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

Ishii, Eiichi   

Major inflows can occur during tunnel/shaft excavations in faulted/fractured rock masses even if pre-excavation grouted. In such cases, post-excavation grouting may be required for reducing the inflows. However, following the diffusion equation for fluid pressure, inflows can naturally reduce to half to one-tenth within the first several days to several weeks when the flow dimensions of the surrounding rock masses are closed to one, while the inflows hardly reduce when the flow dimensions are as high as three. Thus, if the flow dimensions are closed to one, the natural reductions in inflow may be also effectively available for the countermeasures. Nevertheless, the usability of relationship between changes in inflow and flow dimensions has been poorly verified at actual tunnel/shaft-excavation sites. Actual inflows may be strongly affected by different factors including grouting, degassing, excavation processes, and far-field boundary conditions. The author analyzed changes in inflow at six locations where major inflows occurred during tunnel/shaft excavations in faulted/fractured siliceous rocks and investigated flow dimensions in the rocks by packer tests in boreholes. The results confirmed that the inflows for several days to several weeks immediately after the inflows occurred change consistently with the flow dimensions estimated from the packer tests. Thus, even if major inflows were encountered during excavations, the subsequent changes in inflow are predictable based on the flow dimensions, and the flow dimensions can be estimated by preliminary packer tests or from the changes in inflow at the inflow locations for the first several days.