Portuguese Bend Landslide, Palos Verdes Hills, California

Abstract

In the Palos Verdes Hills an ancient complex landslide affects several hundred acres of seaward-dipping tuffaceous Miocene shales. In 1956 about one-fourth of the area of this slide began moving again along 6 to 7 degree slope by earthflow and shifting of large blocks. Movement is continuing at a rate which increases in the winter and decreases in the summer and ranges from 0.03 to 0.1 foot per day. Information derived from studies aimed at stabilization include data from 62 drill holes, survey records of horizontal and vertical movements, and results of soil tests. Possible causes of the present movement suggested by various investigators are (1) loading of landslide head by highway fill and (2) accumulation of excessive ground water derived from cesspools, surface watering, and natural precipitation. Attempts to control sliding by placing 25 four-foot-diameter caissons 20 feet long across the rupture surface were unsuccessful. A plan to accomplish stabilization by removing material from the head and placing it as a rock-protected earth fill at the toe is under consideration. Damage to roads, houses, and other structures is estimated at more than ten million dollars.