地质科技通报 (Jul 2023)
Assessment of fractures geometries and seepage characteristics based on statistical homogeneous zone method
Abstract
Objective Comprehensive analysis of fracture geometries and seepage characteristics is one of the prominent components in site suitability assessment of high-level radioactive waste (HLW) disposal repositories. Methods To provide a sufficient basis for the construction of the Xinchang underground research laboratory (URL) site in the Beishan preselected area of the HLW disposal repository in China, this paper analyzed the development and distribution of fractures and the seepage characteristics in fractured granite. Based on the borehole tests of BS32, BS36 and BS39 carried out in the middle of the Xinchang site and the field investigation of the surrounding fractures, the orientations and linear density of fractures were used to quantitatively classify the homogeneous zones of the BS32, BS36 and BS39 boreholes. Supplemented by the theory of the hydraulic conductivity tensor, the hydraulic conductivities of fractured rock and the principal directions of fluid seepage in fractured rock with different burial depths were obtained. Results The results show that four groups of dominant fractures are developed around BS32, BS36 and BS39 at the Xinchang URL site, with orientations of 279°∠79°, 98°∠76°, 227°∠79° and 36°∠76°, which are especially dominant in the EW and NNE directions. The fractures are mainly shear-stress formed with steep dips (>60°), accompanied by a few tensile fractures, and the fractures are normally distributed. Compared with the field hydraulic test results, the overall comprehensive hydraulic conductivities of the boreholes are in the range of 10-13-10-9 m/s. The main seepage directions are NNE, nearly EW and SE, where NNE and nearly EW are the dominant seepage channels of the fractured rock mass, with larger main permeability tensor values. Two nearly E-W-trending F6 and F7 faults and their corresponding NNE-E-trending secondary faults play a macrocontrolling role in fracture development at the Xinchang site. The permeability of fractured rock is mainly affected by the fracture spacing and aperture, demonstrating a great anisotropy. Conclusion The results can provide necessary data support for the construction of disposal repositories and the numerical simulation of nuclide migration at the Xinchang URL site. In addition, the presented research idea could provide an alternative and practical method for effectively studying the properties of deep fractured rock masses with deep geological disposal of HLW.
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