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Title: Re-evaluation of the Lower San Fernando Dam. Report 3, The behavior of undrained contractive sand and its effect on seismic liquefaction flow failures of earth structures
Authors: Rensselaer Polytechnic Institute. Department of Civil Engineering
Dobry, R. (Ricardo), 1922-
Vasquez-Herrera, Andres
Keywords: Case study
Cyclic loading
Earth dams
Hydraulic fill
Hydraulic structures
Laboratory testing
Soil liquefaction
Residual strength
Seismic stability
Soil dynamics
Soil mechanics
San Fernando Dam
Earthquake effects
Publisher: Geotechnical Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Series/Report no.: Contract report (U.S. Army Engineer Waterways Experiment Station) ; GL-89-2 rept.3.
Description: Contract Report
Abstract: A comprehensive experimental testing program (129 tests) was conducted on several silty sands in order to better understand the factors influencing seismically induced liquefaction flow failures, with emphasis on earth dams and slopes. A commercial sand and two sands obtained from engineering projects in Venezuela (Eastern Shore of Lake Maracaibo) and California (San Fernando Dam) were used. Most of the tests were undrained cyclic torsional experiments of the strain-controlled type, performed on solid cylindrical remolded specimens which had been anisotropically consolidated in a triaxial cell (CyT-CAU tests). In addition, a number of selected CIU, CAU, Cyt-CIU and other types of tests were also conducted, including some experiments on dilative sand. Most of the specimens were compacted using the moist tamping undercompaction method, which produces relatively homogeneous samples. Selected tests on the San Fernando sand prepared using a wet raining sedimentation method were used to verify the influence of a more heterogeneous, layered fabric on soil behavior. The results of the Cyt-CAU and other cyclic and monotonic tests are analyzed in detail from two complementary viewpoints: the shear strength behavior of the soil (including steady-state lines, effective stress conditions at triggering of flow failure, and the pore water pressure needed for triggering) for both monotonic and cyclic loading; and the pore water pressure buildup and number of straining cycles needed for triggering during cyclic loading. The influence of a number of parameters is investigated, including: void ratio, minor effective consolidation stress, coefficient of anisotropic consolidation, cyclic torsional shear strain, sand fabric, and sand type. On the basis of the laboratory results and of a literature review, a conceptual framework is proposed for the evaluation of seismic flow failure of specific engineering projects, including the possibility of undrained and partially drained mechanisms. This procedure is applied to the upstream slope slide induced in the Lower San Fernando Dam by the 1971 earthquake. It is concluded that: 1.) the hydraulic fill sand in the dam was contractive prior to the earthquake, 2.) the seismic strains induced by the shaking were much greater than those needed to trigger flow failure in the critical layer, and 3.) the local factor of safety of the critical layer in the upstream slope after triggering was significantly lower than one, thus explaining the observed failure. Conclusion 1.) means that a purely undrained evaluation (also called Mechanism A in the literature) using the techniques proposed here would have predicted the 1971 upstream failure, as well as the good performance of the downstream slope, which had a local factor of safety close to unity after triggering. NOTE: This file is very, very large. Please allow your browser 5-10 minutes to download the file.
Rights: Approved for public release; distribution is unlimited.
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