Investigation and Repair of Tectonic and Storm-Related Road Damage near the Garlock Fault, California

Abstract

Major damage to the Randsburg Wash Test Range Access Road at the Naval Weapons Center occurred during a thunderstorm the night of August 18, 1983. The damage occurred within zones of weakness related to tectonic strain accumulation across the nearby Garlock fault. Two zones of surface cracking and Assuring, one 732 m long and the other 686 m long, intersect the road. The primary damage, largely erosional, included open fissures up to 30 cm wide, and open and collapsed tunnels with cross-sectional dimensions of 1 to 2 m. Repair of the road involved four major considerations: (1) open fissures extending to an unknown depth, (2) continuing tectonic strain accumulation, (3) localized heavy rainfall and flash flooding, and (4) maintaining the road in a safe and usable condition. These considerations required the incorporation of somewhat novel, non-standard repair techniques. The road repair involved extensive exploratory excavation and engineered backfilling. About 900 m3 of material were excavated to an average depth of 4 m. The stratigraphy included up to 0.7 m of unconsolidated silty sand underlain by 3 m of plastic lacustrine clay and an unknown thickness of coarse, unconsolidated, uniformly graded sand. These materials were replaced by engineered fills designed to: prevent erosion of the subgrade; bridge or collapse into cracks or fissures that may develop; prevent or diminish the effects of reflection cracking; allow drainage; and accommodate tectonic strain. The road-shoulder backfill consisted of 1.5 m of a compacted mixture of arkose and the previously excavated clay overlain by 3.5 m of compacted arkose. The road backfill included a basal, 0.5-m-thick layer of cobbles, an overlying 0.5-m-thick layer of compacted, 19 mm concrete aggregate and 4.5 m of compacted decomposed granite.