Computation Method For Response Of Laterally Loaded Piles And Liquefaction-Mitigation Performance Of Piled Foundation

Pile foundations have been widely used in engineering with the growing construction of civil infrastructures. Piles are commonly used to transfer vertical (axial) forces, arising primarily from gravity (e.g., the weight of superstructures). However, it is not only the axial force that the piles carry; often the piles are subjected to lateral (horizontal) forces and moments. Especially during the earthquake, large lateral forces and overturning moments are imposed on the piled foundations by the inertia of superstructures. As a result, the plasticity can easily occur for reinforced concrete piles as the tensile strength of concrete is quite low. This can lead to bearing failure and cause heavy casualties and severe economic losses. Thus, it is of great demand in seismic design of pile foundation that quasi-static computation method for response of laterally loaded reinforced concrete piles should be established. In addition, the effect of gap formation between the pile and the clay on the response of laterally loaded piles and the mechanism of the gap formation need to be studied and the computation method for the design of laterally loaded pile group also needs to be improved. Finally, liquefaction is a common phenomenon in which the strength and stiffness of a soil is reduced by earthquake shaking and piles are usually used to mitigate the adverse effect of liquefaction, however, little research has be conducted on the liquefaction-mitigation performance of piled foundations. Thus in this thesis, some works related to these issues are conducted and shown as follows:(1) Due to the limitations in the current strain wedge model, a modified strain wedge model is proposed to analyze laterally loaded piles by using finite element software SWPILE compiled in Fortran. A number of full scale pile tests in sand, clay, and layered soils are used to verify the applicability of the proposed method. In addition, some discussions are made on convergence of the proposed modified strain wedge model, the strain wedge depth and the soil strain in the strain wedge for a variation in the lateral load, the sensitivity of some input parameters, the hyperbolic and bilinear stress-strain relationships, and so on. Finally, the influential factors of p-y curves are studied based on the proposed method.(2) On the basis of the modified strain wedge model, layering effects of soils on the response of laterally loaded piles are investigated by comparing pile behaviors in uniform sand and clay soils with those both in double-layer soils and in triple-layer soils (i.e.. sand layer in clay deposit and clay layer in sand deposit). Then, the most influential height of both sand soils and clay soils at the ground surface is studied based on a number of case studies. Finally, the relation between the most influential height and the stain wedge height is investigated.(3) Computation method for analyzing nonlinear behavior of laterally loaded concrete piles is proposed using fiber element. The versatility of the proposed method is demonstrated by a number of full scale pile tests. The effects of material nonlinearity of pile on flexural rigidity, load-deflection curves, and load-maximum moment curves are adequately studied. In addition, the stresses on gauss points are recovered to nodes of the grid of pile section and then depicted in order to study the neutral axis of the pile cross-section and the development of plastic-state area on the pile cross-section with the variation in the lateral load.(4) Simplified computation method for the response of laterally loaded pile group is proposed by the combination of the concept of p-y multiplier proposed by Brown et al.(1988) and group-equivalent pile procedure proposed by Mokwa (1999). The proposed method is verified by a number of full scale pile-group tests. In addition, the effect of p-y multiplier on the calculated p-y curves, the effect of the boundary conditions of pile head on the lateral response of piled foundations and the effect of lateral load and soil depth on the group effect are investigated.(5) A preliminary research is conducted on lateral response of piled foundations at liquefied site. In addition, a fully coupled dynamic effective-stress finite element procedure UWLC is used to reproduce the damage of the two houses due to dune liquefaction. A modified Pastor-Zienkiewicz III model was used to describe the liquefaction behaviors of the younger sand dune layer. The parameters of the model were quantified by the laboratory tests on undisturbed samples and the Standard Penetration Test data. The input motions for the numerical analyses were calculated by SHAKE91using the seismic motions recorded by the vertical array at the service hall of Kashiwazaki-Kariwa Nuclear Power Plant. First, the lateral displacements of the ground surface calculated by UWLC were compared with those calculated by the method of Zhang et al.(2004) to verify the reliability of the UWLC results. Second, the effects of the sand dune slope on the damage to two houses were investigated. Finally, the effectiveness of each countermeasure used for house B and the combinations of two countermeasures was studied. The research highlights the liquefaction-mitigation performance of pile-group foundation.