Ground motions in real earthquakes are three-dimensional, resulting in changes not only in shaking magnitude but also in shaking direction. Thus, soil would experience complex stress paths as the principal stresses change in both magnitude and direction. Some experiments have shown that liquefaction resistance of cohesionless soil is lower under multi-directional shearing than under unidirectional shearing. But these tests were conducted under 1 g; stress state and boundary condition in field were not well reproduced. On the other hand, piles usually have comparable dimensions in two lateral directions, so the effects of multi-directional shaking encountered in actual earthquakes on soil-pile interaction cannot be overlooked. However, due to the limitation of test facilities, all the da...[ Read more ]

Ground motions in real earthquakes are three-dimensional, resulting in changes not only in shaking magnitude but also in shaking direction. Thus, soil would experience complex stress paths as the principal stresses change in both magnitude and direction. Some experiments have shown that liquefaction resistance of cohesionless soil is lower under multi-directional shearing than under unidirectional shearing. But these tests were conducted under 1 g; stress state and boundary condition in field were not well reproduced. On the other hand, piles usually have comparable dimensions in two lateral directions, so the effects of multi-directional shaking encountered in actual earthquakes on soil-pile interaction cannot be overlooked. However, due to the limitation of test facilities, all the data obtained in the past from centrifuge physical model tests are under unidirectional shaking conditions.

The research is aimed at investigating behaviors of sand deposit and sand-pile system under multi-directional earthquake loading. This dissertation describes the results of study, which consists of four major components: (1) a series of centrihge dynamic tests on saturated sand deposits; (2) a series of centrifuge dynamic tests on pile foundations; (3) fully coupled analysis of seismic level ground response; (4) numerical study of soil-pile interaction.

The dynamic model tests conducted in this study by the biaxial shaker at HKUST have produced the first batch of data on the responses of saturated sand deposit and sand-pile system under multi-directional earthquake loading. The experimental results reveal the impacts of multi-directional loading on the liquefaction potential of cohesionless soil and the interaction between soil and pile under earthquakes, as well as the influences of shaking intensity and loading history on the seismic responses of level ground and pile foundation.

Numerical analyses were performed by use of a fully coupled ground response procedure SUMDES, incorporating a comprehensive constitutive model of sand capable of simulating non-proportional loading responses. Based on the numerical studies, it is found that the in situ k₀ condition, and the fact that the mobilized shear stress in soil during earthquake is inversely proportional to the accumulated excess pore pressure, abate the effect of multi-directional shearing on level ground response. The findings prove the theoretical elaboration on this phenomenon made by Li years ago.

Factors influencing the characteristics of soil-pile interaction were studied numerically. Three different numerical approaches were employed to derive p-y relationships from the experimental data. The estimated p-y behaviors reveal the drastic effect of excess pore pressure on the soil resistance to the lateral movement of pile. Using PFC^2D, a program based on the distinct element method, several numerical tests with different loading paths were performed, results of which reflect the path-dependent nature of soil-pile interaction.