STRUCTURAL, STRATIGRAPHIC AND HYDROLOGIC FACTORS CONTROLLING GRAVEL QUARRY SLOPE INSTABILITY, LIVERMORE BASIN, CALIFORNIA

Arroyo del Valle Quarry, located in the Livermore Basin of northern California, exposes Quaternary gravels that have been extensively mined as an aggregate resource. In the subsurface, these coarse-grained braided stream deposits unconformably overlie Pleistocene lacustrine sediments. Within the lacustrine sediments, a bed of highly plastic, sheared, unoxidized clay overlies a marl bed, forming a distinctive marker bed couplet. Structure contours on this marker bed show that these sediments are gently folded into an anticline and syncline in the vicinity of the quarry. Shearing of the unoxidized clay occurred prior to excavation of the quarry pit, and the low residual strength of this clay made it particularly vulnerable to instability when lateral confinement was removed during excavation of the quarry pit. Where the unoxidized clay dips away from the pit, slope inclinometers remained static, but where the sheared unoxidized clay dips toward the pit, slope inclinometers exhibited deflections at depths that consistently corresponded to the depths of the sheared unoxidized clay. Thus, slope stability was controlled, in part, by the basin stratigraphy and geologic structure as well as the location of quarry slopes relative to that geologic structure.

High pore pressures within the unoxidized clay also exerted a destabilizing force. Piezometers installed within a bed of fine sand below the unoxidized clay-marl marker bed revealed a piezometric surface elevation roughly 32 to 41 meters above the sand bed, and piezometers installed within the unoxidized clay revealed a similarly high piezometric surface.

Initial displacements within the sheared unoxidized clay resulted from stress relief upon excavation of the quarry pit and removal of lateral confinement where the unoxidized clay dips toward the pit. Over several years, the slope inclinometer deflection rate declined as the strength of the overlying gravel partially mobilized. Analysis of the critical region between the quarry pit and the anticline axis demonstrated that the factor of safety remained below 1.5 under static conditions, and moderate to large seismic displacements were anticipated. Thus, depressurization wells and an earth-fill buttress were designed and constructed to mitigate deep-seated slope instability at the quarry pit.