Hydraulic stability evaluation of a diaphragm wall in granular soils recharged by an unconfined aquifer

Abstract

Diaphragm walls are normally adopted for deep basements in granular soils because, among other reasons, they act as watertight retaining structures and can form part of the structure foundations. When the water table is high and the excavation is deep there is a likely risk of hydraulic stability problems. In this work two-dimensional flow nets around a diaphragm wall of 20 m depth and 0.8 m thickness are studied for different construction/ excavation stages. The water level behind the wall is the result of the excavation geometry, soil permeability and aquifer characteristics, i.e. a horizontal water level is not imposed. Using a commercial 2D steady flow software, distributions of velocities and hydraulic gradients are determined. Based on the definition of piping safety factor, a maximum excavation depth and a diaphragm wall minimum embedment are determined. Results using an isotropic coefficient of permeability value are compared with the anisotropic case. Hydrodynamic pressures over the wall are also determined and compared with hydrostatic pressures. Finally, it is pointed out that a minimum diaphragm wall embedment should be verified to avoid piping or erosion effects, which can be detrimental for the stability of the project during construction.