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

Abstract

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.

Passive soil pressure on sloping ground and design of retaining structures for slope stabilisation

“…Both the anchored wall and the diaphragm wall were assumed to be perfectly elastic, as in Kourkoulis et al (2011) and Muraro et al (2014), who showed the negligible effects of a large decrease in elastic stiffness and Poisson ratio of the retaining structure (a pile in their case) on the ultimate load.…”

“…They found the simple Mohr-Coulomb constitutive model sufficiently accurate for modelling interface behaviour. The friction at the slip surface interface was initially assumed to equate to the soil friction angle (μ ¼ ϕ), in order to generate the initial stress condition (as in Muraro et al, 2014), then μ was reduced to a value slightly higher than the slope's inclination (μ ¼ α þ 0·001°) to simulate a condition nearing collapse due to the presence of a thin weak layer. The soil-wall friction angle was assumed positive for downward-tilting soil pressure ( Fig.…”

“…The initial stress conditions were assigned according to the method described by Muraro et al (2014), with a userdefined subroutine based on the horizontal coefficient at rest suggested by Eurocode 7 (BS EN 1997-1; BSI, 2004), namely…”

Ultimate lateral load of slope-stabilising piles

“…Figure 7 is a sketch of the failure mechanisms with associated patterns of soil reaction and internal forces along the pile. From equilibrium considerations, as already shown by Muraro et al (2014) dimensionless shear force and bending moment at the interface for mechanisms A and C are obtained as Mode A Fig. 5.…”

“…The latter provided solutions for a single free-head pile embedded in fine-grained soil in undrained conditions (total stress analysis) in the framework of the limit equilibrium method. With a similar approach, Muraro et al (2014) have obtained the solution for the case of a rigid (not yielding) single free-head pile in drained conditions. Ito & Matsui (1975) studied the pile reaction forces when the soil is forced to squeeze between two infinitely long piles, providing two expressions obtained by using either the theory of plastic deformation or the theory of plastic flow.…”