URBAN LANDSLIDE HAZARDS, MILLBRAE CA LANDSLIDE: CASE HISTORY

A 2+-acre landslide (200,000 cubic yards) occurred in March 2000 on a steep, 160-foot-high, 40-year-old cut slope between two rows of residences in Millbrae, California. The landslide, measuring 400’ by 200’ by 55 feet deep with a 60-foot head scarp, encroached upon 6 homes at the base of the slope, damaging 3 and knocking 1 off its foundation. The landslide is along the western margin of an old, large quarry cut into highly sheared and foliated Franciscan Melange bedrock. Following quarry operations, homes were constructed on top and at the base of the 1.5:1 to 2:1 (horizontal to vertical)slopes.

The landslide, which began moving in 1997/98, failed as a translational (with minor rotation) block landslide along a basal failure surface dipping 5 to 10 degrees out of the slope. The lower portion failed as a semi-plastic flow above the toe of the slope. This landslide was not a pre-existing deep bedrock failure and groundwater did not have a significant role in activating this landslide. The failure was due to a gradient that was too steep for long-term stability of the materials, weathering of the bedrock over time and to localized surface instabilities and erosion that ultimately overcame the marginal stability of the existing cut slope.

Due to weak bedrock, steep topography and the location of the houses, conventional remove and replace of the landslide material was not feasible. An innovative mitigation consisting of 5 rows of 50-foot deep, 3-foot diameter reinforced concrete shear pins and removal and replacement of the landslide with geogrid reinforced engineered fill was designed to rebuild the slope to its original configuration.