Reevaluation of the Seismic Performance of Upper San Fernando Dam Using Undrained Shear Strength

The San Fernando earthquake of February 9, 1971, initiated a major slide on the downstream slope of Upper San Fernando Dam. Nearby Lower San Fernando Dam was damaged even more severely, and would have probably led to an uncontrolled release if the reservoir elevation had not been restricted prior to the earthquake. Both dams have been used extensively to calibrate various dynamic models. This paper presents the results of a reevaluation of the seismic performance of Upper San Fernando Dam based on an undrained shear strength model. Previous analyses of Upper San Fernando Dam have focused on various liquefaction approaches based on cyclic pore pressure generation assumptions. The undrained shear strength soil model presented is a different approach, based on the shear strength determined from standard triaxial testing. The underlying assumption of this soil model is that pore pressures increase as a direct result of shearing, rather than cyclic loading. The assumed failure mechanism is a rotational slide, rather than a lateral spread. The approach is a logical extension of a static failure, with the addition of inertial forces and strain-softening of the shear zone. The approach follows fundamental geotechnical principals based on directly measured soil properties. This classic case study was reevaluated after meticulous analysis of a large body of construction data, in-situ data, and laboratory test data. The gradation, density, and shear strengths of the hydraulic fill vary significantly from the core to the outer shell due to the nature of the hydraulic fill placement method. By correlating all available data, a detailed soil profile was developed and input into a FLAC model. The built-in Mohr-Coulomb strain-softening constitutive soil model was used to simulate the undrained shear strength behavior. The results of this approach show good correlation to measured field performance, and are consistent with other observations at the site. 16 pp. 7 references.