Numerical Study of Slope-Stabilizing Piles in Undrained Clayey Slopes with a Weak Thin Layer

This paper presents a numerical study using three-dimensional (3D) finite-element (FE) analyses for slopes that contain a weak thin layer and are reinforced with piles. The presence of a thin weak layer usually has a negative effect on slope stability. In the FE analysis, a strength reduction technique is employed using FE software. In the numerical model, an elastic-perfectly plastic with Mohr-Coulomb failure criterion is used for the soils. The pile is assumed to be an elastic member, without considering failure. Some of the effective factors, such as the optimal pile location and pile length, were verified beforehand using two-dimensional (2D) FE analysis. The spacing effect of the pile, $S/D=4.0$ ($S$ is on-center spacing; $D$ is diameter), is found to be comparable for the 3D model in relation to the 2D model. The appropriate length of the pile used in the 3D analysis is based on the length of piles typically used in engineering practice. It is concluded that proper stabilization can be provided if approximately half of the pile length extends below the weak layer and piles are installed in the middle portion of the slope. The analysis methods are based on a coupled analysis method; that is, both slope stability and pile response are considered simultaneously. Also, the 3D FEM is able to overcome the limitations of the 2D FE model that lacks proper consideration of the boundary effect, the soil movement between the piles, and the spacing between the piles. The effectiveness of the pile-stabilized slope depends on the strength of the soil contained in the interbedded layer. Slope stability analysis for a slope with piles was also performed, whereby three typical failure mechanisms were observed, from translational to rotational failure for the different $C_{u2}/C_{u1}$ ratios ($C_{u1}$ is the undrained shear strength of the slope soil, and $C_{u2}$ is the undrained shear strength of the soil in a thin layer). The presence of the stabilizing piles in such a slope can change the failure mechanisms and the depth of the slip surface. The restricted conditions applied to the pile head are also found to have similar effects in changing the failure mechanisms in a slope. Fixed-head piles are found to provide substantially more improvement to slope stability than free-head piles. However, fixed-head piles are not always recommended, depending on the $C_{u2}/C_{u1}$ ratio and the required factor of safety after being stabilized with piles. This paper also provides a realistic soil–pile interaction model that is subjected to lateral loading on an inclined slope.