MITIGATION OF AN EARTH-SLIDE EARTH-FLOW AFFECTING HUME ROAD AT MILE MARKER 0.45, LOS ANGELES COUNTY, CALIFORNIA

In June of 1995, an earth slide - earth flow started to move along and over the lateral boundary shear zone of the lower part of the Briarbluff-Hume Road landslide complex, which had activated three months earlier. The 580-foot-long, 40- to 80-foot-wide, 28-foot-thick, 0.8 acre slide-flow occurred on a 2.6:1 slope. Maximum rate of movement was greater than six feet per day with total movement greater than 250 feet. At one point, road crews worked around the clock to maintain access to residences serviced by the road which traversed the top of the slide.

Three phases of mitigative efforts and a low rainfall period were required to reduce movement to an acceptable rate. Phase 1, 10/95-5/96 - vertical dewatering wells at the head of the slide, adjacent to Hume Road, constructed in attempt to intercept subsurface water flowing into the slide; result - spring 1996 rains triggered additional fissuring. Phase 2, May 1996 - remove head of slide to rock, construct subdrainage, and compact fill under the road; grade slide-flow to seal fissures, direct drainage, and load the toe; construct axial subdrain, with branches, from compacted fill down center of slide-flow to toe; this drain removed approximately 0.85 acre-feet of water equivalent to 2-3 feet of water within slide in the first month and a half after installation. Phase 3, October 1996 - grade the slide-flow again to seal recent fissures and further load the toe in conjunction with placement of large rip rap on the toe to reduce erosion. Three series of three, nested, piezometers were placed in the slide-flow in December 1996. These showed that the slide-flow was half filled with water perched on the basal shear surface, with a piezometric surface above the basal shear surface near the toe from confined water beneath. Significant movement has not occurred in the subsequent low rainfall years.

The opinions expressed are solely those of the author, and do not represent the opinions of, nor endorsement by, the LACDPW