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.
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