ALTAMONT PASS BEDROCK LANDSLIDE

LIVERMORE AND TRACY, CALIFORNIA

J. David Rogers, Ph.D., P.E., R.G., C.E.G.

On Friday March 24, 1995 portions of an ancient large bedrock landslide reactivated near Milepost 55 on the Union Pacific right-of-way in the western end of Altamont Pass, between Livermore and Tracy, California. The slide appears to have developed within the Clayton shale member of the Great Valley sequence, and more than 100,000 cubic yards of material began moving following heavy winter rains. The style of landslippage appears to have been translational, with the toe thrusting upward with reasonably tight curvature. By the evening of the 24th, the UP mainline tracks had lifted over 20 vertical feet, closing the line until a temporary shoofly could be built onto the abandoned Southern Pacific right-of-way a short distance downslope, which was unaffected by the slide (a cut slope formerly separated the two right-of-ways).

J. David Rogers had recently completed a major reconnaissance mapping program of both soil and bedrock landslides on the Altamont and Brentwood quadrangles, funded by the Contra Costa Water District and the Lawrence Livermore National Laboratory. This mapping had included the ancient slide whose reactivation closed down the Union Pacific railroad mainline in Altamont Pass. Rogers/Pacific provided the railroad with copies of their landslide map, engineering geologic cross sections drawn through the active slide, portions of historic topographic maps produced in 1899, 1915, 1943, 1953 and 1981. These materials were then utilized in mass grading techniques used to correct the slide problem and re-open the mainline to posted speeds.