Finite element modeling of pile-soil interaction: effects of installation and response in sloping ground

Karmaker, Ripon (2022) Finite element modeling of pile-soil interaction: effects of installation and response in sloping ground. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Piles are installed in many civil engineering projects on the level ground and near the slopes in vertical and inclined directions. While axial capacity is the primary concern in foundation design, the effects of lateral load due to ground movement are equally important in many cases, such as slope stabilization by piles and the impact of lateral spreading. Displacement piles are generally installed by jacking or diving. During installation, a large volume of soil displaces around the pile, which could cause soil disturbance and thus affect the subsequent load-carrying capacity. Modeling the pile installation is a challenging task, especially when installed in sensitive clays. The contractive response and structural breakdown during shearing cause strain softening and develop a significant excess pore water pressure that could reduce the effective stress of the soil around the pile close to zero for high sensitive clays. The soil disturbance and penetration resistance, especially the skin friction, highly depend on softening and excess pore water pressure. The rate of shearing could also affect the soil strength and penetration behaviour. In the present study, the installation of the pile in sensitive clays are simulated for undrained condition. Mathematical models for strain-softening and strain-rate effects on the undrained shear strength are implemented using user subroutines. A comparison with the results of a field test conducted previously in Québec, Canada, shows that the present numerical technique can successfully simulate the installation process. The simulation results also show that the simplified approaches (e.g., cavity expansion or strain path methods) cannot model some key aspects of pile installation in highly sensitive clay. For jacking, the shear strength of soil in a small zone around the pile reduces to a small value due to strain softening, and the disturbed soil flows primarily through this narrow zone when the penetration is continued to a larger depth. The plastic shear strain develops over a larger area for low sensitive clays, but its magnitude near the pile is higher in high sensitive clays. The penetration due to impact driving is different from that in jacking. Near the ground surface, each blow results in continuous penetration, although the rate of penetration is negligible at the end. However, at a deeper condition, the hammer impact results in penetration first, and then the pile rebounds some distance. The strain rate effects on undrained shear strength play a significant role in impact driving, as compared to jacking because the rate of penetration is higher during impact, which increases the mobilized shear strength near the pile surface, and therefore soil flow occurs through a relatively larger area. For a given depth of penetration at the end of a blow, the stress distribution around the pile in driving and jacking is comparable. For a sloping ground, the installation of a pile could cause the retrogressive failure of the slope, as reported in some studies. It is extremely difficult to conclude whether such failure was occurred only due to pile driving or a combination of other factors. Two retrogressive landslide cases studies are presented, where, in the first one, the failure of a sensitive clay slope was triggered by pile driving and, in the second one, landslide might have been triggered by toe erosion. However, the post-slide investigations show similar failure patterns, which implies that, although the soil type and triggering mechanisms are different, simulations can be performed using the same numerical technique. In the sloping ground, piles might be located at different locations of the slope—for example, near the toe/crest or on the slope. When slope failure occurs, the upper part of the pile in the sliding mass experience lateral load. Similar loading occurs in lateral spreading. Large deformation FE analyses are performed to calculate the force acting on the pile due to ground movement. The effects of arching on the lateral force in relation to soil behaviour and pile spacing are examined.

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