Developments in excavation bracing systems

Abstract

Modern bracing systems for retaining soil in excavations and their indications and differentiating features are summarized. A recent innovation in a widely used system, cast-in-drilled-hole concrete pile walls, is described. The innovation consists of the use of longitudinal reinforcement distributed non-uniformly around the perimeter of the pile section in order to more efficiently resist bending moment. This non-symmetrical distribution of reinforcement results in a significant reduction in the amount of steel required. Practical situations where the new system has advantages over existing bracing systems are described. Significant reductions in material costs, reduced transportation costs, and simpler site operations result with the innovative system. These benefits are ultimately associated with a reduction in total cost and an extension of the economical range of applicability of this pile wall system. Results of a full-scale experimental demonstration are described, in which a segment of cut-and-cover underground high-speed rail line was constructed using this non-symmetrical piling system. In this segment, conventional symmetrically reinforced and the new non-symmetrically reinforced piles were instrumented. The new system resulted in smaller deformations and did so with savings of longitudinal steel of 44%.