Pore Pressure and $K_{\sigma }$ Evaluation at High Overburden Pressure under Field Drainage Conditions. I: Centrifuge Experiments

This article is the first of two companion papers studying the effect of overburden pressure on the liquefaction behavior of saturated Ottawa sand. A series of four centrifuge tests were conducted simulating a 5-m layer of this sand having two different relative densities, and subjected to overburden effective pressures of ~100 and 600 kPa (1 and 6 atm). The objective was to study the pore pressure response of the soil to base acceleration under low and high pressure, including evaluation of the overburden pressure factor $K_{\sigma }$ for idealized field drainage conditions. The sand layer had a bottom impervious and a top pervious boundary, approximating a common field situation. A novel experimental technique was developed using a dry lead shot layer to provide the necessary high level of pressure. The performances of the sand layer under low and high confining pressure were compared in terms of times histories and profiles of excess pore pressures, cyclic stress ratios (CSR), and cyclic shear strains ${\gamma }_c$, with some of the parameters determined using system identification. It was found that pore pressure dissipation started earlier at shallower depths, and that partial drainage was more significant in the 6-atm than in the 1-atm tests. Field overburden pressure correction factors at 6 atm, $K_{\sigma }$, obtained from the centrifuge tests for ${(r_u)}_{\max }=0.8$ in 10 cycles of shaking and including the partial drainage effect, were found to be higher than 1.0 for both $D_r=45\%$ and 80% This is different from the usual laboratory undrained $K_{\sigma }<1$ based on cyclic triaxial and simple shear laboratory tests and reflected in the current state of practice. The discrepancy is related to the more significant effect of partial drainage and deviation from the undrained assumption at the higher confining pressure for the field drainage and other conditions of these centrifuge tests.