SDOT Landslide Mitigation Program
November 3, 2009
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.
In accordance with landslide policies adopted by Seattle City Council in June 1998, SDOT, SPU and Parks were assigned to assess risks to City-owned facilities and undertook risk-reducing actions and investments in landslide prone areas that could reduce the long-run costs of operating these facilities.
Starting from 1999, Seattle Department of Transportation (SDOT) initiated four engineering studies as a part of the City's landslide mitigation program and completed those studies by early 2001. 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 (completed in 2007)
- Golden Garden Dr. NW (completed in 2008)
- SW Admiral Way (no action based on geotechnical study)
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.
41st Ave NE & NE 142nd Street
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 started pre-design phase which includes two more soil borings along the wall alignment. The construction started in April 2007 and completed in July 2007. A 150 feet long Soldier Pile wall with concrete lagging was construction to support the street.
Golden Garden Dr. NW, South of View Ave NW
The street has been settling over the years. In 2005, SPU took the lead to conduct 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.
In December 2007, a landslide destroyed the section of Golden Garden Dr. NW and the street was closed for more than half a year. SDOT took the lead to hire a consultant for a preliminary study. From the study, a Soldier Pile wall option was selected as a cost effective approach for the emergency project. The construction started in the summer of 2008 and completed at the end of the year.
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.
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 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. The construction started in early October and completed in middle October 2005 through small construction roster program.
10200 Block 47th Ave SW
In January 2006, a landslide occurred along the west shoulder of 47 th Avenue SW on the other side of 10230 47 th Avenue SW. The slide resulted in approximately 18 inches of settlement of the newly constructed asphalt thickened edge along the west edge of the roadway and approximately two feet of undermining of the concrete street. In April 2006, Seattle Public Utility Material Lab conducted a geotechnical study and generated a preliminary geotechnical report dated October, 2006. In 2007, SDOT in house design team started design of a 50 feet long Soldier Pile wall and rock buttress. Besides the wall and rock buttress, the project included reconstruction of approximately 100 feet long concrete street and reinstall new standard guardrail. The construction started in April 2008 and completed in July 2008.
1600 Block 20th Ave E
On December 15 2006, a landslide occurred along the west side of 20th Ave E which undermined shoulder area of the street. As a result, the street was narrowed to one lane in order to protect undermined area from traffic. Starting February, 2007, Seattle Public Utility Material Lab conducted a geotechnical study and generated a preliminary geotechnical report dated March, 2007. Following the recommendation of geotechnical study, SDOT designed a 40 feet long Soldier Pile wall. In addition to the wall, the project included resurfacing the street and install new guardrail. The project was partially funded by FEMA. The construction started in September and completed in October 2008.
Ferry Ave SW and California Way
Heavy rains in early December, 2007 washed out a 40-ft section of road shoulder adjacent to Ferry Ave. SW. The loose material washed downhill onto California Ave SW. Based on geotechnical investigation and analysis, a 48 foot long Solder Pile wall was constructed in 2009 to retain Ferry ave SW on top of the slope. A layer of rock fill with geotextile fabric was installed along the affected slope portion to improve slope stability. The project is jointly funded by FEMA, FHEA and SDOT.
6500 Block Beach Drive SW
A 250 feet long section of sidewalk has been settling up to three feet due to underlying loose fill and steep slope. The sidewalk has been closed since 2007 for public safety. In 2009, SDOT hired a geotechnical consultant for a geotechnical study between 6300 block and 6500 block where most sidewalk and roadway settlement occurs. Based on their investigation and analysis, the 250 feet long closed sidewalk was identified as first priority. Since SDOT does not have fund to build a retaining wall for a permanent repair, a temporary repair with approximately five year service life was decided to accommodate public needs. This repair involve over-excavation of the sidewalk and apportion of the roadway, backfilling it with structural fill and layers of soil stabilization geosynthetic. The project started in the middle of October, 2009 and will be completed in the beginning of November. 2009. After completion of the project, SDOT will monitor possible ground movement with inclinometer and survey for at least one year.
For more information, please contact
Steve Hou, P.E.
Senior Geotechnical Engineer