Fill Placement on Slopes Underlain by Franciscan Mélange

Distress in two housing developments was caused by a deep bedrock landslide triggered by placement of a large upslope fill. The large fill was placed to create a visual barrier between the upslope development and downslope housing developments and to balance the cut and fill quantities for the upslope site. This case history presents some of the ramifications of fill placement on natural slopes surrounded by urban areas, such as overstressing underlying weak material that may exist below the depth of borings typically conducted for single family residences and office complexes; the importance of surface and subsurface information in a formation known locally as Franciscan complex/mélange (which is a block-in-matrix rock formation common to the area); the shear behavior of serpentinite, which is part of the Franciscan complex; and the importance of natural and man-made changes to a slope, such as rainfall, surficial grading, home construction, and fill placement. It also illustrates the importance of locating the critical slope cross section before construction and the proper use of back-analyses in a landslide investigation.