Monitoring and Data Analyses of Pressure Changes and Ground Settlements Induced by Slurry TBM Tunneling in a Semiconfined Aquifer: Case Study in the Netherlands

This paper presents the field monitoring during construction of the Green Heart Tunnel driven by a slurry tunnel boring machine (TBM) in a semiconfined aquifer in which a sand layer was overlaid by a peat layer. Data analyses of the pore-water pressures, slurry pressures, tail-void grout pressures, and ground movements surrounding the tunnel are presented. It was found that the slurry infiltration controls the variation of pore-water pressure only close to the tunnel face. The excess pore-water pressure far from the tunnel face was induced by the elastic storage of the aquifer. It was found that the grouting had little influence on the pore-pressure distribution. Furthermore, because the excess pore-water pressure generated by slurry infiltration might contribute to the grout pressure, the measured value of grout pressure can be smaller than the actual value. Consequently, to obtain the actual value of grout pressure, grout-pressure gauges combined with the pore-pressure transducers should be installed. In the semiconfined aquifer, suction in the overlying peat layer could be induced when the TBM passed through the sand layer. For this condition, the ground movement proportionally increased with the decreasing water pressure.