Unexpected behaviour of a large excavation in saturated sands

Abstract

The paper describes the behaviour of the perimeter wall of a very large excavation in saturated sands. The excavation was extended to a depth of 20m (17m below the water table) and the walls reached an impervious silty clay formation which appears 55 m below the surface. The reinforced concrete walls were stabilized by a single row of anchors grouted in sand, below the water table. Stabilizing berms were also left unexcavated against the walls to allow reaching the maximum excavation depth in the large central area of the site. Later, basement slabs were built and propped against the walls, before the excavation was completed. A disturbing finding was the large horizontal displacements measured in the walls during the early excavation stages. These large displacements extended to large depths, well beyond the excavation depth. However, anchor loads remained essentially constant throughout the excavation and water lowering operations. Computer results were found in reasonable agreement with available wall displacements and anchor loads. A most interesting part of the analysis was the distribution of movements in the active zone of the walls. It was found that significant displacements extended well beyond the expected distance from the wall. In fact, wall displacements and anchor fixed length displacements were similar in magnitude. This result could be interpreted as an indication of impending failure but further finite element analysis carried out to establish the safety factor against failure showed that the entire system was far from limiting conditions. This effect is related to some features of the project, which include the anchor length, the stiffness of the wall and the size of the excavated area.