D. Su

Resistance of Short, Stiff Piles to Multidirectional Lateral Loadings

Lateral loads applied to pile foundations in some cases are multidirectional. However, most of the past studies only considered soil-pile interaction under unidirectional horizontal loadings. This paper describes a comprehensive experimental study on a pile-sand system under both unidirectional and multidirectional horizontal loadings using a computer numerically controlled biaxial motion platform. The displacement paths at the pile head include unidirectional regular paths, cross paths, figure-8 paths, and unidirectional and multidirectional irregular paths with different displacement amplitudes and different aspect ratios of the displacement amplitudes along two horizontal directions (α). The test results indicate that the preloading along one horizontal direction influences the subsequent response along the orthogonal horizontal direction, in terms of the pile resistance and the direction of the force increment vector. In the figure-8 tests, the shapes of the force-displacement curves in most cases differ significantly from that obtained from the unidirectional regular test and different from the unidirectional regular test, the maximum forces appear before the displacements reach the maximum values. In these tests, the direction of the force increment vector always deviates from the direction of the displacement increment vector. According to the results of the regular and irregular loading tests, the lateral resistance of the pile under the multidirectional paths is generally lower than that under the unidirectional path, and the degree of reduction increases with the aspect ratio (α). The ratio of force (rF), defined as the maximum force in the multidirectional tests to that in the unidirectional test, can be expressed as an exponential function of α. Considering that the reduction in the resistance can reach as large as about 30%, overlooking the multidirectional loading effect can lead to unconservative analysis or design in some cases.

Estimation of the Apparent Permeability in the Dynamic Centrifuge Tests

Centrifuge model tests have been widely used to validate analytical procedures, with the assumption that variations of parameters from the prototype to the model are governed by the scaling law. However, the coefficient of permeability in dynamic centrifuge tests seems to be much larger than the value calculated from the scaling law. Permeability may significantly affect the rate of pore-pressure buildup in liquefiable soil and associated deformation during earthquake loading. Therefore, proper estimation of the actual coefficient is very important in interpreting experimental data and validating analytical procedures. Based upon the physical law of conservation of mass, this paper presents an approach for backanalysis of the permeability of saturated sand in the dynamic centrifuge test. The ability and necessity of such an approach was demonstrated by comparing the experimental and numerical results.