On the reliability of 3D numerical analyses on passive piles used for slope stabilisation in frictional soils

The use of passive piles can be an effective method for stabilising unstable slopes. Unfortunately, no rigorous analytical solution has so far been proposed for assessing the ultimate, lateral pile-soil pressure distribution under drained conditions for the design of passive piles in a slope. The present work focuses on the reliability of a finite-element model (FEM) used to assess the ultimate limit state conditions of passive piles in frictional soils. The paper also provides an estimate of the ultimate load of a single pile and a row of piles in a slope of frictional soil. The results are obtained with a series of two-dimensional analyses (evaluating the role of boundary conditions) and three-dimensional analyses on an infinite slope (to evaluate the role of the embedment ratio, the influence of slope inclination and the arching effects in pile rows). The analyses were performed using the Abaqus finite-element code associated with a couple of user-defined subroutines for defining the initial and boundary stress conditions. The computed ultimate loads are compared with theoretical findings obtained from a simple extension to drained conditions of Viggiani's approach to undrained conditions. Depending on pile embedment and soil layer thickness and strength, three rupture mechanisms are discussed from a theoretical standpoint. The FEM converges very efficiently and reliably in one rupture mechanism and for deep pile embedments, whereas convergence is slow and difficult in the other cases and requires a very high elastic soil stiffness.