Effects of flow dimension in faulted or fractured rock on natural reductions of inflow during excavation: a case study of the Horonobe Underground Research Laboratory site, Japan

Abstract

Major inflows of groundwater can occur during excavation in faulted or fractured rock masses, even if pre-excavation grouting is applied; postexcavation grouting may then be required to reduce these inflows. However, the diffusion equation for fluid pressure suggests that inflows may reduce naturally by 50–90% or more within days or weeks when the dimension of the flow in faults or fractures that feed the inflow (flow dimension) is close to 1, but inflow reduction is minimal when the flow dimension is close to 3. Therefore, if the flow dimension is close to 1, the natural reduction in inflow may obviate countermeasures. Nevertheless, this natural reduction being dependent on flow dimension is seldom considered explicitly when planning excavations or countermeasures. To verify the applicability of the relationship between natural changes of inflow and the flow dimension, this study measured changes in inflow at six locations at the Horonobe site, Japan, where major inflows occurred during excavations of tunnels or shafts in faulted or fractured siliceous rocks. The flow dimension at each location was assessed using pre-excavation packer tests in surface-based boreholes. The results confirm that changes in the inflows during the days and weeks immediately after their commencement depended consistently on the assessed flow dimension. Natural reductions in inflow during excavation are predictable based on the flow dimension, which can be estimated using pre-excavation borehole investigations or from the initial changes in inflow during the first several days. This approach may be helpful for improving the efficiency of excavations.