Pore water pressure increase in loose saturated sand at level sites during three-directional earthquake loading.

Abstract

Stresses, strains and pore pressure buildup caused by the ground accelerations are predicted, with due consideration to the nonlinear, anisotropic, elastic-plastic stress path dependent and modulus degrading characteristics exhibited by the soil during undrained cyclic loading.

The computations for Niigata were performed using the computer code POWAP, which is based on the proposed model and was also developed as part of this work. Possible future uses of this program include: a) Evaluation of liquefaction duration and number of equivalent stress or strain cycles of recorded and artificial ground accelerograms, considering the multi-directional character of the shaking. xxii b) Evaluation of acceleration and pore pressure records obtained du ring earthquakes at sand sites currently instrumented in the U. S. and Japan. c) Its use in conjunction with available non-linear, modulus degrading soil amplification codes, capable of computing soil response and pore pressure generation due to an assumed two- or three-component base rock acceleration.

A limited parametric study is also performed for Niigata to provide additional insight on the influence of the in situ shear modulus of the soil, the earthquake acceleration level, and the importance of considering one or two horizontal acceleration components.

The model is verified by correctly predicting the time to the onset of liquefaction occurred in Niigata, Japan, during the earthquake of June 16, 1964.

In order to estimate the pore pressure development, a procedure is proposed, which uses: a) an incremental theory of plasticity formulation to compute seismic stresses and strains, and b) a laboratory based relationship to calculate the pore pressures developed by those seismic strains.

A model is proposed to simulate the undrained (constant volume) behavior during arbitrary 3D earthquake loading of a saturated loose sand layer located at a shallow depth (less than 50 ft) in a level site.