DYNAMIC SIMULATION OF GROUNDWATER INFLOWS DURING TUNNEL EXCAVATION

Most of the analytical and numerical models of tunnel groundwater inflow ignore the excavation process, leading to inaccurate prediction of discharge rate. A secondly development of the groundwater flow simulation program MODFLOW was conducted by using MATLAB software, regarding the continuously driven tunnel as a dynamic boundary condition, to achieve a dynamic simulation of tunnel water inflows. Three kinds of permeability distribution fields were imported into the same model under an ideal engineering condition of uniform drilling speed. The simulation results show that: 1) the heterogeneity of permeability has a great impact on simulation of groundwater flow; 2) the maximum water inflow occurs at the beginning of excavation at any given location, then the water inflow at this location decreases in a negative exponential function, the decay pattern is related to the initial water flow and distribution of the permeability field of the simulated area; and 3) a water inflow event will occur when a higher permeable zone is exposed, and the total water inflows will then gradually decrease until the next event occurs. The relationship between discharge rate and hydraulic properties was further investigated in a high-K zone. It revealed that maximum discharge rate was proportional to hydraulic conductivity (K) and specific storage (Ss). The decaying rate of discharge was time-dependent and also proportional to the value of K and Ss. The water budget analysis demonstrated that water released from storage of the high-K zone was the major source of tunnel discharge at early times.