Landslide Mitigation Program

March 1, 2005

Background

SDOT's landslide mitigation program funds the analysis of landslide prone areas, works cooperatively with other City Departments to develop strategies for decreasing the potential for landslides, and funds mitigation projects. The program helps fund high priority projects as identified by a Landslide Inter-Departmental Team (LIDT). In addition, it provides funding for landslide preventative actions and/or repairs to existing small-scale landslides where there is an imminent hazard to the street system.

Starting from 1999, Seattle Department of Transportation (SDOT) initiated four engineering studies as a part of the City's landslide mitigation program. Those studies included the Retaining Wall Drain Inventory Study, the Retaining Wall Inspection Services, Landslide Risk Assessments, and Slope Reconnaissance in High Priority Sites from the Landslide Risk Assessment study.

Following the above risk assessment study and slope reconnaissance, SDOT closely worked with Seattle Public Utilities (SPU), Parks and City Budget Office to develop criteria for ranking potential city landslide projects related to city-owned facilities and properties.

Landslide Mitigation Projects - Citywide Priorities

In 2002, based on the developed criteria, the LIDT (Landslide Inter-Departmental Team) identified four citywide landslide mitigation prioritized projects :

• Lakeside PL NE (completed in 2004)
• 41st Ave NE & NE 142nd Street
• Golden Garden Dr. NW
• SW Admiral Way

11900 Block Lakeside PL NE
This is one of four citywide landslide priority sites. In early 2002, a landslide damaged a guardrail and threatened to undermine the roadway. Access along the Burke Gilman Trail and Riviera Pl. NE below the slide area could be severely impacted if there is further slide activity. In late 2003, SDOT took the lead and started preliminary geotechnical study by a consultant. Based on recommendation from geotechnical study, a three-teired reinforced Keystone wall was designed by SDOT in-house design force. The construction started in early September, 2004 and completed in October 2004. The wall is approximately 40 feet in length and 20 feet in height. A 150-ft 6-inch diameter drainage pipe was installed to discharge groundwater collected behind the wall down to an existing ditch next to the Burke Gilman Trail. After final inspection, SDOT's Landscape crew planted vegetation in the project area and a street maintenance crew resurfaced the roadway in the area.

Before

After

41st Ave NE & NE 142nd Street (Pre-design)
The street has been settling over the years due to loose fill and groundwater under the street. In late 2003, SDOT started preliminary geotechnical study. The geotechnical report recommended a Soldier Pile wall as a cost-effective solution to stabilize the street. In 2005, SDOT will start pre-design phase which includes two more soil borings along the wall alignment with inclinometer and Geocon transducer to monitor ground movement and water table. Due to limited budget in the program, we anticipate the construction might begin as early as 2007.

Golden Garden Dr. NW and SW Admiral Way
SPU is the lead for both of these citywide landslide mitigation projects which are now in a preliminary study phase. The goal of the study is to better define the risks of landslides in the project area, how much is borne by SPU, SDOT and Parks, and to implement the most cost efficient project that reduces those risks to acceptable levels.

Landslide Mitigation Projects - SDOT Priorities

Besides citywide high priority landslide mitigation projects, SDOT also allocates some of the landslide mitigation program funds to some landslide locations that may be important to safeguard the Right-of-Way but may not be citywide priorities. These projects protect transportation infrastructure which has a real benefit to the general public.

Gilman Dr. W near 14th Ave W
In 2004, SDOT completed a 50 feet long Soldier Pile wall constructed by SDOT city forces on Gilman Dr. W near 14th Ave W where one lane of the street has been closed since early 2002 after a landslide occurred in December of 2001. Some of the concrete panels tilted and moved downslope, which widened the existing large cracks along the center of the street. SDOT contacted SPU material lab in 2003 to conduct a geotechnical study to determine a cost-effective solution to stabilize the slope and street.

Before

After

In 2005, SDOT plans to complete two other SDOT priority landslide mitigation projects. They are:

10100 Rainier Ave S
A steep slope/embankment eroded in October 2003 due to heavy rains. A 30 feet long segment of sidewalk was undermined. The sidewalk has been closed to protect public safety. Since this is an arterial street, SDOT has applied Federal Highway funds for the project. A rock buttress was recommended by a geotechnical study. The construction started in late February and is expected to be completed in March, 2005 .

Newport Way & 38th Ave
In early 2002, a landslide occurred at the bottom of the hill side and covered the sidewalk on Newport Way. Additional movement has required continual maintenance and closure of the sidewalk. A rock buttress was designed by SDOT geotechnical engineer to improve the slope stability. Currently, SDOT is working with SPU for drainage improvement design and seeking an opportunity to build this project through small construction roster program in 2005.

For more information, please call

Steve Hou, P.E.
Senior Civil/Geotechnical Engineer
(206) 615-0951
email: steve.hou@seattle.gov