高放废物地质处置库北山预选区新场岩体地应力场分布规律

DISTRIBUTION CHARACTERISTICS OF GEOSTRESS FIELD IN XINCHANG ROCK BLOCK OF CANDIDATE BEISHAN AREA FOR HIGH LEVEL RADIOACTIVE WASTE REPOSITORY IN CHINA

甘肃北山预选区是我国高放废物地质处置库首选预选区。为掌握区域岩体地应力大小、方向及其分布规律,在预选区新场岩体内进行4个钻孔的水压致裂地应力测量。基于钻孔岩芯编录和钻孔电视成像结果,在0-700 m范围内共开展81段压裂试验,并在19个测试段内进行印模定向试验,成功获得地应力量值和最大水平应力方向数据。根据压力–时间关系曲线,采用3种不同方法判读瞬时关闭压力参数,保证水平主应力测量结果的精度与可靠性。测量结果表明,新场岩体地应力随着深度的增加而增大,在测量深度范围内,最大水平主应力小于25 MPa,且最大水平主应力普遍高于垂直应力,揭示新场岩体应力场以构造应力为主导的特点。最大水平主应力与垂直应力比率随深度的变化规律表明,构造应力的作用随着深度的增加逐渐减弱,垂直应力作用加强。此外,最大水平主应力的优势方向为NEE,与区域构造应力场方向基本一致,从而证实应力测量结果的准确性。研究成果可为高放废物处置工程的开挖设计及稳定性评价提供科学指导,并为我国西北地区应力场分布规律研究提供重要参考资料。

Beishan area in Gansu Province is the first potential area for the construction of Chinas high-level radioactive waste（HLW） repository. To know the magnitude,orientation,and distribution characteristics of geostresses in the regional rock block,geostress measurement using a single loop hydraulic fracturing system is carried out in the Xinchang sub-area of the Beishan area. Based on the information in borehole log and acoustic borehole televiewer image,a total of 81 hydro-fracturing tests and 19 fracture impression tests are successfully conducted in four sub-vertical boreholes at depths ranging between 0 and 700 m below the ground surface. The obtained stress data provide a comprehensive view of the state of geostresses in the Xinchang sub-area. To make the hydro-fracturing results accuracy and reliable,three different methods are used to determine the instantaneous shut-in pressures according to the pressure-time history records. The measurement results indicate that the geostress components increase with depth. Within the measurement depth range,the maximum horizontal principal stress is generally larger than the vertical stress,and has a maximum value close to 25 MPa,reflecting that the regional stress filed is dominated by tectonic horizontal stress rather than by the overburden load. On the other hand,the relationship between the ratio of the maximum principal stress to vertical stress and depth shows that there exists a possible transition of regional stress field from tectonic stress dominated to weight stress dominated in deep ground. In addition,the orientation of the major horizontal stress in the Xinchang sub-area is dominantly in the NEE direction,which is in general agreement with that of regional tectonic stress field deduced from movement of the earths plates. This confirms the reasonability of the geostress data obtained from the present stress measurement campaigns. The measurement results provide stress boundary conditions for the design and stability assessment of HLW disposal systems in the Beishan area,and provide meaningful geostress information for further research on stress field distribution of northwestern China.