STABILIZATION OF A PORTION OF THE HONBY LANDSLIDE COMPLEX, SANTA CLARITA, CALIFORNIA

The informally named Honby landslide complex consists of a group of large bedrock failures that have occurred within the Sunshine Ranch Member of the Plio-Pleistocene Saugus Formation in the northwestern portion of Los Angeles County. The bedrock failures within the complex range in area from 1 acre to 1/2 mile2. The failure mechanisms of the landslides are dissimilar, ranging from bedding-plane failures along a locally extensive claystone to across bedding failures related to fracturing and faulting of the bedrock. A portion of the landslide complex required stabilization of a 19 acre, 1.5x106 yd3 landslide for the construction of a 6-lane roadway. Constraints to remediation design included an immovable communications facility at the landslide head, the fixed location of the road right-of-way and dual 30" high-pressure natural gas pipelines along the western lateral boundary and toe. Investigation of the landslide included the completion of 10 bucket-auger borings up to 110 feet deep. Subsurface data indicated that the landslide was a combination-type failure that incorporated a fault-controlled cross-bedding mechanism at the head and block-glide bedding-plane parallel mechanisms at its base. The landslide ranged in thickness from 100 feet at the mid-section to 60 feet near the toe. Loss prevention techniques involved the protection of the adjacent properties by installation of a slope inclinometer, visual survey of existing buildings using photography and hand drafted logs, and construction of 20 soldier piles at the head of the landslide. Survey points were also used along the grade beam at the top of the soldier piles. Recommendations in the grading plan review included a proposed grading sequence to minimize slope failures. An alternative grading sequence was selected to minimize construction costs. During excavation of the backcut below the communications facility, three failures occurred which increased the difficulty of earth removal operations. During grading of the buttress backcut, mapping revealed steep northward-dipping faults, requiring the partial redesign of the buttress including a midway shear key. Construction of the buttress fill above the roadway incorporated geogrid-reinforced 11/2:1 fill-slopes 150 feet high that were covered on the surface with geomat to help retard erosion.