Abstract

The Anchorage Metropolitan area sustained catastrophic damage and loss of life during the 1964 Prince William Sound earth-quake (MW = 9.2). Current geologic and geo-physical data indicate that the area is subject to future major earthquakes related to the underlying subduction zone and possibly to nearby shallow faults. A major cause of destruction during the 1964 earthquake was ground failure. The cause of most of these failures has been directly linked to the Bootlegger Cove Formation, although there has been disagreement as to whether the failures were due to seismically induced liquefaction of sands or fabric collapse of sensitive silty clays. Our research has focused on one of these landslides and consisted of insitu geo-technical testing, undisturbed sampling, and laboratory testing of acquired samples. Based upon standard and electric-cone penetration tests, we conclude that the sand strata within the formation are non-liquefiable. Static and cyclic laboratory tests of the cohesive facies of the formation demonstrate that for peak horizontal accelerations above 0.2 g the shear strength of some facies decreases 25% to 30%. Further, we find that if the seismic loading continues on the order of a few minutes the strain-softened facies may undergo additional strength reduction to a residual shear strength ranging from 28% to 33% of the original undrained strength. This model for soil failure can be applied to all of the major Anchorage landslides, which we believe resulted from strength degradation of a cohesive silty clay facies under long-duration accelerations between 0.2 and 0.4 g.

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