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.
The present study examines the use of nonlocal regularisation in a coupled consolidation problem of an excavated slope in a strain softening material. The considered boundary value problem allows for a thorough evaluation of the nonlocal regularisation approach, as it does not entail any kinematic restraint on the slip surface development. The examined nonlocal strain softening constitutive model requires the specification of one additional parameter, the defined length DL, which essentially modifies the rate of softening. In addition, the optional radius of influence parameter, RI, can be specified to reduce the number of local strains referenced in the nonlocal strain calculation and thus increase the efficiency of the analysis. The nonlocal strain softening constitutive model reduces significantly the mesh dependency of cut slope analyses for a range of mesh layouts and element sizes in comparison to the conventional local strain softening approach. The nonlocal analyses are not entirely mesh independent, but the predicted time to failure and horizontal displacement over time are much more consistent compared to analyses that employ the local strain softening constitutive model. Further investigation, computing the Factor of Safety of various mesh arrangements showed that for drained conditions the nonlocal regularisation eliminates the mesh dependence shown for the same analyses by the conventional local strain softening model. The impact of the two nonlocal parameters, DL and RI, on the numerical predictions is also parametrically examined. The parameter DL modifies the softening rate of the soil and therefore its selection should be based on simulating a realistic softening rate for the examined soil material. The RI parameter was found to reduce significantly the computational cost at the expense of affecting the development of the secondary slip surfaces. Overall though, the results and the critical slip surface were similar for all considered values of RI
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