Engineering Geology Case Histories Number 2
Prepared for the Division on Engineering Geology of the Geological Society of America, Engineering Case Histories 2 includes 11 case histories covering tunnel construction, foundation grouting, dam-site studies, landslide causes, and more.
A series of landslides occurred during construction of new alignment of California State Highway 37 near Sear's Point, Sonoma County, California, in 1949-1951. The site of the landslides is a quarter to half a mile southwest of the junction of State Highways 37 and 48, approximately 25 miles north of San Francisco. The purpose of the investigation upon which the report is based was to determine the causes of the landslides.Construction of the 1.2 miles of four-lane, divided highway was commenced in October 1949 and was completed on July 3, 1951. Excavation of the road cut, 1,500 feet in length with a maximum depth of 75 feet, was begun on November 16, 1949. The design of the excavation called for cut slopes of 1 1/2 to 1 and provided for benches 20 feet wide, 34 feet above the edges of the shoulders.During the course of the roadway excavation, repeated landsliding occurred which hampered construction in the early stages and became progressively more troublesome as the cut was deepened to final grade. Indications of the incipient landslide condition came on January 25, 1950, during the early part of excavation. At that time slumping occurred on the north slope of the road cut in the place that was to become the principal slide area. A total of five landslide areas ultimately formed in the 1500 feet of road cut. The largest of these five slide areas had a toe width of 400 feet and was the result of the