Numerical analysis of collapse in a deep excavation supported by ground anchors

On 3 October 2013, a global failure occurred in one of the stabilised sections of a deep excavation in Shahrak-e gharb in Tehran. The excavation was supported by ground anchors with enlarged reinforced concrete thrust blocks and sprayed concrete facing. The failure occurred despite the system passing conventional limit equilibrium design and satisfying requirements in terms of allowable displacements. The case of Shahrak-e gharb was modelled using Abaqus in order to understand the soil deformation leading to the failure condition. It was found that a near-surface zone of weaker fill material was responsible for the failure. As the displacements during construction and the soil failure mechanisms were well matched by the numerical model, the case study was considered to be a validation case for the numerical model and a parametric study on the extent to which anchoring bypassed the weaker surface soil was performed. This study showed that proper embedment into competent soils is important as excessive strains can still develop if the anchor bond zone is insufficiently confined, potentially leading to progressive collapse.