THE HIDDEN COMPLEXITY OF A DEEP-SEATED LANDSLIDE COMPLEX IN RICHMOND, CALIFORNIA

Detailed subsurface investigation of landslides often reveals complexity that is not apparent at the surface. A case in point comes from the proposed Castro Ranch development, located in the eastern San Francisco Bay Area. Our photogeologic mapping led to recognition of a landslide complex consisting of several large, deep-seated landslides that failed away from a linear strike ridge and into an incised drainage. Based upon geomorphology, we initially interpreted these features as deep-seated, translational landslides consisting entirely of displaced blocks of Tertiary sedimentary rock. Based upon that interpretation, we compiled a preliminary cross section that showed a shallowly dipping basal rupture surface that projected to the present elevation of the incised stream. During subsurface investigation (downhole logging of large-diameter borings), we found that the depth of the basal rupture surface was much greater than predicted by the preliminary cross section. Thus, the preliminary landslide model did not fit the subsurface data. The borings within the downslope half of the landslide complex encountered displaced sedimentary rock over a planar rupture surface, but within the upslope half, the borings encountered a thick succession of unconsolidated colluvium, consisting of multiple stacked deposits of sandy silt with angular siltstone fragments separated by buried soil horizons. These deposits appear to fill a deep graben that is not expressed at the surface. After completion of the subsurface investigation, it became apparent that the landslides failed into the incised stream channel and displaced the channel laterally, raising the channel elevation. The present channel elevation led to the initial misinterpretation of the landslide rupture surface elevation on the preliminary cross section. Multiple debris or earth flows generated on steep slopes above the landslide complex filled the graben with unconsolidated deposits. These details were revealed only through careful subsurface investigation using downhole logging methods.