Tunnelling-induced ground settlements in a groundwater drawdown environment - A case history

In this paper a case history of conventional tunnelling in which tunnelling-induced groundwater drawdown caused excessive surface settlements is presented. The measured ground surface settlements were first analyzed to identify the settlement characteristics at the site in relation to the tunnelling-induced groundwater drawdown. The measured settlement data revealed a considerably larger settlement affected zone than for cases with no groundwater drawdown, with a tendency for slow settlement stabilization. Also revealed in the measured piezometric data is that a significant portion of the total groundwater drawdown, around 65%, was completed prior to the top heading arrival. This suggests that pre-grouting is of paramount importance in controlling groundwater inflow in tunnelling situations with highly permeable water bearing ground. In addition a three-dimensional stress-pore pressure coupled finite element analysis was performed which confirmed the direct link between groundwater drawdown and excessive settlements. The practical implications of the findings from this study are also further discussed.