USE OF DETAILED MAPPING AND MONITORING OF A LANDSLIDE TO PERMIT SAFE OCCUPATION OF SURROUNDING RESIDENCES: AN EXAMPLE IN SAN LEANDRO, CALIFORNIA

A 60,000 m3 translational landslide occurred in San Leandro, California during the El Niño winter of 1997/98, forcing evacuation of eight homes. Careful investigation and monitoring permitted safe reoccupation of homes and construction of cost-effective control measures.

Detailed engineering geologic mapping and observations made in boreholes and trenches indicated that the landslide was a new failure that occurred on a cutslope in gabbro bedrock. The gabbro was highly fractured and sheared due to its position within the Hayward fault zone. The landslide occurred on approximately February 7, 1998 and moved at a nearly constant maximum rate of 0.4 m/day during the nine days thereafter. Initial displacement slowed, then quickly stopped, soon after heavy rains ceased. Slide movement typically recurred with each significant rainfall event. Monitoring of slope inclinometers indicated that the basal rupture surface occurred from 9 to 15 m below the ground surface. The local groundwater table was identified within a small portion (less than a few percent) of the landslide body a few days after initial failure and dropped beneath it within ten days. Increased pore-water pressures in fractures and pre-existing shear zones through which the rupture surface propagated apparently triggered the failure. The landslide posed a significant hazard to residents situated below from catastrophic failure onto the homes, rockfall/slide off the toe, or rupture through utilities and/or residential foundations.

Earthwork and monitoring measures implemented during the fall of 1998 permitted relatively safe occupation of homes during the winter of 1998/99. Removal of slide debris from the head, placement of buttress fill on the toe, improvement of surface drainage and construction of rockfall-protection fences reduced the danger of renewed movement. Installation and bi-weekly monitoring of inclinometers, groundwater wells and topographic survey monuments enabled us to predict reactivation of the landslide and issue appropriate warnings. Long-term control measures implemented during 2000/01 permitted safe, permanent occupation of six of the eight homes. These measures consisted primarily of construction of tied-back, shear-pin retaining walls, surface-drainage improvements and horizontal subdrains.