FAILURE ANALYSIS OF A ROCKSLIDE ON SUNSET RIDGE FIRE ACESS ROAD, SAN GABRIEL MOUNTAINS, CALIFORNIA

We studied a road cut along the Sunset Ridge fire access road that failed during the early 1990s. Debris that blocked the road for several years has been excavated, exposing a NE-facing triangular scar in fractured crystalline rock. The present-day slide scar is 30 m high at its apex, and widens to 36 m at road level. Our objectives were to reconstruct the pre-slide geometry and determine the structural and mechanical conditions that led to failure.

Fracture analysis entailed systematic mapping of foliation, joints and faults along accessible reaches of the slide area. Gently NW-dipping foliation striking into the slope face was not a major factor in failure. Two intersecting sets of NE-dipping joints and faults played important roles in activating the slide block, originally rhombus-shaped in cross section. A steeper set coincides with a prominent fault (N36W/64-80NE) that bounded the rear of the block and facilitated its release. A shallower set (N30W/25-45NE) represents adversely dipping planes that projected out of the original road cut. A corresponding basal slide surface (N40W/28NE) is exposed 5 m above road level. The SE edge of the slide block was released along a steep cross fault (N20E/75SE). Thickness of the block tapered from 4m on the SE side to zero at the bounding ridgeline on the NW.

We surveyed the slide area with a Trimble total station, used ArcGIS to create a topographic map with 2 ft contour interval, and plotted fracture orientations precisely on this base map. The volume of the slide block was determined by subtracting topography of the existing slide scar from a projected surface representing the original slope and road cut. Assuming shear failure along the basal slide plane, we set up stability equations to determine combinations of cohesion and friction angle needed for failure (safety factor = 1). Preliminary calculations yield friction angles between10 and 45 degrees for geologically reasonable cohesion and water conditions.