Centrifuge and Large-Scale Modeling of Seismic Pore Pressures in Sands: Cyclic Strain Interpretation

Centrifuge modeling of pore pressure buildup in a sand deposit as a result of shaking is evaluated by comparison with a large-scale experiment. In large-scale Test SG-1, a 5.6-m-thick, mildly sloping deposit of hydraulic fill clean Ottawa sand of Dr=40%, was subjected to 5 s of low-intensity base shaking (<0.02g) that induced excess pore pressures short of liquefaction. Three centrifuge experiments using various soil deposits and saturation fluids were conducted and compared with the large-scale test. One of these centrifuge simulations used the same Ottawa sand and Dr=40% of the prototype, a viscous pore fluid, and dry pluviation deposition, which created a soil fabric stiffer than the prototype. The other two centrifuge simulations used silty sand saturated with water. The pore pressure buildup in one of the silty sand tests was in good agreement with the prototype, while the other two centrifuge deposits did not develop any excess pore pressure. The various responses in the four tests are explained by various levels of cyclic soil shear strain using a cyclic strain approach. The cyclic strains in the two tests with no pore pressure were smaller than the threshold strain needed to start pore pressure generation in sands (<0.01%), while the cyclic strains in the two tests that built up pore pressure were greater than the threshold (0.03%>0.01%). In addition, two more experiments using Ottawa sand were also conducted: a large-scale level-ground test and a centrifuge sloping test subjected to a greater excitation. The results of the six experiments including the two additional tests are very consistent, further verifying the validity of the strain approach and threshold strain for both level-ground and mildly sloping-ground sites.