Landslide Policy Action Report

Revised June 26, 2001

In accordance with landslide policies adopted by Seattle City Council in June 1998, one of the recommended actions is to invest in a preventive program to protect public facilities in Landslide Prone Areas.

The goal is to invest in a preventive program to protect public facilities in landslide prone areas. SDOT is assessing risks to City-owned facilities and will undertake risk-reducing actions and investments in landslide prone areas that can reduce the long-run costs of operating these facilities.

Seattle Transportation initiated four engineering studies in April 1999. The status of the studies is as follows:

Retaining Wall Drain Inventory Study

The City of Seattle owns and maintains approximately 500 retaining walls. Most were built during the initial grading or widening of streets, and several walls were built after a landslide damaged a street. Retaining walls typically have drain systems that control subsurface water build-up behind the walls. Walls have weep holes or drain pipes connected to an existing main line, or a combination of both.

Drain lines require maintenance. Management of subsurface water in steep slope and slide prone areas plays a big role in slope stability. Slope movement may cause drain lines to break that may cause a concentration of water, which in turn aggravates slope stability.

As a slide preventive measure, we researched our records and located all drain lines in our retaining walls. Knowing these locations, we can better manage drainage maintenance and also expedite slide prevention measures and risk assessment of retaining walls in slide prone areas.

Our consultant started this contract in early July 1999 and completed it in September 1999.

Retaining Wall Inspection Services

The City of Seattle’s approximately 500 retaining walls would stretch more than 24 miles if connected end to end. Most of these retaining walls were built prior to 1970, although some were built as early as 1900.

In early 1990 the Roadway Structures division made a preliminary inspection of all retaining walls in its inventory. After this inspection, the retaining walls were grouped into four categories:

1) Retaining walls that are in good condition.
2) Retaining walls that need minor to major maintenance.
3) Retaining walls that need frequent monitoring.
4) Retaining walls that need replacement.

Retaining walls that were in good condition ten years ago may not be in the same condition now. The retaining walls need to be inspected again.

Our consultant started this contract in early October 1999 and completed it in February 2000.

Landslide Risk Assessments

The purpose of this contract is to prioritize potential landslide mitigation projects so the City can reduce the long-term operating costs of City-owned facilities in landslide prone areas and improve public safety.

To accomplish this our consultant will interview city staff to develop a framework for risk analysis and prioritization for preventive actions/investment decisions.

This contract was started in September 1999 and completed it in January 2000.

Slope Inspection and Analysis in High Priority Sites

Based on landslide risk assessment, our consultant will conduct a detailed slope inspection and cost-to-benefit analysis to adjust high priority sites accordingly. SDOT will work with other departments to develop a priority-implemented landslide mitigation program.

The scope of work will start in mid-year 2000.

For more information, please call:

Lingjun (Steve) Hou, Senior Civil Engineer, (206) 615-0951, email: steven.hou@seattle.gov.