Modelling transient discharge into deep-buried tunnels in karst area based on a coupled discrete-continuum model

Karst aquifers are characteristic of complex hydrogeological conditions and highly developed void structures, often causing serious water and mud inrush, karst collapse and other geological disasters during construction of deep-buried tunnels. Accurate prediction of groundwater discharge into tunnels has become a key issue for tunnel engineering in karst formations. Motivated by safety assessment of tunnel construction in the Jiayan Water Diversion Project located in Guizhou Province, China, we perform groundwater flow simulations to predict transient discharge into the long, deep-buried tunnels. Two numerical models, an equivalent porous medium model and a coupled discrete-continuum model, are used. In the coupled discrete-continuum model, both the tunnels and karst conduits are treated as discrete channels, which are coupled with the surrounding matrix through water exchange. Based on hydrogeological data and site characterization, the model parameters are calibrated by using the PEST inversion algorithm. It is shown that numerical predictions of tunnel discharge during construction agree well with the measurements, indicating reliability of the numerical models. Comparison between the two models shows that the coupled discrete-continuum model can more accurately reflect the changes in the groundwater level and can better estimate the sources of groundwater discharge into the tunnels during the construction. This work suggests that the coupled discrete-continuum model is advantageous over the equivalent porous medium model as the former is a better representation of the karst system. It is demonstrated that the simulation methodology provides a reliable method for assessment of water discharge into tunnels and the optimization of construction scheme. The results are of practical significance for safe construction of tunnels in karst areas.