Excessive groundwater inflow during TBM tunneling in limestone formation

A sudden increase of groundwater inflow of 670 tons a day was experienced during tunneling in a limestone formation at around 53 m below the ground surface. The limestone cavities were expected to be located at 5-10 m or more above the tunnel crown in geotechnical exploration. However, limestone cavity network was encountered at the tunnel depth, and large groundwater inflow was experienced. Because of the excessive groundwater inflow into the tunnel, the groundwater level dropped to GL-32 m from the original level of GL-16 m, and consequently maximum of 23 mm of ground settlement and damage of adjacent structures took place. In this study the change of groundwater regime during tunneling in limestone area containing cavity network was analyzed through 3-dimensional numerical analysis, and the mechanism of groundwater level drawdown followed by ground subsidence was analyzed by comparing measurements and numerical analysis results. And also the analytical solution for determining the range of cutoff grouting was proposed to minimize groundwater inflow during excavation in a fractured zone.