Undrained Strength Characterization of Compacted Earthen Embankment Core Material

B.F. Sisk Dam is a 116.4 meter high zoned embankment structure located near Los Banos, California on the west side of the Central Valley, and is currently the subject of two investigations - one studying potential remedial actions to decrease the risk associated with potential seismic activity in the region, and a second in support of feasibility level designs for increased water storage and embankment crest raise. The impervious core of the embankment is comprised of heavily compacted sandy lean clay. As part of the current investigations, several bore holes were drilled from the crest of the dam and intact samples of the core material were retrieved. A comprehensive laboratory testing program was undertaken including one-dimensional consolidation and K_o-consolidated undrained static triaxial shear testing. This paper presents the details of the testing program, depicts typical results, and discusses the proposed undrained strength definitions for stability modeling. Proposed undrained strength definitions for modeling used an undrained strength ratio for normally consolidated core materials and SHANSEP analysis for overconsolidated core materials. For very shallow embankment materials, the SHANSEP strength approach was modified to reflect lower strengths produced by the effective stress friction angle.