MITIGATION ALTERNATIVES FOR ROCK SLOPE INSTABILITY ABOVE THE SANTA ANITA DAM ACCESS ROAD, LOS ANGELES COUNTY, CALIFORNIA

In March 1992, cracking of a gunite-covered cut slope in diorite provided initial evidence of slope deformation above the access road to Santa Anita Dam in the San Gabriel Mountains. In January 1993, the deformation culminated in a large rockslide that covered about 180 linear feet of the road. The rockslide was preceded by 24.75 inches of rainfall over an 18-day period. A wedge-type rockslide that covered the road occurred here in 1938, thirteen years after road construction on Forest Service land. A public road traverses the natural slope 200 feet above the top of the 190-foot-high, 47-degree cut.

We evaluated the geologic feasibility of seven alternatives that address the need for safe passage of personnel. The alternatives were: 1) constructing a bypass tunnel, 2) constructing a roof structure over the road, 3) placing a wire-rope net over the slope, 4) maintaining the status quo, 5) excavating debris collection benches, 6) applying shotcrete to the slope, and 7) constructing an impact retaining structure. The rock mass is very hard, closely to widely fractured, and slightly weathered to unweathered, with multiple faults offsetting igneous dikes. The 1993 rockslide originated on the upper third of the slope, which is composed of finer-grained, more weathered rock with a closer fracture spacing than the lower slope. The morphology of the slope is dominated by three major ridges with intervening chutes containing ramps, which is conducive to funneling rocks into the chutes and launching rocks into the air. Joints and joint-fault intersections are commonly oriented adversely to the slope, and some structures are continuous across the slope. The Colorado Rockfall Simulation Program and structural analyses for Markland's Test were used in evaluating mitigative alternatives. Because the elimination of rock slides was found to be impractical unless a tunnel was constructed, the mitigation of rock fall hazards was emphasized to protect personnel using the road.

Alternatives 1, 2, and 3 were found to be feasible, and we recommended construction of a wire-rope net as the most cost-effective measure providing reasonable protection.

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