Design of reinforcing piles to increase slope stability

TLDR

The study proposes a systematic pile‑reinforcement design for slopes that determines the required additional shear force, evaluates each pile’s maximum resisting shear, and selects pile type, number, and placement to achieve a target safety factor. The method employs stability analyses to compute needed shear, a computer program (ERCAP) to model pile response to lateral soil movement and calculate resisting shear as a function of pile diameter, flexibility, and soil‑movement depth, and engineering judgment to integrate these results for pile selection. Applying the approach to a Newcastle highway bypass, the authors installed 64 bored piles 1.2 m in diameter, 6–12 m long, spaced 3.2–6.0 m across a 250 m slope cut, demonstrating the ERCAP‑derived pile behavior characteristics.

Abstract

This paper describes an approach for the design of piles to reinforce slopes, involving three main steps: (1) evaluating the shear force needed to increase the safety factor to the desired value; (2) evaluating the maximum shear force that each pile can provide to resist sliding of the potentially unstable portion of the slope; and (3) selection of the type and number of piles, and the most suitable location of these piles within the slope. For step 1, stability analyses can be used to assess the required additional shear force for stability. Step 2 involves the use of a computer analysis for the response of a pile to laterally moving soil. This analysis can be implemented via a computer program ERCAP, and enables the resisting shear force developed by the piles to be evaluated as a function of pile diameter and flexibility and the relative depth of the soil movement in relation to the pile length. Step (3) involves the use of engineering judgement in conjunction with the analysis results from steps 1 and 2. The paper describes the ERCAP analysis and the characteristics of pile behaviour it reveals. The application of the approach to a highway bypass problem in Newcastle, Australia, is described in detail. In the final design, a total of 64 bored piles 1.2 m in diameter were used over a total length of slope cutting of about 250 m. The pile lengths ranged between 6 and 12 m, with the spacings varying between 3.2 and 6.0 m. Key words : analysis, boundary element, piles, soil–pile interaction, slope stabilization, soil mechanics.