Continued regression of the Mussel Rock Landslide headscrap would endanger critical infrastructure along Skyline Drive.
The Mussel Rock Landslide is a deep seated ancient landslide that involves the movement of about 9 million cubic yards of material. A number of secondary landslides have developed in recent times associated with sliding of the soil behind the steep headscarp left by the main landslide. This continued movement caused the removal of 5 homes within the 900 Block on the western side of Skyline Drive. Continued regression of the landslide headscarp would endanger Skyline Drive and could undermine and break critical water and sewer force mains buried under the pavement. These force mains are critical elements of the Daly City infrastructure.
TERRA was retained by the City of Daly City to assess the likelihood that the landslide headscarp will continue to move to the east and undermine and damage the street and water and sewer force mains, estimate the timing of such an event, and evaluate alternatives for protecting the street and force mains.
TERRA reviewed available geotechnical and geologic data, performed detailed geologic mapping of the landslide headscarp, investigated the soil and groundwater conditions using cone penetrometer tests, and analyzed the stability of the headscarp under static and earthquake loading. We evaluated the likelihood that the landslide would continue to progress and damage the force mains, considered alternative remedial actions and recommended short and long-term protective measures.
Our analyses suggested that it would take another 25 to 50 years for the headscarp to regress within 25 feet of the force mains and we recommended that the City monitor the rate of regression and install a soil nail/shotcrete wall to stabilize the face of the slide before it moved within 25 feet of the force mains.