Measurement and Assessment of the In-Situ Stress of the Shazaoyuan Rock Block, a Candidate Site for HLW Disposal in Northwest China

For the assessment of the long-term stability of deep geological repositories of high-level radioactive waste (HLW), the information on in-situ stress field is a key parameter. We recently performed a great number of hydraulic fracturing tests within 1000 m vertical depths in the Shazaoyuan rock block, a new candidate site for HLW disposal in northwest China to clarify its stress field. The results show that the maximum ( σ H ) and minimum ( σ h ) horizontal stresses increase in gradients of 0.0193 ± 0.0018 MPa/m and 0.0143 ± 0.001 MPa/m, respectively, and the largest value is less than 30.65 MPa within the tested depths. The Shazaoyuan rock block is characterized by a thrust stress regime ( σ H > σ h > σ v ) at depths above 426 m, whereas strike-slip ( σ H > σ v > σ h ) within 426–1000 m depths interval where σ v is the vertical stress. The stress field of the Shazaoyuan is rotated by 90° with respect to the larger regional trend, but parallel to the NW-striking fault system inside the site. Our findings manifest that the Shazaoyuan stress field which shows NW-oriented σ H and greater horizontal stresses is a rare local stress field generated under the NE regional forces. One possible explanation for this dramatic rotation is the potential perturbation of the NW-trending fault system on the regional stress field. Finally, we discuss the geomechanical stability of the site and find that it is currently in stability state, being suitable for HLW disposal. This study provides important information for site suitability assessment of HLW disposal, also a reference for stress field prediction in similar geological settings.