There was no time to waste. The City of San Diego declared an emergency after detecting movement on the active Rose Canyon Fault, out of concerns that a landslide could hit the Mount Soledad homes directly above it.
The city quickly mobilized forces to address the hazard because the area had experienced four landslides since 1961—and El Nino was in the forecast.
At the request of the City, Harris & Associates was retained by Leighton & Associates to provide civil engineering services for the emergency project on an accelerated schedule, coordinating efforts of civil, environmental, legal, geotechnical and construction professionals. Harris ran weekly meetings that successfully organized the team into a single-minded effort to avoid potential damage to the community.
The main focus of the restoration effort was resident safety. The area that showed movement had extensive erosion, with eroded vertical walls in the slot canyon up to 50 feet high. To address this issue and minimize long-term maintenance, the team:
Performed geotechnical analysis that revealed three fault lines and multiple fractures within the slope repair. This data led to a decision to import over 25,000 yards of material as fill. Careful soil management and stockpiling were crucial because safety required a slow, deliberate reconstruction of the fill slope in the narrow, steep canyon. The fill included an agronomic soil cap to improve safety for the slope and support revegetation.
Developed a solution to collect surface water flow, since a legal issue prevented the building of concrete ditches. Low-maintenance cobble swales were used instead as a natural method of repair.
Realigned and extended the storm drain downhill where it could discharge safely. The new drain minimizes potential impacts from slope creep and rainfall infiltration.
The team had to move so quickly that “after the fact” permits were necessary. This required close coordination with the U.S. Army Corps of Engineers, the Regional Water Quality Control Board and California Department of Fish and Wildlife.
Harris and its partners were able to successfully improve the eroded slot canyon beyond its original condition and stabilize the landslide potential as much as possible within the defined scope of slope restoration.
Beyond the Blueprints
Harris needed to map soil conditions to confirm that construction of a slope would not disturb the site’s delicate equilibrium. Since there was no subsurface geologic information, geotechnical analysis required the drilling of 30” diameter exploration borings up to 143 feet deep. A geologist was then lowered into each hole to acquire the essential information to characterize landslide potential.