Time series modelling: applications for groundwater control in urban tunnelling

摘要

Water ingress to tunnels may result in pore pressure drawdown and consolidation settlements in areas above tunnels foundedon soft soil deposits, potentially causing damage to buildings and infrastructure. To limit pore pressure drawdown, requirementsare set on water ingress to bedrock tunnels. To meet these requirements, pre-excavation grouting is often performedto reduce the hydraulic conductivity of the rock mass surrounding the tunnel. Real-time pore pressure monitoring may beused to document pore pressure drawdown during construction. However, the effect of tunnel water ingress can be difficultto distinguish from natural pore pressure fluctuations. This paper presents a tunnel case in Oslo, Norway, where time seriesmodelling was applied to local pore pressure data using the transfer function model framework. The input to the models wasdaily meteorological data considering precipitation and evapotranspiration, and the output was simulated pore pressure levelswith impulse response functions. The models were optimised with data from before tunnel excavation, and simulations wererun during the tunnel excavation period. Simulated pore pressure levels were compared with observed pore pressure levelsto assess tunnelling-induced drawdown. Model uncertainty ranges were used to produce upper, lower, and best estimates ofthe drawdown. The findings show that time series modelling with transfer function models may be used in tunnel projects tocontinuously assess the impact on the local groundwater environment, for better evaluation of the pre-grouting performance,and for quantifying both the temporary and long-term drawdown with increased accuracy.