Mine pit slopes at Arroyo del Valle Quarry in the Livermore Basin of northern California expose late Quaternary sandy gravel. In the deep subsurface, the gravel unconformably overlies gently folded lacustrine sediments. Within the lacustrine sediments, a bed of sheared, unoxidized clay overlies a marl bed, forming a distinctive marker bed couplet. Structure contours on this marker bed show an anticline and a syncline in the vicinity of the quarry. These northwest-striking folds are aligned parallel to regional Quaternary fold and thrust belt structures. Where the marker bed dips toward the pit, slope inclinometers consistently deflected toward the pit at the depth of the unoxidized clay. Where the marker bed dips away from the quarry pit, no slope inclinometer deflections were recorded. Thus, slope instability was controlled by the site stratigraphy, geologic structure, and the location of quarry slopes relative to that geologic structure. High pore-water pressures within the unoxidized clay also contributed to slope instability. Shearing of the unoxidized clay occurred prior to excavation of the quarry pit, and the resulting low residual strength of this high-plasticity clay made it particularly vulnerable to incipient landsliding when lateral confinement was removed during excavation of the quarry pit. Analysis of the critical region between the quarry pit and the anticline axis showed that the static factor of safety remained below 1.5. Seismic displacement analyses indicated that moderate to large displacements would be anticipated. Thus, depressurization wells and an earth-fill buttress were designed and implemented to mitigate deep-seated slope instability.

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