THE ENGINEERING GEOLOGY OF HUME ROAD, LOS ANGELES COUNTY, CALIFORNIA

Between 1993 and 2005, the 1.2-mile-long Hume Road in the Santa Monica Mountains was repeatedly closed, or at best, partially blocked by 11 very small to medium-sized landslides. The last episode of landsliding in March 2005 closed the center portion of the road permanently. New connector roads were constructed to bypass the section of closed road. Landslide types include slumps in colluvial soil, an earth flow with a maximum rate of movement of six feet per day, translational failure in rock, and partial reactivation of currently-inactive complex landslides. Approximately 85% of the road is underlain by large, old landslides, and 16% was underlain by historically active landslides. Maintenance costs are estimated to have exceeded two million dollars per year since 1993.

Mitigation strategies have included round-the-clock maintenance, restricted road use, relocation of the road, soldier pile and lagging walls, use of GeoFoam as fill, minor non-engineered to massive engineered grading, dewatering, and French drain construction. Movement of the largest landslides could not be stopped, and so the road must be regraded and resurfaced whenever boundary scarps deform the road. Landslides activate in the spring during above-normal rainfall years, except where our mitigative activities have caused movement. Constraints include steep topography, lack of shallow bedrock, and the preference to try and remain within road easement.

The area is underlain by Cretaceous to Paleocene, marine and terrestrial clastic rocks that have been extensively fractured. Miocene thrust (detachment?) faulting form daylighted weak zones along which some of the failures occur.

The opinions expressed are those of the author, and do not reflect the opinions of, nor endorsement, by the LACDPW, its employees or agents.