We conducted a geotechnical investigation, prepared construction plans and specifications, and provided construction observation and testing services for the repair of a landslide on residential property in Marin County, California. The landslide repair involved restoration and stabilization of the failed hillside, replacement of damaged portions of the concrete driveway, collection and controlled discharge of storm water runoff, and removal of a septic tank leach field.
 
 

During the original grading for the residential property in the 1950's, a ravine was filled to create a level yard and parking area. Within this same fill, sanitary leach fields were constructed. Effluent from the leach fields provided a significant source of water. to saturate the slope. Following a period of heavy rains in January 1993, a 40 foot high fill slope adjacent to the residence failed. The failure was a result of increased hydrostatic pressures from water flowing beneath the fill soils already saturated from the leach fields. This fill material was swept 500 feet down a ravine onto a road below, plugging a 24" concrete culvert.
 
 
 
 

In order to construct a stable slope to near its original contours (1:1 slope inclination), extensive use of geogrid reinforcement in the compacted fill was utilized. Also, a series of subdrains designed to intercept and drain groundwater from behind the fill were incorporated into the constructed embankment. Erosion control measures included interception and collection of runoff, installation of jute netting on the completed slope repair, placement of riprap at the subdrain and storm drain discharge points as well as placement in the ravine below, and hydroseeding of all disturbed areas.