Excavation-Induced Groundwater Evolution of Noncircular Tunnels in Mountainous Regions: Analytical and Numerical Investigation

Abstract

Excavation damage induced by the drill-and-blast method (DBM) of the surrounding rock zone in tunneling intensifies the loss of groundwater and causes serious hydroecological imbalance. Traditional theoretical analysis concerning tunnel seepage fields leads to an underestimation of groundwater inflow due to the ignoring of the factor of increased permeability in the DBM damage zone. Meanwhile, the composite liner widely used in DBM tunnels with the waterproofing and drainage system is difficult to simulate numerically. Taking into account the damaged zone, this work presents an analytical solution based on the axisymmetric modeling method suitable for deep tunnels to determine the groundwater inflow, permeability coefficient, and seepage force. The proposed solution combining the equivalent perimeter method is extended to noncircular tunnels. Comparisons of the numerical results with those of a previous study indicate that the proposed approach provides realistic predictions for the groundwater inflow and external water pressure. Introducing the concept of an equivalent liner, a series of numerical simulations for the center-diaphragm (CD) method are performed, in which 14 steps are divided for the tunnel construction and operation. The results indicate that the external water pressures significantly decrease after an upper-section excavation of the left drift and increase after support closure. The seepage velocity of the primary support and second liner reaches the maximum near the arch skewback. The maximum seepage volume Q occurs after a left-drift upper-section excavation and the maximum groundwater table drawdown h occurs before right-drift support closure. The impact of rainfall on h is far greater than that on Q. For damaged and grouting zones, the effects of permeability on Q and h are more significant than those of thickness. Furthermore, a significant advantage of the CD method compared with the full-face and bench methods is deduced for tunnels crossing unfavorable strata.