Numerical modelling of rows of discrete piles used to stabilise landslides under long-term conditions in clays

Abstract

A literature review found no rigorous solution for the ultimate lateral pile-soil pressure ( ) in a soil characterised by a frictional failure criterion, and that the popular empirical methods to estimate give profiles with depth that differ significantly. Most existing solutions for the lateral pile capacity in a group are for soil characterised by an undrained shear strength failure condition. Plane strain and constant overburden finite difference analyses (in FLAC3D) were used to model flow of soil around a pile but did not appear to give sensible solutions for a frictional soil. The ultimate pile-soil line load from three-dimensional analysis in FLAC3D behaved as physically expected; passive wedges formed close to the surface giving lower normalised resistance than at greater depths. A number of parametric analyses were carried out using the three-dimensional model to investigate the variation in the ultimate pile-soil line load with the soil strength and pile-soil interface strength. Larger values of initial earth pressure coefficient K0 led to enhanced values of and the mechanisms for this was further investigated by analysing the soil stresses mobilised around the pile as the soil was pushed with the pile. Limit equilibrium pile failure mechanisms were developed from conditions of force and moment equilibrium for the pile based on failure in the soil. Pile limit equilibrium conditions were determined for three failure modes to understand the relationships between pile shear force, bending moment and pile embedment length ratio. Three-dimensional numerical (FLAC3D) models were used to verify the limit equilibrium failure mechanisms. The limit equilibrium equations were found to provide unconservative predictions for the force that the pile can provide to stabilise a slope, compared with the FLAC3D analysis. The program Alp (which models the pile as a beam on springs) gave results that were close to the limit equilibrium calculations. Three-dimensional FLAC3D models were modified to investigate the conditions over which the derived limit equilibrium pile failure mechanisms could reasonably be applied. The centre-to-centre pile spacing was varied from 1 d to 10 d, where d is the diameter of the pile, to understand the pile-soil interaction for a row of piles using the FLAC3D model. When the pile spacing was less than 2 d, the pile stabilising force was the same as for a solid retaining wall. Beyond about 4 d, the piles were found to act individually.

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ORCID for J.A. Smethurst: orcid.org/0000-0001-8175-985X

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