During the past two decades, the use of bored piles, or drilled piers, has been shown on several occasions to be an effective deterrent to exessive slope movements. The technique is applicable when either the resisting forces of the slope are to be decreased or the driving forces to be increased. These situations arise when cutting a steeper slope or adding a surcharge above a slope, respectively. The movements of landslides can also be controlled by drilled-in piers or piles.
Several investigators have studied the problem of a single pier subject to lateral soil movement, given certain simplifying assumptions with regard to the soil model, soil-pile interface, and repartition of the lateral loads. It is commonly assumed that a plane strain condition prevails, and the piers are analyzed in two dimensions. This does not allow the effects of pier size and spacing to be represented, although finite-difference solutions have shown these to be very significant. Current design practice uses piers spaced closely together in a continuous barrier. Part of the face is usually exposed to form a retaining wall, and in some cases the piers are tied back at the top. The design methods are similar to those used for sheet-pile walls and usually valid for relatively shallow overburden depths. This methodology results in a conservative sizing and spacing of the piers.
In this paper a finite-element based methodology is proposed to model the three-dimensional effects involved in the stabilization of surcharged slopes with drilled piers. The soil is modeled with eight nodes, 24 degrees of freedom isoparametric elements. The piers are represented by three-dimensional spar elements. The program can be used to determine the effects of piers' position, size, spacing, and stiffness on slope movements. This program is the first step towards a more general methodology that will consider soil nonlinearity and creep, and make provisions for slippage around the pile.