Exponent contributed to the investigation of a large block-glide landslide that occurred on June 2, 1999 during mass grading of a new ocean-front golf course in Palos Verdes, California. Beneath the new golf course was a 1950s vintage 27-inch-diameter sanitary sewer trunk line made of reinforced concrete pipe having tongue and groove ends. During construction, grading equipment operated over the top of the pipe at several locations. Through a review of construction documents, Exponent discovered that the minimum cover over the pipe in areas of active grading was approximately 18 inches, much less than the contract-required 5-foot minimum cover intended to protect the pipe. Furthermore, the sewer pipe had hand-mortared joints in the section of the pipeline where active grading took place.
Exponent performed full-scale equipment load testing on a reconstituted section of the sewer pipeline to quantify the effects of the grading operations on the integrity of the pipe. Exponent found that the pipe sections remained in an elastic state and were undamaged as a consequence of loading away from the joints. However, when the equipment wheel or track loads were directly above the joints, the hand-packed mortar was observed to incrementally and progressively crack and spall, creating large gaps between the tongue and groove ends. These gaps would allow sewage to exfiltrate and flow into the loose sand pipe backfill. Exponent determined that water exfiltrating from damaged joints in the trunk line was a substantial contributor to ground water seeping down through an ancient landslide graben to the basal slide plane. The primary source of water causing the large block-glide landslide was found by others to be water seeping from improperly lined ponds on the golf course.