Pore pressure reduction and settlements induced by deep supported excavations in soft clay
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As a part of the large research project, "LimitingDamage" (BegrensSkade), common causes of damage or unexpectedly large settlements connected to ground and foundations works have been investigated. It can be seen that drainage is a common cause for settlements when performing deep excavations, causing decrease in pore pressures and consolidation of soft soil. The article presents monitoring data from numerous building sites, showing that there is often a substantial reduction in pore pressure levels at bedrock. The reduction in pore pressure can be observed hundreds of meters from the excavation. The general effects have been documented before, but the analysis of data in the project "LimitingDamage" shows a clear systematic reduction in pore pressures for the majority of excavation projects. The effects depend on the hydrogeological properties, as well as the extent and duration of the construction work and the mitigation efforts undertaken. The data and observations from the case studies suggest that the risk of drainage is substantial in the conventional methods and procedures commonly used. Excavating to bedrock level, if under groundwater level, can cause substantial reductions in pore pressure. In addition, it is concluded that drilling for tie-back anchors and bored piles can increase the risk of drainage. The risk of settlements caused by drainage and pore pressure reduction can be reduced during the early design phase of a project by undertaking the correct type of investigations and understanding the hydrogeology. Furthermore, one may select construction methods, which reduce risk of drainage. Measures may then be designed in order to mitigate the effects, followed by implementation and monitoring during the construction phase.