Evaluation of groundwater flow during excavation of shafts by inversion of tilt data

Nakatani, Katsuya*; Matsuki, Koji*; Arai, Takashi*; Omura, Kazuo*; Takeuchi, Shinji; Arai, Yasushi; Horimoto, Seiki*

Groundwater flow at the site of MIU (Mizunami Underground Research Laboratory) in the Tono district, Japan, was evaluated by a new inverse method using tilt data measured by four tiltmeters during excavation of shafts. The features of the method are that the fluid volume change per unit volume of rock (dv) is assumed to vary in a quadratic manner with the coordinates in elements within a region of groundwater flow, that the values of dv are set to zero at the boundaries of the region and that the sum of squared second derivatives of dv are adopted as constraining conditions which are weighed and added to a squared error in the tilt. An inverse analysis was performed by assuming three sizes for a whole region of groundwater flow. It was shown that there are three regions where the volume of groundwater decreases and two regions where the volume of groundwater increases regardless of the size of the whole region of groundwater flow and that groundwater came mainly from the sedimentary rocks lying between two faults. The latter is consistent with the previous prediction that these faults have a low permeability and may act as a flow barrier. Furthermore, the results of the inverse analysis predict that there may be other hydrogeological structures which prohibit groundwater flow. Thus, the inverse method proposed by the authors for evaluating groundwater flow from tilt data has proved to be reliable in spite of the assumption that the rock mass is a homogeneous and semi-infinite body.