Effect of Toe Excavation on a Deep Bedrock Landslide

Observations, data, and analyses that were used to investigate the cause of distress to a single-family residence located adjacent to a major highway cutslope are presented herein. The investigation revealed that the distress in the single-family residence was caused by a deep, large excavation-induced landslide. The excavation, which was made to widen an existing highway, helped trigger the landslide by exposing geologic structures on the cutslope and by unloading the toe of the slope. This case history illustrates some of the ramifications of large highway excavations in natural slopes surrounded by urban areas, e.g., exposing significant geologic features such as shear zones, faults, and folds; the importance of investigating and explaining signs of movement at both the top and toe of a slope; the impact of rainfall on the movement of a large slide mass; and that large slide masses can undergo slow, episodic movement instead of sudden, large movement.