Database for Deep Excavations in Soft Clay with Focus on Groundwater Drainage and Installation Effects

Abstract

An extensive database consisting of measurements of ground movements and pore pressures from 48 deep excavations is presented. The ground conditions for all cases are soft, normally consolidated clays, often underlain by a coarser layer of soil, i.e., confined aquifer, on top of bedrock. Traditionally wall and ground movements caused by deep excavations have been predicted without assessing effects of groundwater drainage or the influence of installation of tie-back anchors or foundation piles. This study clearly shows that the observed ground movements exceeded expected values based on induced shear movements and significant deformations occur at far distance from the excavation. The cause of the deformations is largely consolidation settlements due to pore pressure reduction, with a zone of influence recorded at up to 400 m distance from the excavation. The construction methods and opening of drainage paths to the confined aquifer is as important for pore pressure reduction, as exposing the confined aquifer itself. In addition, the groundwater drawdown is shown to be dependent on the ground conditions and mitigation measures. It is also shown that the pore pressure reduction and zone of influence may be very small if no drainage path is created, i.e., in case of strutted excavations and no piles or driven piles, or when the bedrock surface is not exposed. The drilling of tie-back anchors and piles are also seen to influence ground movements, especially at a distance of 2–4 times the excavation depth. Charts produced from the database enable estimation of expected groundwater drawdown including zone of influence. In addition, they can support decisions on construction methods, need for mitigating measures and extent of monitoring program in relation to deep excavations.