In-situ stress measurements and regional stress field assessment of the Beishan area, China

Knowledge of the in-situ stress field in the rock mass at depth is important to provide stress boundary conditions for underground excavation design, stability evaluation, and the optimization of ground support systems. In the present study, in-situ stress measurement using hydraulic fracturing method is conducted in the Beishan area in Gansu Province, China, a preferred candidate site for Chinaʹs high-level radioactive waste repository. Information on the stress magnitudes and orientations in three sub-areas is obtained from 136 measurement points in nine sub-vertical boreholes at depths ranging from 30 to 700 m, and the distribution characteristics of the in-situ stress field is analyzed. The pressure–time records obtained from the test intervals with pre-existing fractures are discussed according to the acoustic borehole televiewer (BHTV) images. The measurement results indicate that the regional stress field is dominated by horizontal stress within most of the measurement depth ranges. A piecewise distribution of the principal stresses reveals approximately three stress domains with increasing depth. At shallow and medium depths, the stress fields are characterized by σH > σh > σv and σH > σv > σh, respectively, where σH, σh, and σv are maximum horizontal, minimum horizontal, and vertical stresses, respectively. With further increase of depth, the maximum principal stress changes from horizontal to vertical (i.e. σv > σH > σh). Fracture impression results reveal that the maximum horizontal stress is dominantly oriented in the NNE–NEE direction, which is in agreement with the orientation of regional tectonic stress field generated from movement of the Earthʹs tectonic plates. In addition, it is found that the orientations of σH are affected by the geological structures significantly.