Evaluation and assessment of inflow rates in tunnels excavated in jointed rock mass.

摘要

The approach used in current engineering practice in estimating groundwater inflow rate during tunnel excavation assumes that the variance in the measured packer hydraulic conductivity values reflects the hydraulic conductivity distribution along the tunnel alignment. However, the field packer test results may not adequately reflect the significance of key parameters affecting groundwater inflows which include tunnel depth, in-situ stress, geologic features (highly pervious zone), degree of weathering, drawdown of groundwater level, and rock joint characteristics. This thesis presents an approach to discern the impact of those various parameters on the hydrological flow regime around tunnels in fractured rock masses.; The effects of key parameters affecting groundwater inflows were investigated using 2-D Distinct Element Method Program which can take into account the coupled hydro-mechanical behavior within the rock fractures. The main concluding points of the study can be summarized as follows: (1) The hydro-mechanical behavior of the ring of tighter rock around the opening is similar to that of a semi-permeable liner, and the rate of water inflow into an unlined rock tunnel taking into account the tighter rock zone can be estimated using an analytical approach similar to that used to calculate inflow into a concrete lined tunnel surrounded by a homogeneous medium. (2) An assessment of the in-situ geology needs to be made to determine the potential of highly pervious features which can have hydraulic conductivities that are orders of magnitude larger than the other discontinuities and cause significant increase of the inflow rate. (3) When the drawdown of groundwater level takes place during tunnel excavation, the inflow rate can be approximately estimated by using the excavation-induced lowered groundwater level above a tunnel instead of the initial groundwater level. (4) Representative value of the jointed rock mass hydraulic conductivity in estimating water flow rate into a tunnel can be obtained from packer test results with applying appropriate adjustment factors corresponding to the depth of excavation, pore pressure values, spacing and nature of the joints likely to be intersected by the opening.