Dynamic Analysis of the Slide in the Lower San Fernando Dam during the Earthquake of February 9, 1971

During the San Fernando, California earthquake of February 9, l97l, a major slide occurred in the upstream slope of the Lower San Fernando Dam. Based on data obtained from tests on undisturbed samples, a detailed dynamic analysis of the seismic stability of the embankment is presented. It is concluded that an analysis procedure incorporating: (1) Consideration of the initial static stresses in the embankment: (2) the use of a dynamic finite element analysis procedure to determine the dynamic stresses induced in individual elements of the embankment by the earthquake; (3) the use of cyclic loading triaxial compression test data to determine the response of the soil elements to the induced stresses; and (4) consideration of progressive failure effects provides a satisfactory basis for assessing the stability of the embankment. This type of analysis indicates the development of a zone of liquefaction along the base of the upstream shell that would be sufficiently extensive near the end of the earthquake shaking to lead to a condition of instability.