Stabilisation of excavated slopes in strain-softening materials with piles

Abstract

The use of a row of discrete vertical piles is an established method, successfully used to remediate failure of existing slopes and to stabilise potentially unstable slopes created by widening transport corridors. This paper challenges the assumptions made in current design procedures for these piles, which treat the pile only as an additional force or moment and simplify soil-pile interaction. Two-dimensional planestrain finite-element analyses were performed to simulate the excavation of a slope in a stiff clay and the interaction of vertical piles within the slope. A non-local strain-softening model was employed for the stiff clay to reduce the mesh dependency of the solution. An extensive parametric study was performed to systematically examine the impact of pile position, dimensions (length and diameter) and time of pile construction on the stability of a cutting in London Clay, which was chosen as a representative strainsoftening material. A variety of different failure mechanisms were identified, depending on pile location, dimensions and time of construction. The variability of the pile and slope interaction that was modelled suggests that an oversimplification during design could miss the critical failure mechanism or provide a conservative stabilisation solution. Given the prevalence of stiff clay slopes in the UK, increased capacity requirements of transport infrastructure and the age of slopes in this material, an informed and more realistic design of stabilisation piles will become increasingly necessary.

Behaviour of discrete piles used to stabilise a tree-covered railway embankment

“…There is no indication of deeper-seated movements, either interacting with or developing below the pile row, as seen in 2D finite element analyses of a pile wall stabilised cut slope in clay presented by Summersgill et al, (2018); at Mill Hill the slope inclinometer tubes extend below the toe of the piles, and do not show any displacements forming below the structure.…”

Nonlocal regularization of an anisotropic critical state model for sand

“…(8). Note that several other weight functions have also be proposed in the literature but one in Galavi and Schweiger [7] is found to give the best regularization results for soils with strain softening [28].…”

“…In previous studies, some of the state variables affecting the strain-softening and flow rule are assumed to evolve with the nonlocal plastic strain. This is called the partially nonlocal approach [7,28]. Though it is less rigorous than the fully nonlocal method which assume that both stress strain are nonlocal, it is found effective in regularizing most soil models with strain-softening.…”

“…In particular, the nonlocal method is found effective and convenient in regularizing strain-softening models for soils (e.g. [7,21,22,25,27,28]). In a fully nonlocal constitutive model, the stress, strain and state variables should all be considered as nonlocal variables.…”

“…The nonlocal method has mainly been used in soil models with simple strain-softening rule. For instance, the variable controlling the strain softening is a function of the plastic shear strain only in Galavi and Schweiger [7], Summersgill et al [28] and Mánica et al [22]. Therefore, the model can be easily regularized by assuming that strain-softening variable is dependent on the nonlocal plastic shear strain.…”