.  As a result of the January 1997 landslide, several properties on the downhill (west) side of 23rd Avenue were also damaged by the earth movement.

The landslides that occurred in this area prior to 1960 reportedly were related to grading of 22nd and 23rd Avenues, presumably caused by cutting into the slope on the east side of these streets.  The instability that took place following 1960 was generally related to filling on private properties on the west side of 23rd Avenue, or cutting into the slope on private properties east of Delridge Way. 

The subsurface conditions in this area consist of a silt-clay colluvium that is up to 25 feet thick, located over stiff to hard clay.  Groundwater levels are typically high because this area is at and near the toe of a slope.  The sand-clay contact (Tubbs, 1974) has not been mapped in this area.  The contributing factors to instability are the soil conditions on this slope (colluvium over stiff to hard clay), undercutting or filling on the slope, and high groundwater levels/seepage.  The landslides were triggered by heavy rainfall that resulted in surface runoff and infiltration into the slope soils.

To improve stability for 23rd Avenue S.W. at the Dakota Street right-of-way (not a through street), a buried, drained, secant-type soldier pile wall was constructed along the west edge of the street.  The wall length was about 110 feet.  Repaving the street east of the new wall included provisions to control surface drainage.  With wall construction, stability was improved for 23rd Avenue; however, instability could still occur downhill from the wall, particularly on private properties where owners should obtain professional advice for improving stability on their sites.

Recommended actions in this area would include homeowner education and storm drainage systems maintenance and/or improvement, including the improvement of storm drainage from private properties uphill from 23rd Avenue.  Finger drains could also be considered to improve stability for the toe of the hillside upslope of 23rd Avenue.

11.9.2   Admiral Way

The Admiral Way Stability Improvement Area is the east-facing slope situated as shown on Figure B-9.  In this area, a total of 26 shallow colluvial and deep-seated landslides have been recorded for this area, beginning in 1917.  Some of the landslides occurred on the steep slope uphill from S.W. Admiral Way, and others took place on the steep slope downhill.  The most recent instability occurred uphill from Admiral Way in early 1998, which resulted in the City constructing remedial measures including a rock buttress near the top of the slope, and a 120‑foot-long, drained soldier pile and concrete lagging wall (6 to 8 feet high) along the toe of the slope on the west side of Admiral Way.

The subsurface conditions consist of colluvium on the steep slopes overlying glacially overridden native soils.  In some areas, fill may be present, such as for backyards.  The original construction of Admiral Way likely included some fills along the east side and cutting along the west.  A 6- to 8-foot-high rail (trolley) and concrete lagging toe wall is present along much of the west side of Admiral Way.  A portion of this wall failed at the time of the 1998 landslide, and other portions of the wall are bulging or have been overtopped by slope erosion debris or previous landslides.  The slopes both west and east of Admiral Way exhibit active signs of creep.  Based on available subsurface information, the colluvium on the slopes is 10 or more feet deep.  The sand-clay contact (Tubbs, 1974) extends through this area.

The factors that contribute to instability in this area are steep topography, relatively deep colluvium on the slope, high groundwater levels/seepage, storm water runoff, and cutting and filling.  The triggering mechanism is generally heavy rainfall with surface water runoff and infiltration.

Stability improvements that could be considered are trench subdrain installations, wall construction, storm drainage systems maintenance and/or improvement, and homeowner education.  Subsurface drainage is probably the most cost-effective method for improving slope stability in the area.  Interceptor trench subdrains parallel to contours uphill from Admiral Way, or trench subdrains at intervals perpendicular to contours (finger drains) could be effective.  Such subdrains should extend through the colluvium and into the glacially overridden soils.  Stronger, higher walls for toe support and increased catchment area for slide debris could also be considered to protect Admiral Way.  Refer to Table 3-1, for estimated lengths of subdrains and wall that could be considered for budgeting purposes in this area.  A comprehensive study and improvement to storm drainage is recommended for east of 35th Avenue S.W. and north of S.W. Spokane Street.  The instability downhill of Admiral Way occurred mostly on private properties where homeowner education and prudent development practices should be followed.

11.9.3   Fairmount Gulch

The Fairmount Gulch Stability Improvement Area consists of a large, steep-sided ravine that extends from Harbor Avenue to the southwest where Admiral Way crosses the ravine on a high bridge; refer to Figure B-9.  Eleven landslides have been recorded for this area, mostly on the east-facing slope of the ravine.  Ten landslides were listed as shallow colluvial with three of them debris flows, and one was not identified as to type, although it was likely also a shallow colluvial landslide based on the database comment.  The earliest recorded landslide occurred in 1937, and instability has been reported through the years.  Only one event involving instability (tension cracks in backyard) was reported due to the 1996/1997 storm. 

The subsurface soils in this area, based on geology mapping and our experience in this area (no explorations reviewed), consist of colluvium overlying glacially overridden soils.  The overridden soils consist of sand over clay, and the sand-clay contact (Tubbs, 1974) is present at lower elevations in the ravine.  The landslides reported in this area are primarily failures in colluvium and/or yard fills placed by private property owners.  One landslide was reported in 1995 to be related to road fill placed for Belvidere Avenue S.W.  This street is located in the ravine near the sand-clay contact.  The factors contributing to instability are steep topography, loose fill and/or colluvium on the slope, high groundwater levels with associated seepage particularly near the sand-clay contact, and heavy rainfall (triggering cause) that saturates the loose soil.

It is recommended that work by the City to improve stability include maintaining existing storm drainage facilities and improving them when indicated by future observations in this area.  Springhead drains installed at known seepage points could reduce infiltration and saturation of colluvial soils by groundwater springs and seeps.  Homeowner education is recommended to include providing information regarding prudent construction and site drainage practices, and obtaining professional advice for improving stability for existing property, additions, or new construction.

11.9.4   Harbor Avenue

Sixty-one landslides have been recorded for the Harbor Avenue Stability Improvement Area.  Most of the landslides were reportedly of the shallow colluvial type (48), while a few were listed as deep-seated (8) and groundwater blowouts (5).  These landslides have generally occurred on the easterly- and northerly-facing steep slopes in this area; refer to Figure B-9.  The earliest recorded landslide occurred in 1916 and instability has occurred continually through the years, including 1998.  A number of landslides occurred in this area during the 1996/97-winter storm (13) including a large deep-seated landslide at California Way S.W. and Ferry Ave S.W., which closed California Way S.W. for several months. 

There are three general areas of instability in this area: the east-facing slope between Victoria Avenue S.W. and Harbor Avenue S.W., the east-facing slope between Palm Avenue S.W. and California Way S.W., and the north to northwest-facing slope between California Lane S.W./California Way S.W. and Alki Avenue S.W./Harbor Avenue S.W.  Shallow colluvial landslides and debris-flows dating back to about 1933 have impacted structures at the toe of the slope, east of Victoria Avenue.  The slope below Palm Avenue exhibits abundant groundwater seepage near the sand-clay contact and is the location for two relatively large deep-seated landslides that occurred in early 1996 and in early 1997:  the 1300-block of California Way, and California Way/Ferry Avenue, respectively.  The pavement along the east side of Palm Avenue was cracked and had settled at the time of our visit in 1998.  The City improved the stability of the slope in the 1300-block of California Way by constructing a drainage blanket retained by a 45-degree earth slope reinforced with geogrids.  This repair was the first use of reinforced slopes with geosynthetics in Seattle.  The City improved the stability of the California Way/Ferry Avenue landslide using subsurface drainage, crib walls, grading, and buttressing.  A buried, soldier pile wall approximately 110 feet long, was also constructed along the east side of California Way to improve roadway stability.  The structures at the base of the northerly-facing slope below California Lane and California Way have been impacted by gradual bluff regression and sloughing since 1955.  Structures on the bench, in the vicinity of California Lane, have been impacted by at least two deep-seated landslides.

The subsurface soils in this area, based on geologic mapping and our experiences in this area, consist of colluvium overlying glacially overridden soils.  The glacially overridden soils consist of slightly silty sand (outwash) over silty clay (glaciolacustrine).  The sand-clay contact (Tubbs, 1974) is present at approximately elevation 100 feet (±20 feet) throughout the Harbor Avenue Stability Improvement Area.  A bench of variable width exists at the top of the clay unit with roughly 10 to 60 feet thickness of colluvium accumulated from up-slope sources.  The factors that contribute to instability in the Harbor Avenue area include steep topography, loose colluvium over glaciolacustrine clay, high groundwater levels/seepage, and cutting at the toe of the slopes.  The landslides reported in this area typically initiate at or near the bench with debris traveling down the lower clay slope.  

Recommended improvements that could be considered by the City and private landowners in this area consist of storm drainage systems maintenance and/or improvement, road fill replacement, springhead drain installation at identified seepage points, retaining/catchment wall installation, trench subdrain installation, and homeowner education.  Surface drainage in the vicinity of California Lane could be improved in order to reduce infiltration into the thick colluvial soils along the bench.  We recommend that existing and new drainage facilities installed in the area by the City or private landowners be checked and maintained on a regular basis for proper functioning.  Interceptor trench subdrains may be appropriate along the bench area in the vicinity of California Lane and downslope of Palm and Victoria Avenues.  Consideration could also be given to removing the fill portion of Palm Avenue and replacing it with compacted material to reduce settlement and pavement cracking and resulting surface water infiltration into downslope soils.  Retaining/catchment walls would be effective along the toe of the lower slope below Victoria Avenue and at the northernmost tip of the Stability Improvement Area.  Furthermore, installation of springhead drains could be considered in discrete areas of acute groundwater seepage along the steep slopes to prevent saturation of the colluvial soils by spring water.  We accordingly recommend that property owners in this area obtain geotechnical advice regarding precautions to reduce the risk to properties, including catchment walls at the base of the slope and surface drainage at the top of the slope. 

11.9.5   Alki Avenue

The Alki Avenue Stability Improvement Area is a northwesterly-facing slope situated as shown on Figure B-9.  In this area, a total of 106 landslides (deep-seated, groundwater blowout, and shallow colluvial) have been recorded in this area since 1916.  Approximately one-third of the landslides occurred along the upper bluff, just west of Sunset Avenue S.W.  The others occurred along the lower bluff behind the properties along Alki Avenue S.W.  While approximately 33 slides were reported in this area due to the 1996/97 winter storm, the most notorious landslide occurred in the spring of 1974 where a large-scale deep-seated event threatened properties along the 1400-1700 blocks of Alki Avenue S.W. 

The subsurface conditions consist of thick accumulations of colluvium (up to 50 to 60 feet thick) on a midslope bench and thin rinds blanketing the steep slopes, as shown on Figure 3-1.  Underlying the colluvium is an upper unit of glacially overridden outwash sand (Esperance Sand) with glaciolacustrine clay (Lawton Clay) and older, pre-Vashon silt/clay and sand below.  Some fills placed for roads and residential construction may be present along the upper sand bluff just west of Sunset Avenue and in the vicinity of California Lane and Bonair Drive, where the construction of these streets likely included fills along the west margins of the roads.  Near the 1300-block of Sunset Avenue, the City installed a buttress fill and a drained soldier pile and lagging wall to mitigate landslides that occurred on the upper steep slope just below Sunset Avenue.  Several other remedial measures in this area included crib walls and soldier pile and lagging walls to protect structures along the upper bluff, and catchment walls and surface drainage behind structures at the toe of the lower slope.

The contributing factors to instability in this area are steep topography, colluvium on the slope, high groundwater levels/seepage, cutting and filling, and heavy rainfall and associated infiltration (triggering mechanism). 

With respect to the instability during the spring of 1974, Shannon & Wilson, Inc. (Shannon & Wilson) was retained by the City to conduct a geotechnical study of the area.  Based on geologic reconnaissance and subsurface borings, a report dated July 1975 recommended two conceptual preliminary design alternatives.  One alternative was to design a large earth buttress (including subdrains) on the bench.  The other alternative was to construct a large, tied-back cylinder pile wall on the bench in conjunction with trench subdrains.  Because of the great depth of colluvium on the bench, such measures to improve stability would be extensive and expensive.  Upon further exploration and evaluation in 1999, a scheme of horizontal drains and deep trench drains was chosen to increase stability of this slope and particularly the bench area.  To help fund this work, the City applied for and received a Federal Emergency Management Agency (FEMA) grant.  Although some improvement in stability conditions is anticipated above and below the bench area, some risks of instability would still be present.  Property owners above and below the bench area would still need to seek geotechnical advice and take precautions to reduce the risk to their properties. 

Other improvements that the City could consider consist of storm drainage systems maintenance and/or improvement, subdrain and springhead drain installations along the bench area outside the project area described in the preceding paragraph, and homeowner education.  Homeowner education is probably the most cost-effective method for improving slope stability in this area.  Property owners in the Alki Avenue Stability Improvement Area should avoid making improper cuts and fills, maintain existing drainage systems, seek geotechnical advice, and take precautions to reduce risk to their properties.

It is recommended that the City consider evaluating, repairing, and maintaining existing City-owned drainage pipes that have been installed over the years in this area (a drainage map is available in City files).  Furthermore, it is recommended that the City coordinate efforts (expeditious processing of permits or other cooperative effort as described in Sections 1.5 and 10.2 in this report) with private property owners along Alki Avenue, relative to building catchment walls along the toe of the slope for protection of the structures. 

11.9.6   Boyd Place/Chilberg Place

The Boyd Place/Chilberg Place Stability Improvement Area consists of a west-facing steep slope, as indicated on Figure B-9.  Seven landslides are recorded for this area, mostly in the vicinity of the Boyd Place S.W. and Chilberg Place S.W. intersection.  Three landslides were listed as shallow colluvial and four, at the Chilberg/Boyd intersection, were listed as deep-seated.  The earliest recorded landslide occurred in 1964 and consisted of a setdown along the Boyd Place right-of-way.  Other landslides in the vicinity of this intersection, along Boyd Place, have occurred in 1971, 1974, and 1997.  During the 1997 earth movement, and probably as a result of this instability, a water main ruptured, exacerbating the situation.  Remedial measures included an 85-foot-long wall installed along the west side of Boyd Place to retain the road fill and a 55-foot-long wall was installed along the east side of Boyd Place to retain the cut slope.  These two walls consisted of soldier piles with concrete lagging.  An 83-foot-long reinforced concrete retaining wall was also constructed along the downhill side of Chilberg Place to provide support for that roadway.  The City also installed catch basins and other drainage improvements in the vicinity.

The subsurface conditions in this stability improvement area generally consist of colluvium overlying glacially overridden native soils.  In some areas, existing fill is present, such as for backyards and roads.  The glacially overridden soils consist of outwash sand overlying glaciolacustrine silt and clay.  The sand-clay contact is located in the vicinity of the intersection of Chilberg and Boyd Place.  Associated groundwater seeps and springs exist in this area.

The factors that contribute to instability in this area are steep topography, abundant groundwater seeps and springs associated with high groundwater levels, storm water runoff, and cutting and filling.  The triggering mechanism is generally heavy rainfall with surface water runoff and infiltration into downslope soils.

It is recommended that actions by the City in this area consist of maintaining and/or improving storm drainage systems, particularly in areas outside of the 1997 Chilberg/Boyd Place project area.  Homeowner education is also recommended. 

11.9.7   Jacobsen Road

The Jacobsen Road Stability Improvement Area is a west-facing slope situated as shown on Figure B-9.  In this area, a total of 18 landslides (deep-seated, shallow colluvial, and groundwater blowout) are recorded in the database since 1933.  Some of the landslides occurred on the steep slope on the east side of S.W. Jacobsen Road, and others, including several deep-seated landslides, have occurred on the steep to moderate slope along the west side of Jacobsen Road.  The most recent instability occurred downhill from Jacobsen Road in early 1997, which resulted in severe structural damage to two residences west of Jacobsen Road.  Remedial measures have been planned by private property owners to improve slope stability and repair the damaged structures.  The City placed an asphalt curb to prevent surface water from infiltrating the slope soils west of Jacobsen Road. 

The subsurface conditions consist of colluvium on the steep slopes overlying glacially overridden native soils.  In some areas, existing fills may be present, such as for residences along the west side of Jacobsen Road.  The original construction of Jacobsen Road likely included some fills along the west side and cutting along the east.  The sand-clay contact with its associated groundwater seepage exists along the downslope side of the southern portion of Jacobsen Road, and crosses to the uphill side of Jacobsen Road to the north.  The slopes on both sides of Jacobsen Road exhibit signs of soil creep.  Based on available subsurface information, we estimate that the colluvium on the slopes is 10 or more feet deep. 

The factors that contribute to instability in this area are the steep topography, relatively deep colluvium on the slope, high groundwater levels/seepage, and cutting and filling.  The triggering mechanism is generally heavy rainfall with surface water runoff and infiltration. 

Stability improvements that the City could consider consist of surface drainage maintenance and/or improvement, interceptor trench subdrain installation, and homeowner education.  Surface drainage is probably the most cost effective method for improving slope stability along the west side of Jacobsen Road.  An interceptor trench subdrain along the east (upslope) side of Jacobsen Road may be appropriate, unless a suitably functioning subdrain is already in place.  Installation of curbs and gutters to prevent surface water from Jacobsen Road from flowing onto and infiltrating the downslope areas west of the roadway could also be considered.  Information should be provided to property owners regarding proper cutting and filling, and controlling their on-site drainage systems.

11.9.8   Beach Drive/Atlas Place

The Beach Drive/Atlas Place Stability Improvement Area, as shown on Figure B-9, consists of the following:  1) an upper, west-facing steep slope between 49th and 50th Avenue S.W. (east of Atlas Place) and Atlas Place S.W.; 2) a bench approximately 300 feet wide (upon which Atlas Place is constructed); and 3) a lower, west-facing moderate slope west of Atlas Place that extends down to Beach Drive S.W.  Twenty-five landslides have been recorded for this area.  Six landslides were listed as deep-seated and the others were the shallow colluvial type.  The earliest recorded landslide occurred in 1927, and instability has been reported through the years.  Four landslides occurred during the winter storm of 1996/97, including a deep-seated event in the 5900-block of Atlas Place. 

The subsurface soils in this area, based on geologic mapping and our experience in this area (no explorations reviewed), consist of colluvium overlying glacially overridden native soils.  The overridden soils consist of sand overlying clay, and the sand-clay contact (Tubbs, 1974) is present at roughly the same elevation as the bench.  The slope instability reported in this area is located along the steep slope west of Atlas Place, along the steep slope east of the 6500-block of Beach Drive, and along the west shoulder of Beach Drive. 

The landslides that have been reported in this area occurred primarily in colluvium and/or road cuts and fills for both Beach Drive and Atlas Place.  Instability along the west margin of Beach Drive appears to result from fills placed during the construction of the roadway.  Ponding water and road-settlement were observed along Beach Drive during our field reconnaissance.  Shallow colluvial landslides along the east (uphill) side of Atlas Place appear to be the result of cutting into the slope without any slope retention measures.  Surface water is also contributing to instability between Beach Drive and Atlas Place.  The City placed an asphalt curb along the west side of Atlas Place to prevent surface water from infiltrating the downslope areas.  In summary, the factors contributing to instability are the steep topography, cutting and filling, surface water, and high groundwater levels/seepage. 

Actions the City could consider consist of improvement of the Atlas Place street grade with curbs, gutters, and storm drainage facilities; removal and replacement of existing loose soils along the west side of Beach Drive, and education of property owners in this area.  Improvement of the Atlas Place street grade would include the retention of the cut-slopes, a possible interceptor trench subdrain along the centerline of the roadway, and provisions for surface drainage along the full length of the roadway.  Springhead drains would be effective in capturing groundwater seeps and springs along the cut slope east of Atlas Place.  Improving stability of Beach Drive could include removal of the existing fill soils and replacement with lightweight, structural fill material.  It is recommended that homeowner education include proper methods for controlling on-site drainage systems and discharging drainage in accordance with City regulations.

11.9.9   47th Avenue S.W.

The 47th Avenue S.W. Stability Improvement Area is a steep, west-southwest-facing slope situated as shown on Figure B-9.  In this area, one deep-seated, one groundwater blowout, and 19 shallow colluvial landslides have been recorded since 1955.  Some of the landslides occurred on the steep slope uphill from 47th Avenue and others took place on the steep slope downhill of 47th Avenue.  Others occurred in the vicinity of Maplewood Place S.W. (private road) located near the southern edge of this stability improvement area  The most recent instability took place at the intersection of 47th Avenue and Maplewood Place during the 1996/97 winter storms.  This resulted in the City constructing remedial measures, including a gabion wall on the east side of 47th Avenue, a soldier pile and lagging wall along the west side of Maplewood Place, and drainage improvements. 

The subsurface conditions consist of colluvium on the very steep slopes overlying glacially overridden native soils.  Fills are present in some areas such as for residential backyards, based on the landslide descriptions.  The overridden native soils consist of limited occurrences of glacial till overlying outwash sand with glaciolacustrine clay below.  The sand-clay contact (Tubbs, 1974) is present east of 47th Avenue at elevation 170 feet (±30 feet).  The landslides that have been reported in this area are primarily failures in colluvium resulting from surface water runoff and groundwater seepage near and downslope from the contact.  Numerous groundwater seeps and hydrophitic vegetation exist along the east (uphill) side of 47th Avenue. 

The factors that contribute to instability in this area are steep topography, improper cutting and filling, high groundwater levels with associated seepage particularly near and downslope from the sand-clay contact, and improperly directed surface water.  For example, improper cutting into the toe of the slope on both private and public properties, or private utility failures (water and sewer lines) reportedly influenced approximately five of the recorded slides in this Stability Improvement Area. 

Stability improvements that we recommend the City and private property owners consider are surface drainage systems maintenance and/or improvement and homeowner education.  The City could consider placing a curb/gutter along the west side of 47th Avenue S.W. to prevent infiltration of surface water into downslope areas, particularly upslope of Maplewood Place, a private road.  Furthermore, it is recommended that the City facilitate the processing of permits regarding design, access, and construction efforts with private property owners along Maplewood Place, with respect to catchment wall construction along the toe of the cut slope for protection of the structures.  A soldier pile retaining wall could also be considered along the west margin of 47th Avenue upslope of the Maplewood Place dead-end to improve stability for the street.  It is recommended that homeowner education emphasize the need to obtain professional advice before cutting and/or filling along any slopes.  Private property owners in this area should control their on-site drainage systems and discharge drainage in accordance with regulations, since improperly channeled water decreases slope stability.

11.9.10   Seola Beach

The Seola Beach Stability Improvement Area consists of a moderately steep to steep-sided ravine that extends from Puget Sound to the north-northeast for approximately one mile; refer to Figure B-9.  All of the landslides recorded in the database for this area are on the west side of the ravine.  The east side of the ravine is outside of the Seattle City Limits.  Along the west side, a total of six landslides (shallow colluvial and deep-seated) have been recorded, beginning in the spring storm of 1986. 

The subsurface conditions based on geologic mapping and our experience in the area (no explorations reviewed) consist of colluvium overlying glacially overridden outwash sand and gravel.  There is no lacustrine clay exposed in this area below the outwash sand and gravel.  Therefore, the sand-clay contact is not mapped in this area.  The landslides that have been reported in this area primarily occurred in colluvium and/or yard fills placed by private property owners.  One landslide/debris flow was reported in 1986 to be related to the rupture of a sewer main on the upper plateau, north of the south end of Seola Beach Drive S.W.  The runout of debris reached Puget Sound. 

The factors contributing to instability are moderately steep to steep topography, private backyard fills and/or colluvium on the slope, and heavy rainfall (triggering cause) that saturates the loose soil and causes failure.

In the long-term, there do not appear to be practical remedial measures that the City could take to prevent the natural occurrence of landslides in this area other than homeowner education.

12.0  Magnolia/Queen Anne

While Magnolia and Queen Anne are two distinct topographic highs, they share similar geology conditions and, therefore, are treated as a single study area.  Perkins Lane West, located along the southwestern margin of Magnolia, is similar to Alki Avenue in West Seattle in that it contains a very high density of historical reported landslide events. 

12.1  Site Description

Magnolia and Queen Anne are two distinct topographic highs separated by Interbay, a north trending linear depression (refer to Figure B-10).  Magnolia, offset north with respect to Queen Anne, reaches a maximum elevation of 375 feet.  Queen Anne, similar in size to Magnolia, reaches a maximum elevation of 400 feet.  The area is bordered by Puget Sound and Elliot Bay to the west and southwest, the Lake Washington Ship Canal to the north, Lake Union to the east, and downtown Seattle to the south.  Steep slopes surround both Magnolia and Queen Anne.  The bluff along the west side of Magnolia, extending from Smith Cove to the Lake Washington Ship Canal, is locally armored against wave action and is the steepest slope in Magnolia.  Kinnear Park and the slope west of Aurora Avenue are among the steepest slopes on Queen Anne.

12.2  Soil Stratigraphy

Soils deposited during the most recent glaciation of the central Puget Lowland dominate the surface and subsurface geologic conditions in the Magnolia and Queen Anne study area.  Because Magnolia and Queen Anne lie north of the Seattle Fault, Tertiary bedrock is buried below roughly 3,000 feet of glacial and non-glacial sediments.

The primary geologic units involved with landsliding in Magnolia and Queen Anne are the Vashon glacial deposits.  The glacial soils consist of all ranges in particle size from clay to boulders and may be divided into four broad categories: glaciolacustrine deposits (Lawton Clay), advance outwash (Esperance Sand), lodgement till (Vashon Till), and recessional outwash.

Colluvium is also present along the lower portions of the hillsides in the Magnolia and Queen Anne study area.  Particularly thick accumulations of colluvium occur along the Perkins Lane West area of Magnolia.  Colluvium also forms a thin rind on steep slopes all around Magnolia and Queen Anne. 

12.3  Groundwater

Groundwater plays a key role in slope instability in Magnolia and Queen Anne.  The contact between advance outwash sand and underlying glaciolacustrine silt and clay is exposed in slopes around both Magnolia and Queen Anne.  Prominent springs associated with this contact occur throughout these areas including Perkins Lane W., Kinnear Park, 15th Avenue W., and Westlake Avenue N.  Figures B-11 through B-19 illustrate the location of the sand-clay contact.

12.4  Landslide Types

12.4.1   High Bluff Peeloff

High bluff peeloff-type landslides occur in only a few discrete areas in the City of Seattle.  A map showing the distribution of high bluff peeloff landslides in the Magnolia and Queen Anne study area is presented in Figure B-11.  Areas where the slopes are near vertical resulting from either wave action at the base of the slope or the presence of resistant lodgement till, or both, are present in Kinnear Park, Lawtonwood, and along the southwestern shoreline of Magnolia.  With the exception of Kinnear Park and portions of Perkins Lane W., there is little or no armoring along the toe of the slope below the high, steep bluffs.  The high bluff peeloff landslide located at the northern tip of Magnolia likely occurred as a result of undercutting by wave action at the base of the bluff.  In 1997, a high bluff peeloff landslide occurred along a short section of steep bluff east of the northern portion of Perkins Lane.  The high bluff peeloff landslides along the southwest margin of Magnolia occurred on the steep bluff above (east of) Perkins Lane W.  The very steep, bare bluff south of the southern end of Perkins Lane West has a long history of high bluff peel-off type landslides, but the City files do not have information on these events because they generally have little effect on structures or transportation routes.

12.4.2   Groundwater Blowout Landslides

A map showing the distribution of groundwater blowout landslides in the Magnolia and Queen Anne area is presented in Figure B-12.  The contact between the advance outwash (Esperance) sand and the glaciolacustrine silt and clay (Lawton Clay) is also shown.  As stated previously, without accurate reporting and analysis of the landslide event, it is difficult to distinguish between a shallow colluvial landslide and a groundwater blowout landslide.  Several landslides described in the historical records as shallow colluvial landslides may, in fact, be groundwater blowout landslides.  In Magnolia, the Works Progress Administration (WPA) completed several projects designed to capture and redirect groundwater for slope stabilization purposes.  The WPA projects are marked on Figure B-10 with a pick and shovel symbol. 

12.4.3   Deep-Seated Landslides

A map illustrating the locations of deep-seated landslides in the Magnolia and Queen Anne areas is presented on Figure B-13.  The highest density of deep-seated landslides is located along the west side of Queen Anne and Perkins Lane W.  Along the west side of Queen Anne, several deep-seated landslides were reported, including a very large area of instability that was active from 1951 to 1956 in the vicinity of 12th Avenue W. and W. Blaine Street.  The Perkins Lane W. landslides generally occur below the bluff, in a relatively thick colluvial wedge as shown on the Idealized Geologic Conditions West Magnolia profile, Figure 3-2.  The colluvial wedge overlies a hard surface of Lawton Clay that commonly slopes toward Puget Sound.  This contact between the colluvial wedge and the hard Lawton Clay creates groundwater conditions conducive to landsliding.

12.4.4   Shallow Colluvial Landslides

Figure B-14 shows the distribution of shallow colluvial landslides in Magnolia and Queen Anne.  It is our opinion that groundwater along southwest Magnolia and southwest Queen Anne significantly contributes to shallow colluvial landslides as well as groundwater blowout landslides.  Furthermore, based on the spatial distribution of shallow colluvial and groundwater blowout landslides along the southwest margins of Magnolia and Queen Anne, the flow direction of groundwater perched atop the glaciolacustrine silt and clay may be toward the southwest.  Landslides plotted from the database are conspicuously absent from the north margins of both Magnolia and Queen Anne even though the sand-clay contact surrounds both hills.  Many landslides have occurred in Discovery Park, but these were not reported because they have little to no affect on structures or transportation routes.

The east side of Queen Anne represents an area where proper development can increase the stability of a hillside by incorporating proper buttressing and consequent drainage improvements.  For example, the undeveloped steep slope west of Westlake Avenue N. is susceptible to landsliding resulting from uncontrolled drainage.  Where several condominiums were recently built along Westlake Avenue N., Dexter Avenue N., and Aurora Avenue North, the potential for shallow colluvial sliding has been reduced substantially because of the incorporation of tied-back retaining walls, subsurface drainage, and surface drainage improvements.

12.5  Landslides with Debris Flows

A map showing the distribution of debris flow landslides in the Magnolia/Queen Anne area is presented in Figure B-15.  Areas where debris flows are common include Kinnear Park, Perkins Lane W., and along Magnolia Way W.  Near Perkins Lane W., the landslides with debris flows generally originate in the depressions along the undulating slope crest of the upper lodgement till bluff.  Near Kinnear Park and Magnolia Way, steep slopes with relatively unimpeded runout zones dominate these areas.

12.6  Timing of Landslides

A map of Magnolia and Queen Anne showing the distribution of landslides by decade is presented in B-16.  It illustrates that both the west and east sides of Queen Anne and the Perkins Lane W. area of Magnolia are chronic landslide areas.  

12.7  Severe Storm-Related Landslides

A map illustrating the distribution of landslides in Magnolia and Queen Anne during the four most notable landslide winters ( 1933/34, 1971/72, 1986/87, and 1996/97) is presented in Figure B-17.  The most notable trend in the quantity and distribution of the severe storm-related landslides in this area is the large number of 1996/97 landslides in Magnolia.  Conversely, while the 1933/34 precipitation year is believed to be comparable to that of 1996/97 (based on information received from the City), very few 1933/34 landslides are documented in the database in Magnolia.  The severity of the 1933/34 storm was partially responsible for the large number of WPA projects in Seattle (notice the proximity of the 1933/34 events to the WPA project locations along Perkins Lane W. and Westlake Avenue N. on Figure B-10); therefore, the landslides resulting from this storm may not be sufficiently documented.  

12.8  Potential Slide Areas

A map illustrating the coincidence of historical landslides in the Magnolia/Queen Anne study area with the potential slide areas is presented in Figure B-18.  Approximately 81 percent of the historical landslides in the Magnolia/Queen Anne area fall within the currently mapped Potential Slide Areas, as described in Section 20.0 of this report.  Landslides outside of the Potential Slide Area occurred along the upper and lower slopes along the northern portion of Perkins Lane W. in Magnolia and along the east flank of Queen Anne, west of the existing Potential Slide Areas as indicated in City documents.

12.9  Stability Improvements

This section presents possible stability improvements that could be made by the City to protect utilities, drainage features, streets, and other City facilities in the Magnolia/Queen Anne area.  Furthermore, this section includes measures that could be made by adjacent property owners in conjunction with the City to improve the stability of an entire landslide or unstable slope.  We further present comments regarding educating private property owners on steps they may take to improve stability.

The Magnolia/Queen Anne area has been divided into nine smaller Stability Improvement Areas where landslide activity has been prevalent, in order to describe various improvements and homeowner education suggestions.  As shown on Figure B-19 (Appendix B, Map Folio), the nine areas are as follows:

Perkins Lane North

PerkinsLane South

32nd Avenue W.

W. Galer Street

Magnolia Way

Kinnear Park

West Queen Anne

Northwest Queen Anne

East Queen Anne

For each area, we summarize the general subsurface conditions, landslide types and causes, and present actions that could be considered for improving stability.

12.9.1   Perkins Lane North

The Perkins North area, located as shown on Figure B-19, is notorious for instability.  It consists of those properties in and north of the 1900-block of Perkins Lane W. Properties and instability south of the 1900-block are presented subsequently under the Perkins South Stability Improvement Area.

In the Perkins Lane North area, 111 landslides have been reported.  They have occurred throughout the years, being first recorded in 1930 and extending through January 1998.  All four types of landslides have been recorded:  high bluff peeloff (11), groundwater blowout (4), deep-seated (40), and shallow colluvial (56).  The high bluff peeloffs and the groundwater blowouts have been recorded primarily on the uphill side (east) of Perkins Lane.  The other two types of landslides (deep-seated and shallow colluvial) have reportedly occurred on both sides of Perkins Lane.  On a number of occasions, landsliding has damaged the roadway and frequently debris has come down onto the lane.  A number of houses on both sides of Perkins Lane have been destroyed by landslides. 

In the 1900-block along Perkins Lane (south end of this designated area), a 110-foot-long portion of the lane was rebuilt with lightweight fill material (bottom ash from Centralia, Washington) in 1983.  This work was contracted by homeowners in this area in order to repair a landslide that destroyed a portion of Perkins Lane and prevented vehicle access to properties to the south.  A deep subdrain trench was incorporated into this repair effort.

Perkins Lane is located at the western edge of Magnolia, overlooking Puget Sound (refer to Figure B-19).  The lane, reportedly constructed in 1926 and 1927, is situated on an uneven midslope bench.  From the top of Magnolia Bluff to the east, near the location of Magnolia Boulevard W., the ground surface slopes steeply to precipitously down to the west.  The midslope terrace on which Perkins Lane is built slopes moderately to steeply down to the Puget Sound shoreline on the west.  A majority of the shoreline beaches are protected by rock seawalls or concrete bulkheads.  The right-of-way of Perkins Lane is normally 40 feet wide (locally 60 feet); however, the asphalt-paved lane is rarely wider than 20 feet and no sidewalks are present.  Drainage ditches and catch basins are commonly included in the paved section.  To the south of W. Raye Street (approximate center of this designated improvement area) a rail and concrete lagging toe wall, about 4 feet high, is locally present. 

The subsurface conditions in this area are illustrated on Figure 3-2, Idealized Geologic Conditions, West Magnolia Bluff.  As shown, there are five geologic units; however, not all the units are present everywhere and may not be of similar thickness as indicated.  Near the south end of this improvement area, Vashon till is exposed in the bluff, but toward the north, the till is absent and advance outwash sand dominates the hillside.  The elevation of the sand-clay contact varies along Perkins Lane.  Draped over much of the hillside is colluvium (relatively loose), which is commonly thicker at and to the west of Perkins Lane.  On the steep, unvegetated portions of the bluffs, soil loosened by weathering is present.

With respect to groundwater seepage, that which occurs at the contact between the recessional outwash (not always present) and Vashon till is minor.  The more prolific springs emanate from the sand/clay contact.  Seepage can also occur from pervious sand layers within the till or clay units.

The primary factors that contribute to instability in this area are steep to moderately steep slopes, colluvium or weathered soil on the slopes, and high groundwater levels with associated seepage near the sand/clay contact.  The predominant triggering mechanism is heavy rainfall with storm/surface water runoff and infiltration.

As a result of the 1996/1997 storms, 16 landslides were identified by the City between the 1900 and 3400 blocks of Perkins Lane W.  As a result of these landslides, the City contracted for design and construction of remedial measures, which were made in the latter part of 1997.  The stability improvements consisted of drainage improvements, rock buttresses, and catchment/ retaining walls.  The drainage improvements included finger drains, intercept trench subdrains, springhead drains, and directional drains.  The improvements apparently are generally performing as anticipated, although some additional effort is recommended below.

Additional stability improvements that could be considered to protect the lane are catchment/retaining walls at two locations:  3400-block and 2800-block south of W. Barrett Street.  Additional finger drains may be appropriate in the 2800-block and 2300-2400 blocks.  It is recommended that existing storm drainage facilities be maintained, possibilities for improving drainage explored, and homeowner education take place.  In particular, drainage from private properties should be suitably controlled so as not to reduce stability.

12.9.2   Perkins Lane South

This Stability Improvement Area consists of the 1700 and 1800 blocks along Perkins Lane W., extending from Magnolia Boulevard W. (to the east) down to the shore of Puget Sound; refer to Figure B-19.  In this area, 17 landslides have been recorded in the Seattle Landslide Database:  listed as high bluff peeloff (7), deep-seated (6), and shallow colluvial (4).  The earliest recorded landslide occurred in 1934.  Since then, landslides have been reported in the 1960s through 1990s.  The high bluff peeloff landslides have occurred primarily from the high bluff on the east side of Perkins Lane.  The recorded deep-seated landslides have generally occurred in colluvium located at and west (downhill) of Perkins Lane.  One of these deep-seated landslides (1972) damaged the west shoulder of the roadway and was repaired by the City with pit-run sand and gravel backfill.  The deep-seated landslides that were recorded in 1996 and for the 1996/1997 storm also involved the movement of bluff soils uphill from the lane.  The shallow colluvial landslides also occurred in colluvium located on the downhill side of the residences west of Perkins Lane.

The site topography is generally similar to that described for the Perkins North area.  In this Perkins South Stability Improvement Area, Perkins Lane is also situated on a midslope bench.  This area slopes from Magnolia Boulevard steeply to precipitously some 75 to 100 feet downward to Perkins Lane on the west.  The private properties to the west of Perkins Lane slope moderately to steeply downward an additional 80 to 90 feet (vertical measurement) to the Puget Sound shoreline.  Most of the shoreline, except toward the north, is protected by some type of seawall (rocks or timber piles).

The subsurface conditions in this area consist of Vashon till or till-like soils exposed in the bluff to the east of Perkins Lane, and a relatively thick layer (up to 25 feet or more) of colluvium beneath and downslope of Perkins Lane.  Both the till/till-like zone, which contains sand layers, and the colluvium overlie hard clay.  The Esperance Sand that normally overlies the clay is absent, based on those borings made in this area.  Groundwater is present in sand layers within the till and clay, and is also present in the relatively loose colluvium.

Recent instability in this area was first reported to the City in February 1996.  The movement detected at that time involved City property at and uphill (east) of Perkins Lane and the southernmost four private properties on Perkins Lane, although subsequent evaluation indicated that slope movements originated in the colluvium to the west of Perkins Lane.  As a result of this instability that exhibited slope movement through June of 1996, the City graded the bluff back to a flatter inclination and began to install pumping wells along Perkins Lane in order to reduce groundwater pressures.  Before the completion of pumping well installation and additional remedial work, the 1996/1997 storms occurred and slope movement again took place in this area.  Separate movement then took place to the north to include two more residential sites at the south end of Perkins Lane.  Six houses in the 1700-block of Perkins Lane (no houses are in the 1800-block) have now been destroyed by the landsliding.  In general, the slope at and west of Perkins Lane in the 1700 and 1800 blocks has dropped 20 or more feet.  

The primary factors that contribute to instability in this area are the loose nature of colluvium at and downslope of Perkins Lane, the steep bluff above (east of) Perkins Lane, and possibly preexisting planes of weakness behind the bluff face.  The movements were triggered by heavy precipitation and high groundwater levels.  The available data on movement in this area indicate that movement in the colluvium removes lateral restraint for the bluff soils which, in turn, move.

There is litigation in progress with respect to landsliding in this area.  Thus, even preliminary improvement recommendations would not be appropriate at this time; however, as a general statement, improvements to stability likely would include subdrainage installations, lightweight fills, retaining walls, etc.

12.9.3   32nd Avenue W.

In the 32nd Avenue W. Stability Improvement Area, as designated on Figure B-19, eight landslides are indicated.  High bluff peeloff and shallow colluvial type landslides were reported.  The landslides in this area have taken place along the east- and south-facing slope of the ravine, upslope (west) of 32nd Avenue W. and north (upslope) of Logan Avenue W, along the toe of the slope.  Instability in this area was reported as early as 1965.  The most recent instability took place during the 1996/97 winter storm, when two shallow colluvial and one high-bluff peeloff type landslides reportedly occurred.  The high-bluff peeloff landslide was located upslope of Logan Avenue (undeveloped) and impacted the back of a residence along the shoreline of Elliot Bay.  Both January 1997 shallow colluvial landslides took place along the east-facing slope uphill of 32nd Avenue. 

The topography in this Stability Improvement Area generally dictates the distribution of landslide types.  Three high-bluff peeloff landslides occurred (two in 1968 and one in 1997) on the south-facing, near-vertical bluff just north of Logan Avenue.  The instability that took place west of 32nd Avenue was typified by shallow colluvial landslides generally resulting from groundwater seepage, surface water runoff, and filling along the top of the slope, east of Magnolia Boulevard W. 

The subsurface conditions in this area consist of colluvium on the moderate to steep slopes west of 32nd Avenue, overlying glacially overridden native soils.  In some areas, fill material is present, such as for residences along the east side of Magnolia Boulevard, upslope and west of 32nd Avenue.  The overridden soils consist of dense to very dense lodgement till over glaciolacustrine silt and clay.  Although the sand-clay contact (Tubbs, 1974) is mapped in this area, there is no outwash sand present above the glaciolacustrine silt and clay.  Instead, glacial lodgement till directly overlies the silt and clay.  In the vicinity of Logan Avenue, there is little to no colluvium on the south-facing steep bluff. 

The landslides reported in this area primarily occurred in colluvium, weathered bluff soils, and/or fills placed by private property owners.  Two shallow colluvial landslides upslope (west) of 32nd Avenue reportedly were related to saturated, loose soil and triggered by heavy rainfall.  Three high-bluff peeloff landslides were reported along the south-facing slope behind the beach houses along Logan Avenue.  Landslides resulting from residential fills in the 1500-block of Magnolia Boulevard (1965, 1967, 1986) comprise the remaining three landslides reported in this area.  The factors contributing to instability are steep bluffs, loose fill and/or colluvium on the slope, and high groundwater levels/seepage in sand and gravel lenses within the till and lacustrine silt and clay soils.   Heavy rainfall that saturates the loose soil generally triggers the failures.

It is recommended that action by the City include surface drainage maintenance and/or improvement and homeowner education.  Furthermore, springhead drains could be considered along the east-facing slope west of 32nd Avenue to prevent groundwater seepage from infiltrating the downslope colluvial soils.  With respect to the south-facing bluff north of Logan Avenue, there is little the City can do to prevent instability; however, educating the homeowners on the dangers for property and structures at the toe of a steep and unstable bluff should be done.  Homeowner education should emphasize the risks of making fills on slopes and present prudent practices, such as maintenance of private drain systems and construction of catchment walls to protect against landslide debris damage.

12.9.4   W. Galer Street

The W. Galer Street Stability Improvement Area consists of a south-facing, steep slope situated as shown on Figure B-19.  This slope is the eastern extension of the steep bluff north of Logan Avenue W., as described in the previous section.  In this area, ten landslides representing all four types have been recorded, beginning in 1928.  Six landslides were listed as shallow colluvial, two as high-bluff peeloff, one as deep-seated, and one as groundwater blowout type.  All of the landslides occurred between Galer Street (at the toe of the slope) and Magnolia Blvd (north and uphill of Galer Street).  One event, a deep-seated landslide in 1969, undercut the south shoulder of Magnolia Boulevard, which resulted in the City constructing remedial measures.  These measures included a 200-foot-long rail and wood lagging wall along the south shoulder of Magnolia Boulevard to protect the street and placement of rubble along the toe of the slope to protect against beach erosion.  The most recent instability (March 1997) blocked Galer Street with landslide debris.  Five landslides were reported in this area during the 1996/97 winter storm. 

The subsurface soils in this area consist of variable thicknesses of colluvium (thinner in the steep bluff areas and thicker at the toe of the slope) overlying glacially overridden native soils.  In some areas, existing fill may be present, such as along the downslope margin of Magnolia Boulevard.  The overridden soils consist of dense to very dense lodgement till overlying interbedded silt and fine sand, in turn overlying glaciolacustrine clay.  Although the sand-clay contact (Tubbs, 1974) is mapped in this area, there is no outwash sand present above the silt and clay.  During our field reconnaissance in March 1997, we noted seepage coming out of the interbedded silt and fine sand.  Large streams of water were also seen issuing from cracks (joints) in the till about 10 feet below the top of the bluff at the easterly side of the Stability Improvement Area.  Furthermore, during the March 1997 field reconnaissance, we noted that surface water at the top of the slope appears to flow toward the southwest corner of Magnolia Park; however, surface water does not flow over the top of the bluff. 

The factors that contribute to instability in this area are a combination of groundwater seepage occurring 10 to 20 feet below the top of the slope, and cutting at the toe of the slope (along Galer Street, and by beach erosion).  Heavy precipitation appears to be the triggering mechanism for the instability in this area. 

It is recommended that the City consider providing drains for the springs located along the slope.  Such drainage could consist of springhead drains and a deep trench subdrain north of the top of the slope.  It is recommended that homeowner education emphasize maintenance of shore protection along the beach area south of the residences along W. Galer Street, and that the City maintain and/or improve storm drainage facilities along Galer Street and Magnolia Boulevard.

12.9.5   Magnolia Way

The Magnolia Way Stability Improvement Area is the east-southeast-facing slope situated as shown on Figure B-19.  In this area, a total of ten shallow colluvial and groundwater blowout landslides have been recorded, beginning in 1940.  All of the landslides occurred on the steep slope north of the Magnolia Bridge and east of Magnolia Way W.  The most recent instability occurred during the 1996/97 winter storm.  The largest was a deep-seated type landslide that destroyed some of the supports for the Magnolia Bridge.  This landslide resulted in the City constructing remedial measures including a 30-foot-high, permanent tieback soldier pile wall with concrete lagging located near the top of the original hillside extending for 260 feet north of the bridge.  Another, slightly smaller landslide occurred farther north in the vacated W. Blaine Street right-of-way (an east-west street located near the center of the Stability Improvement Area).  Stability for the head-scarp of this landslide was improved with a large soldier pile and wood lagging retaining wall, yet surficial sloughing still exists along the north and south margins of the landslide scar (observed during field reconnaissance in July 1998).

The subsurface conditions consist of loose to medium dense colluvium and recessional outwash sand overlying glacially overridden native soils.  In some areas, fill may be present, such as for backyards.  The overridden soils consist of glacial lodgement till overlying interbedded sand and silt which, in turn, overlies glaciolacustrine silt and clay.  The sand? clay contact (Tubbs, 1974) is mapped in this area.  Groundwater levels are typically high and groundwater seeps and springs exist throughout this Stability Improvement Area. 

The contributing factors to instability are the steep topography, high groundwater levels and associated seepage and springs, and surface water and roof runoff.  The landslides were generally triggered by periods of heavy rainfall that resulted in heavy runoff and infiltration into the slope soils. 

Recommended action in this area includes homeowner education that strongly emphasizes the proper control of on-site drainage systems, particularly downspout discharge, and discharge of drainage in accordance with government regulations.  The City could also consider improving storm drainage systems (curbs and gutters, etc.).

12.9.6   Kinnear Park

The Kinnear Park Stability Improvement Area is the southwest-facing steep slope situated along the southwest side of Queen Anne Hill, as shown on Figure B-19.  Twelve landslides have been recorded for this area, generally on the steep slope/bluff above Elliott Avenue W. (located at the toe of the slope) and downslope of 9th Avenue W. and W. Olympic Place (near the top of the slope/bluff) along the east margin of the Stability Improvement Area.  All four types of landslides are recorded in this area: deep-seated (2), groundwater blowout (2), shallow colluvial (7), and high bluff peeloff (1).  The earliest recorded instability occurred in 1933 (two landslides).  The high bluff peeloff-type landslide occurred during the 1996/97 winter storms toward the southern portion of the Stability Improvement Area where the slope is steepest.  Debris from this landslide as well as from two other landslides along the southern steep bluff area (downslope of Olympic Place) impacted several structures along Elliott Avenue W.  Other areas of instability include several landslides that occurred along the downslope side of 9th Avenue (in the northern portion of the Stability Improvement Area); and along the W. Prospect Street right-of-way (near the center of the Stability Improvement Area).  Older landslides (1950 and 1933), reportedly influenced by sewer breaks, occurred along the lower portion of the slope in the vicinity of the VanBuren Avenue W. right-of-way.  Previous landslide repairs conducted by the City in this area ranged from simple removal of landslide debris to bio-engineered vegetation mats with interceptor drains (in the vicinity of W. Prospect Street).

The subsurface conditions consist of variable thicknesses of colluvium overlying glacially overridden native soils.  In some areas, existing fill may be present, specifically in the northern portion of the Stability Improvement Area.  The glacially overridden soils consist of an upper layer of lodgment till (jointed) overlying interbedded silt and gravelly, fine sand which, in turn, overlies glaciolacustrine clay.  Abundant groundwater seepage occurs from the interbedded silt and sand zone.  The moderate to steep slope west of 9th Avenue exhibits active signs of creep and appears to be an ancient landslide scar.  Development in this area has included various amounts of fill material that appears to contribute to the instability west of 9th Avenue. 

The factors that contribute to instability in this area are steep topography (specifically in the southern portion of the Stability Improvement Area), high groundwater levels/seepage, and cuts and/or fills (particularly in the northern portion of the area).  Based on field reconnaissance shortly after the 1996/97 winter storms, there were little to no signs of surface water runoff over the top of the steep bluff area. 

Stability improvements that the City could consider consist of interceptor-trench subdrain installation, storm drainage systems maintenance and/or improvement, and homeowner education.  These improvements are discussed further in the following paragraphs. 

Measures the City could consider to reduce the rate of bluff regression include constructing an MSE wall, a geotextile-reinforced soil slope, or flattening the slope face in combination with an interceptor trench drain.  Constructing an MSE wall or reinforced soil slope would be a long-term solution to bluff regression above the wall base elevation, and would be less expensive than a concrete pile wall.  An alternative solution would be to flatten the slope face in conjunction with installing an interceptor trench north of the slope crest.  This latter alternative should provide sufficient groundwater drainage while reducing the volume of excavated and imported material and, thus, is less expensive than the other two alternatives.  With respect to reducing the risk of damage when landslides occur, the City could consider removal of selected trees that may impact structures along Elliott Avenue and removal of precariously perched soil blocks on the slope (coordinate with private property owners).  Private parties planning new construction or stability improvements for existing homes in the northern portion of the Stability Improvement Area (west of 9th Avenue) should obtain professional advice.  Retaining/catchment walls could also be considered along the toe of the slope to protect downhill properties adjacent to Kinnear Park and along Elliott Avenue.  It is recommended that homeowner education emphasize proper control of on-site drainage systems, particularly in the north half of the Stability Improvement Area.

12.9.7   West Queen Anne

The West Queen Anne Stability Improvement Area is the west-facing slope situated north of the Kinnear Park Stability Improvement Area, as shown on Figure B-19.  The hillside has a steep upper and lower slope separated by a mid-slope bench.  The hillside above the bench is about 100 feet high and has slopes between 20 and 50 degrees with the horizontal.  The steepest part of the slope is at the top, just below the houses along 11th Avenue W. and 12th Avenue W.  The hillside below the bench is also about 100 feet high and has slopes between 25 and 45 degrees. 

In this area, a total of 23 landslides have been recorded, beginning in 1909.  Three types of landslides are documented in this area, consisting of: 4 groundwater blowout, 7 deep-seated, and 12 shallow colluvial landslides.  Some of the landslides occurred on the steep slope just below 11th and 12th Avenues, while others took place on the steep slope below the mid-slope bench.  Instability during the 1996/97 winter storms occurred on both the upper and lower slopes in this Stability Improvement Area.  In particular, 1996/97 landslides affected the City (Seattle Parks Department) maintenance facility along 15th Avenue W., the intersection of W. Galer Street and 11th Avenue, and a soldier pile retaining wall behind a residence along 11th Avenue (south of the intersection of Galer Street).  In 1989, several horizontal drains were drilled and connected to a catch basin located mid-slope in the vicinity of 12th Avenue to improve stability of the slope for new residences near W. Blaine Street (near the center of the Stability Improvement Area).  The City performed repairs to 12th Avenue (at W. Garfield Street), where a reactivation of the 1951 deep-seated landslide occurred during the 1996/97 storm. 

In 1998, deep-seated instability occurred between Garfield Street and Galer Street, and extends down to the Magnolia (Garfield Street) Bridge on-ramp.  A combination of subsurface drainage improvements, retaining walls, and grading has been implemented by the City in this area.

The subsurface soils in this area consist of colluvium and recessional outwash sand overlying glacially overridden native soil.  The thickness of the colluvial layer varies considerably over the slope; it is as thin as 1.5 feet along the steep slopes and as thick as 13 feet at the midslope bench and at the toe of the lower slope.  The overridden soils consist of lodgment till and outwash sand in the upper steep slope, which overlies glaciolacustrine silt and clay.  The top of the clay unit occurs at the mid-slope bench.  Abundant groundwater seeps and springs exist at the top of the lower slope at the edge of the bench.  Water also ponds along portions of the mid-slope bench, the overflow of which was reported to have contributed to several landslides along the lower slope.

Contributing factors to instability are steep topography, loose soil conditions, high groundwater levels/seepage and springs, pond overflow from the bench onto the lower slope, residential and road fills along the top of the upper slope, and heavy precipitation (triggering cause).  Furthermore, based on a review of previous Shannon & Wilson reports, abandoned pipes and tightlines discharging storm-water runoff onto the mid-slope bench from upslope sources may also contribute to instability.

Stability improvements that the City could consider in this area consist of interceptor trenches, control of surface water runoff onto downslope areas, and homeowner education.  Several springhead drains could be installed at points of known seepage, although the installation of a deep interceptor trench along the outer edge of the midslope bench would be a more positive method of improving long-term stability of the lower slope area.  The City could consider installation of a catchment/retaining wall at the toe of the lower clay slope to protect City utilities and other structures from damage by landslide debris originating from the bench or along the lower slope.  Improperly directed surface water runoff, including discharge from tightlines and other abandoned utilities, should be eliminated if found.  Ponded water on the midslope bench should also be eliminated.  Homeowner education should emphasize proper control of on-site drainage systems, specifically eliminating downspout discharge onto downslope soils.

12.9.8   Northwest Queen Anne

The Northwest Queen Anne Stability Improvement Area is the north-facing slope located at the northern tip of Queen Anne Hill, as shown on Figure B-19.  In this area, a total of 14 shallow colluvial and deep-seated landslides have been recorded, beginning in 1922.  Some of the landslides occurred on the moderate to steep slope between W. Emerson Street (uphill and south of W. Nickerson Street) and W. Nickerson Street (at the toe of the slope).  Others took place on the northwest-facing slope between 13th Avenue W. (generally uphill and east of 15th Avenue W.) and 15th Avenue W. (west and near the toe of the slope).  The most recent instability occurred along Nickerson Street in October 1997 along the edge of an unimproved alley south of a residence.  Approximately 10 of the recorded landslides in this Stability Improvement Area were reported to be related to improper fills and/or cuts by both public and private property owners.  For example, the 1922 landslide (located at the northern tip of the Stability Improvement Area) was reportedly related to the grading of Nickerson Street, which resulted in the City constructing a rail (trolley) and concrete lagging toe wall along the south margin of Nickerson Street.  Other reported landslides include a rockery failure behind an apartment building, instability related to the grading of Emerson Street without proper slope retention, and several reactivations of a large deep-seated landslide in the alley west of 13th Avenue. 

The subsurface conditions consist of colluvium on the moderate to steep slopes overlying glacially overridden outwash sand and glaciolacustrine silt and clay.  In many areas, fill may be present, such as for roads and private structures.  The sand-clay contact (Tubbs, 1974) is mapped in this area.  The construction of Nickerson Street and Emerson Street likely included several cuts along the south margins of the roadways, and Emerson Street may have some fills along the northern margin. The factors that may contribute to instability in this area are cuts and fills made by both public and private property owners, and high groundwater levels and associated seepage.

Homeowner education would be appropriate to emphasize obtaining professional advice for improving stability for existing homes, additions, or new construction, and controlling on-site drainage.

12.9.9   East Queen Anne

The East Queen Anne Stability Improvement Area consists of an east-facing steep slope generally between Dexter Avenue N. (uphill and west of Westlake Avenue N.) and Westlake Avenue N. located at the toe of the slope (east of Dexter Avenue N.); refer to Figure B-19 for location.  Twenty landslides have been recorded for this area, all of which are located on the steep slope above Westlake Avenue N.  Nineteen landslides were listed as shallow colluvial and one was deep-seated.  The earliest recorded landslide was in 1926, and instability has been reported through the years.  Five shallow colluvial landslides occurred during the 1996/97 storm, and at least two of them deposited debris onto the southbound lanes of Westlake Avenue.  Three landslides along Dexter Avenue (2500-block) were reported in 1933, 1954, and 1969 to be related to the grading of Westlake Avenue by the City in 1920.  Subsequent instability along the downslope side of the 2500-block of Dexter Avenue was reported in 1978, 1982, 1986, and 1997.

The subsurface conditions in this area consist of a silt-clay colluvium overlying stiff to hard clay.  Groundwater levels are typically high because this area is at or near the toe of the slope.  The sand-clay contact (Tubbs, 1974) is mapped west (upslope) of this Stability Improvement Area.  The contributing factors to instability are the steep topography, the soil conditions on this slope (colluvium over stiff to hard clay), undercutting or filling on the slope, and high groundwater levels/seepage.  The landslides were triggered by periods of heavy rainfall that resulted in surface runoff and infiltration into the slope soils.

Stability improvements the City could consider consist of a deep interceptor-trench subdrain installation (locally), catchment/toe wall installation, and homeowner education.  For example, installation of a deep interceptor-trench subdrain extending south along portions of 8th Avenue and 9th Avenue (rights-of-ways that are closest to the slope crest), keyed into the stiff to hard, overridden silt and clay, may be effective in reducing groundwater levels downslope locally.  In addition to reducing groundwater levels, a method for decreasing the landslide risk to Westlake Avenue would be a catchment/toe wall along the west side of Westlake Avenue in areas most frequently impacted by sliding.  It is recommended that homeowner education emphasize minimizing cuts and fills along the slope as well as properly controlling on-site drainage, including downspouts and surface water runoff. 

13.0  MADRONA

Madrona represents one of the oldest neighborhoods in Seattle.  Therefore it has some of the oldest recorded landslides in the database. 

13.1  Site Description

The Madrona study area is located along Lake Washington in east Seattle.  The study area extends from E. Madison Street to Coleman Park, located south of Interstate 90.  Madrona lies on the east-facing slope of Mount Baker Ridge, which is an elongated ridge that extends south from Madison Park to Mt. Baker Park (south of Colman Park).  The Madrona area is moderately incised by several short, steep gullies that are occupied by intermittent streams in Colman Park, Madrona Park, Leschi Park, and Frink Park.  The slopes of Madrona are flatter than the West Seattle and Magnolia/Queen Anne study areas.  Because Madrona is located along the relatively placid Lake Washington, there are no high bluffs or near-vertical slopes resulting from shoreline erosion at the base of the slopes.  Furthermore, both Lake Washington Boulevard and Lakeside Avenue protect the west shoreline of Lake Washington from erosion.  Portions of these roads and at least one row of residences or a strip park are located on the former lake bottom that was exposed when the lake level was lowered about 10 feet in 1916 for the Lake Washington Ship Canal project.  Refer to Figures B-20 through B-28 for Site Plans of the Madrona Area. 

13.2  Soil Stratigraphy

The soils that underlie the Madrona study area are products of the most recent glaciation of the central Puget Lowland.  Because the Seattle Fault extends through the Madrona study area, the depth to Tertiary bedrock increases from less than 300 feet on the south side of the fault to 1,000 to 3,000 feet on the north side of the fault.  There are no bedrock outcrops in the Madrona study area.  The primary geologic units involved with landsliding in the Madrona area are the pre-Vashon glacial and nonglacial deposits, Vashon glacial deposits, and colluvium.

13.3  Groundwater

Groundwater plays a key role in slope instability in the Madrona study area.  The contact between advance outwash (Esperance) sand and glaciolacustrine silt and clay (Lawton Clay) is between elevations 160 feet and 250 feet and extends roughly from Madison Park, south to Colman Park (refer to Figures B-21 through B-28).  Because most slopes in Madrona are moderately inclined and have thick accumulations of colluvium, groundwater from the sand-clay contact generally does not flow to the surface, but rather moves downslope within the colluvium.

13.4  Landslide Types

13.4.1   High Bluff Peeloff Landslides

Because the Madrona study area has no steep bluffs, there are no high bluff peeloff landslides documented in the database for this area.

13.4.2   Groundwater Blowout Landslides

Only one groundwater blowout landslide is documented in the Madrona study area (refer to Figure B-21).  Although this landslide does not occur near the sand-clay contact, its elevation is consistent with the elevation of the top of the impermeable clay/silt unit.  Furthermore, groundwater does not necessarily emerge from a single stratigraphic horizon due to gradational changes between advance outwash and lacustrine units.  Based on the spatial distribution of the shallow colluvial landslides in the Madrona study area with respect to the sand-clay contact (see Figure B-23), the effect of groundwater on landsliding is significant, in our opinion.  An explanation for the lack of documented historical groundwater blowout landslides in the Madrona area may, therefore, be that springs and seeps are rarely observed emerging from discrete points along the sand-clay contact.  Water from the sand-clay contact apparently travels along the colluvium-clay contact, contributing to landsliding of the colluvium.  The location of the resulting landslide may therefore be some distance downslope of the sand-clay contact.  Without exposures of the glacial soils or other evidence of groundwater seepage, it is difficult to identify a groundwater blowout landslide. 

13.4.3   Deep-Seated Landslides

A map illustrating the distribution of deep-seated landslides in the Madrona study area is presented in Figure B-22.  The highest densities of deep-seated landslides in the Madrona study area occur in the vicinity of South Dose Terrace and the 1100 block of Lake Washington Boulevard S.  With the exception of a deep-seated landslide that occurred in 1959 on S. Judkins Street, all the deep-seated landslides in these two areas occurred prior to 1936.  The conspicuous absence of deep seated landslides after 1940 may be a result of increased development and related surface and subsurface drainage improvements.

13.4.4   Shallow Colluvial Landslides

Shallow colluvial landslides in the Madrona study area are shown on Figure B-23.  This type of slope instability is the most common in Madrona.  The highest densities of shallow colluvial landslides are located near the 1200 block of Lakeside Avenue S., Lake Dell Avenue E., Madrona Drive, and McGilvra Boulevard E. 

Based on "The Preliminary Geologic Map of Seattle and Vicinity, Washington" (Waldron, and others, 1962), nearly all of the historical shallow colluvial landslides in the Madrona area occur on slopes underlain by glaciolacustrine silt and clay.  In other words, nearly all of the shallow colluvial landslides documented in the Madrona area lie at, or topographically below, the sand-clay contact, as shown on Figure B-23.

13.5  Landslides with Debris Flows

Because the slopes in the Madrona study area are flatter than those in the other two study areas, the occurrence of debris flows is limited.  A map showing the distribution of landslides with debris flows in the Madrona study area is presented on Figure B-24.  The highest concentration of debris flows in the Madrona study area occurs in the 200-block of Lake Dell Avenue E. where the undeveloped slopes are among the steepest and longest in Madrona. 

13.6  Timing of Landslides

A map displaying the distribution of all landslides by decade in the Madrona area is shown on Figure B-25.  It suggests that the likelihood of landsliding in the Madrona area has not changed over time.  Shallow colluvial landsliding is the primary mode of ground failure in Madrona in more recent years.

13.7  Severe Storm-Related Landslides

A map showing the distribution of landslides resulting from the four major storm events occurring in the Madrona study area in the past century (1933/34, 1971/72, 1986/87, and 1996/97) is shown on Figure B-26.  Landslides from the winter 1986 storm account for nearly 50 percent of the total number.

13.8  Potential Slide Areas

A map displaying the distribution of historic landslides in the Madrona study area with respect to the existing City maps showing Potential Slide Areas, as described in Section 20.0 of this report, is shown on Figure B-27.  It shows that most of the landslides occurred within the existing Potential Slide Areas (79 percent).  Exceptions include landslides in the vicinity of the 200-block of Lake Dell Avenue E., south of Colman Park, and near the 2000-block of Lake Washington Boulevard S.

13.9  Stability Improvements

This section presents possible stability improvements that could be made by the City to protect utilities, drainage features, streets, and other City facilities.  Measures are also presented that could be made by the City and adjacent property owners to improve the stability of an entire landslide or unstable slope.  We present further comments regarding educating private property owners on steps they may take to improve stability.

In order to describe various improvements and homeowner education suggestions, the Madrona area has been divided into seven smaller Stability Improvement Areas, where landslide activity has been prevalent.  As shown on Figure B-28 (Appendix B, Map Folio), the seven areas are as follows:

Hillside Drive

32nd Avenue E.

Madrona Drive

Madrona Park

Lake Dell

Lakeside North

Lakeside South

For each area, we will summarize the general subsurface conditions, landslide types and causes, and present actions that could be considered for improving stability.

13.9.1   Hillside Drive

For the Hillside Drive Stability Improvement Area, as designated on Figure B-28, five landslides are indicated.  Deep-seated (2), shallow colluvial (2), and groundwater blowout (1) landslides have been recorded in this area since 1946.  The landslides in this area have taken place on the east-facing slope between 36^th Avenue E. (uphill and west of Hillside Drive E.) and Hillside Drive E., as well as along the east shoulder of Hillside Drive E.  The most recent instability took place approximately 50 feet downslope of the 600-block of 36th Avenue and flowed across Hillside Drive in the spring of 1997.  The debris from this event also impacted several properties on the downhill (east) side of Hillside Drive. 

The landslides that occurred in this area prior to 1950 were related to instability along the downhill (east) side of Hillside Drive, possibly in fill either placed for residences or during the grading of Hillside Drive.  During our field reconnaissance for this phase of the study, we observed settlement and areas of ponded water in the same location as the pre-1950 landslides along the east portion of Hillside Drive. 

The subsurface conditions in this area consist of a silt-clay colluvium over glacially overridden native deposits.  The glacial deposits consist of an outwash sand unit and a lower glaciolacustrine clay unit.  Although the sand-clay contact is not mapped in this area, several groundwater seeps and springs exist along the west (uphill) side of Hillside Drive E.  The factors contributing to instability are the soil conditions on the slope (as much as 25 feet of silt-clay colluvium), high groundwater levels with associated seeps and springs, settlement of fill, and ponded water along the east side of Hillside Drive E.

To improve stability for the Hillside Drive area, we recommend that the City consider improving the surface drainage systems along Hillside Drive, installing springhead and/or finger drains, and promoting homeowner education.  Replacement and compaction of existing fill areas and installation of a new curb along the east side Hillside Drive may be the most cost-effective way to reduce ponding, runoff, and infiltration of surface water on downslope areas.  We further recommend that the City consider recording the location and amount of paving placed along Hillside Drive in order to evaluate specific areas along Hillside Drive where fill settlement is a potential problem.  Springhead and/or finger drains could be effective in reducing instability associated with groundwater seeps and springs along the uphill side of Hillside Drive.  It is recommended that homeowner education emphasize prudent construction practices and controlling on-site drainage systems.

13.9.2   32nd Avenue E.

In the 32nd Avenue E. Stability Improvement Area, as designated on Figure B-28, six landslides are indicated.  Both deep-seated and shallow colluvial landslides occurred.  The landslides in this area have taken place along the west-facing slope generally between 34th Avenue E. (uphill and east of 32nd Avenue E.) and 32nd Avenue E. near the toe of the slope.   Instability in this area was reported as early as 1910, shortly after the grading of 32nd Avenue in 1907.  One landslide reportedly was related to piping (soil movement) in trench fill for the water service to a residence in the 300-block of 34th Avenue.  The most recent instability took place during the severe winter storm in 1972, which damaged several houses along the west side of 33rd Avenue E. in the 1700 block.  This landslide was reported to be related to an excavation downslope of the affected residences. 

The subsurface conditions consist of relatively thick colluvium on the moderately-steep slopes overlying glacially overridden native soils.  The sand-clay contact extends through this area.  The original construction of 32nd Avenue likely included cutting into the toe of the slope between E. Denny Way and E. John Street.  After several failures of a wood bulkhead prior to 1941, the City constructed a 4- to 6-foot-high concrete bulkhead/toe wall along much of the east side of 32nd Avenue between Denny Way and John Street. 

The factors that contribute to instability in this area are the thick colluvium, high groundwater levels with associated seepage along the sand-clay contact in the vicinity of the 33rd Avenue right-of-way, piping of soil in trench fill, and cutting and filling.

In the long term, there does not appear to be any additional remedial measures that the City could take to prevent the natural occurrence of landsliding in this area other than an evaluation of the existing bulkhead along 32nd Avenue, particularly where the wall appears to be slightly bowed, and homeowner education. 

13.9.3   Madrona Drive

The Madrona Drive Stability Improvement Area is the east-facing slope situated as shown on Figure B-28.  In this area, six landslides have been reported since 1935 in the 1500‑block along the east side of Madrona Drive.  Two other landslides occurred in 1951 and 1960 just uphill from Lake Washington Boulevard E.  Some of the landslides were related to improper fills along the east side of Madrona Drive in the 1500 block, and others were related to groundwater seepage.  Two shallow colluvial landslides during the 1960s (the two youngest slides reported in this area) damaged structures located in a small ravine at the toe of the slope. 

The subsurface conditions consist of an estimated 5 to 15 feet of colluvium on the steep slopes overlying glacially overridden outwash sand and lacustrine silt and clay.  Although the sand-clay contact (Tubbs, 1974) is mapped at higher elevations and west of this Stability Improvement Area, groundwater seepage, likely from pervious zones within the silt/clay soils, exists along the steep slope east of Madrona Drive.  In some areas, fill may be present along the east side of Madrona Drive.  Settlement along the east margin of the street was observed during our field reconnaissance in October 1998 where the curb and sidewalk settled to below the street surface in some locations.  This settlement permits surface water from Madrona Drive to flow onto downslope properties.  In 1936, the WPA performed extensive drainage work in the vicinity of the 1500 block of Madrona Drive.

The factors that contribute to instability in this area are fills along the east side of Madrona Drive, steep topography, and high groundwater levels/seepage.  The triggering mechanism is heavy rainfall with surface water runoff and infiltration. 

Stability improvements that the City could consider consist of improving storm drainage and educating homeowners.  For a distance of about 500 feet, drainage improvements could be considered along the east margin of Madrona Drive (curbs/gutters/catch basins, etc.).  It is recommended that homeowner education emphasize the management of surface water using suitable drainage systems (such as discharge to storm drains) to reduce the risk of shallow colluvial and deep-seated landslides that may damage downslope and upslope properties. 

13.9.4   Madrona Park

In the Madrona Park Stability Improvement Area, located northwest of Madrona Park, six shallow colluvial landslides are indicated, refer to Figure B-28.  The landslides took place along the east-facing slope between 36th Avenue  (two blocks west and uphill of Madrona Park) and 38th Avenue/Newport Way, located near Madrona Park.  Instability in this area was reported as early as 1914.  The two most recent landslides in this area took place in the winter of 1986.  A contributing cause of one of these landslides was reported to be surface water runoff from the storm drain at the 38th Avenue dead-end.  Infiltration of this surface water into the colluvial soils adjacent to a residence apparently triggered a shallow colluvial landslide that undermined the foundation of a residence.  The property owner obtained professional geotechnical advice and constructed a 6- to 10-foot-high concrete retaining wall in the vicinity of the headscarp. 

The two landslides that occurred in this area prior to 1960 were reported to have been related to excavations for Newport Way (December 1914) and 38th Avenue (December 1956).  The instability that took place following 1960 was generally related to storm-water drainage and over-watering during the summer. 

The subsurface conditions in this area consist of silt-clay colluvium located over glacially overridden outwash sand and glaciolacustrine clay.  The sand-clay contact is mapped in this area, generally between 36th and 38th Avenues.  Abundant groundwater seepage was observed along Newport Way and 38th Avenue during our field reconnaissance in October 1998.  The factors contributing to instability are the wet soil conditions on the slope (high groundwater levels/seepage), undercutting of the slope, and surface water runoff, specifically near the stairway along the 38th Avenue right-of-way extending down to Newport Way. 

Recommended actions for consideration in this area include storm drainage systems maintenance and/or improvement, installation of springhead and/or finger drains, and homeowner education.  The storm drain and surface water catchment area located at the dead end of 38th Avenue could be improved by increasing the capacity of the catch basin to prevent overflow of ponding water onto the downslope stairway area.  Springhead and/or finger drains installed along the west (uphill) side of Newport Way and 38th Avenue (located just west of Madrona Park) could be effective in reducing instability associated with groundwater seeps and springs.  It is recommended that the gutter area along the west side of Newport Way be cleared of vegetation and other debris to facilitate proper drainage.  Homeowner education is recommended to specifically address the need to check and clean vegetation and other debris from public stormdrains in the vicinity of their homes.

13.9.5   Lake Dell

The Lake Dell Stability Improvement Area consists of a south- and east-facing steep slope situated as shown on Figure B-28.  In this area, 16 shallow colluvial and deep-seated landslides have been recorded beginning in 1897.  Some of the landslides occurred upslope of Lake Dell Avenue E. and others took place on the moderately-steep slope downhill.  Lake Dell Avenue, cut prior to 1920, traverses this steep slope.  The most recent instability occurred uphill from Lake Dell Avenue in March of 1997, related to a cut made for a private driveway in the 200-block.  Eight of the landslides in this area reached Lake Dell Avenue and at least four of these blocked portions of Lake Dell Avenue. 

The subsurface conditions consist of colluvium on the steep slopes overlying glacially overridden outwash sand and glaciolacustrine silt and clay.  The sand-clay contact (Tubbs, 1974) extends through this area.  In some areas, fill may be present, such as for backyards.  The original construction of Lake Dell Avenue likely included some cuts along the west and north side and filling along the east and south.  Two 1933 landslides that occurred near the 100-200 block of Lake Dell Avenue were attributed to surface water runoff onto downslope areas; however, we observed adequate curbs and gutters along the east and south side of Lake Dell Avenue during our field reconnaissance in October 1998. 

The factors that contribute to instability in this area are the steep topography, road and private property cutting and/or filling, high groundwater levels and associated seepage near the sand-clay contact, and private storm-water discharge.  Many of the reviewed landslides were reportedly triggered by periods of heavy rainfall that resulted in surface runoff and infiltration into the slope soils.

Recommended action for consideration by the City in this area includes retaining/ catchment wall construction, storm drainage systems maintenance and/or improvement, springhead drains installation, and homeowner education.  A retaining/catchment wall along Lake Dell Avenue on the oversteepened, upslope side of the road could be effective for increasing upslope stability and preventing landslide debris from blocking the roadway.  Several springhead drains placed at known points of groundwater seepage and springs could be effective in reducing instability along the uphill side of Lake Dell Avenue.  It is recommended that homeowner education emphasize management of on-site drainage systems to prevent improperly directed surface water runoff. 

13.9.6   Lakeside North

The Lakeside North Stability Improvement Area consists of an east-facing moderately-steep slope generally between 35th Avenue S. (uphill and west of Lakeside Avenue S.) and Lakeside Avenue S. at the toe of the slope, along the shore of Lake Washington, as shown on Figure B-28.  In this area, a total of eight landslides are recorded, consisting of three deep-seated and five shallow colluvial landslides, since 1928.  The most recent instability occurred in 1991, which was related to an excavation by a private property owner downhill from the 300-block of 35th Avenue.  Other instability was reported in the vicinity of the stairway in the S. Jackson Street right-of-way in 1948 and 1958.  In the vicinity of S. Leschi Place and Lakeside Avenue S., two landslides were reported, one of which overtopped the concrete bulkhead on the west side of Lakeside Avenue in 1991. 

The subsurface conditions in this area consist of a silt-clay colluvium located over stiff to hard clay.  The sand-clay contact is located upslope of this Stability Improvement Area.  The contributing factors to instability are the soil conditions on this slope (colluvium over stiff to hard clay), road cuts, and groundwater seepage. A 15- to 20-foot-high concrete retaining wall exists along the road-cut on the west side of the 400-block of Lakeside Avenue.  Above the wall, west of Lakeside Avenue, an upper road runs parallel to Lakeside Avenue.  The cut-slope along the west side of the upper road was wet from high groundwater levels/seepage and appeared to be unstable in the vicinity of the Jackson Street stairway.

Stability improvements that the City could consider consist of wall construction and homeowner education.  A catchment/retaining wall along the west side of the upper roadway in the vicinity of the Jackson Street right-of-way could be considered along with improvement of the surface drainage in this area.   A catchment/retaining wall along the cut-slope along the 300-block of Lakeside Avenue, north of Leschi Place, could also be considered to prevent landslide debris from blocking Lakeside Avenue.  It is recommended that homeowner education emphasize prudent construction practices and management of on-site drainage systems. 

13.9.7   Lakeside South

The Lakeside South Stability Improvement Area consists of the east-facing slope as shown on Figure B-28.  In this area, deep-seated and shallow colluvial landslide types make up 16 of the 17 total landslides reported since 1925.  One landslide was not identified as to its type.  The landslides in this area took place along the east-facing slope generally between 32nd Avenue S. (uphill and west of Lake Washington Boulevard S.) and Lakeside Avenue S. (at the toe of the slope, east of Lake Washington Boulevard S.).  There are two general areas of instability.  The first area, upslope of Lake Washington Boulevard (1100-block), consists of mostly pre-1955, deep-seated type landslides.  The second area, located upslope of Lakeside Avenue (1300-block), consists of both recent and older, deep-seated, and shallow colluvial landslides.  The most recent instability was a reactivation of a 1983 landslide that took place along the 1300-block of Lakeside Avenue in 1986.  Upslope of the 1100-block of Lake Washington Boulevard, several old (pre-1940) deep-seated landslides reportedly affected structures along the shore of Lake Washington.  The WPA performed extensive subsurface drainage work in the vicinity of S. Judkins Street and Lake Washington Boulevard S. during the 1930s.  With the exception of three records of instability during the 1986 winter storm, the Lakeside South Stability Improvement Area has been relatively stable for the past 15 years.

The subsurface conditions in this area consist of colluvium overlying glacially overridden glaciolacustrine silt and clay. The sand-clay contact (Tubbs, 1974) is mapped uphill from Lake Washington Boulevard.  The contributing factors to instability are the high groundwater levels with associated seepage and wet soil conditions, cutting at the toe of the slope and filling near the top, and probable pre-existing, ancient landslide blocks in the vicinity of the 1100-block of Lake Washington Boulevard S.

To improve the stability of the Lakeside South Stability Improvement Area, the City could consider surface drainage maintenance and/or improvement, catchment/retaining wall construction, finger drains or springhead drains, and homeowner education.  It is recommended that the storm-water gutter along Lakeside Avenue, at the toe of the slope, be cleaned and maintained.  Abundant groundwater seepage exists in this area.  Therefore, several finger drains or springhead drains along Lakeside Avenue could be effective in reducing groundwater seepage along the slope.  A drained catchment/retaining wall could be constructed at the toe of the steep slope along the west side of the 1300-block of Lakeside Avenue, to prevent shallow colluvial and occasional deep-seated landslides from encroaching onto the southbound lanes of Lakeside Avenue.  It is recommended that homeowner education emphasize proper construction methods, and maintenance of on-site private and public storm drainage systems.