Advantages of employing multilevel monitoring wells for design of tunnels subjected to multi-aquifer alluvial

For tunnels being excavated through multiple aquifer formations, having precise knowledge of the aquifers' hydraulic head becomes essential for determining groundwater inflow into the tunnel and analyzing its stability, specifically using multilevel monitoring systems. In the multi-aquifer alluvial section of the Glas tunnel (Iran), since the hydraulic head calculations were based on the data obtained from single-piezometer boreholes, the excavation risk was assessed to be at high level and the tunnel seemed to be unstable, thus an incorrect conclusion was derived from the misleading data. To take cost mitigation measures into account, it was necessary to calculate the hydraulic head at tunnel level accurately. By installing nested and clustered wells the mean hydraulic head was measured to be 70 m, significantly different from the 90 m previously determined by boreholes. Considering the updated value, the groundwater inflow and bulkhead load, formerly calculated as 0.65 m3/s and 9.5 bars, were determined to be 0.49 m3/s and 7.5 bars, respectively.