An adaptation of Limit Equilibrium Methods for the design of soil-nailed walls facings

Abstract

Soil nailing is a technique developed in France during the 1970s for the retainment of excavations. The nails are steel bars introduced in the soil and preventing the soil mass to fail. Such structures are often designed through classical slope stability analysis software, i.e. based on Limit Equilibrium. The tension in the nails is generally considered equal to the maximum tensile forces admissible in the reinforcement. Yet, the service loads in the reinforcements are generally smaller than the calculated ones, especially at the bottom of the excavation because of the construction phasing. This is critical for the design of the facing and Limit Equilibrium based software need to be adapted by considering the construction phasing to modulate the mobilization of reinforcements. Based on the study of soil nailed walls through real-scale experiments, centrifuge and numerical modelling, an improvement of the limit equilibrium classical design is proposed. The software used was PROSPER, developed by Laboratoire Central des Ponts et Chaussées in the 1990s. The particularity of PROSPER is to derive the reactions of the nail by imposing a displacement of the failing soil mass. This displacement is generally considered as homogenous along the failure surface. However, considering a high displacement for the top nails (fully mobilized) and a small one for the bottom nails (partially mobilized) provide a relevant distribution of soil nail tensile forces. A distribution is proposed for such displacement and this design approach has been tested on an experimental wall, providing an efficient and time-saving design of soil-nailed walls.