水利枢纽工程区受河谷地形的影响显著,应力场分布复杂。针对东庄水利枢纽工程,采用三维水压致裂法进行地应力测试,结合应力场反演分析,揭示了深切河谷区岩体的应力分布规律,探讨了地质特征对应力场的影响。对比二维水平应力和三维空间应力特征,空间主应力与水平主应力矢量存在较大的夹角,在地形复杂工程中三维地应力测试更具代表性。泾河河谷区应力场具有明显的分区特征,靠近河谷位置主应力明显偏大,属应力集中区,主应力方向与河谷走向呈大角度相交;而地下厂房区最大主应力与自重应力接近,应力方向主要受西北两侧边坡地形的联合影响,处于应力平稳区。同时,河床底部高程地下水长期连通流动产生溶蚀作用,形成了厂房区底部相对集中的溶孔、溶隙等形迹,为应力释放提供了通道,导致局部应力降低和不均匀分布。

[Objective] In engineering areas with minor topographic relief, horizontal stress can be approximately used as the stress condition for underground engineering design, and 2D hydraulic fracturing method is generally applicable for in-situ stress testing. However, in deep valley areas with significant topographic relief, planar stress is no longer sufficient to represent the principal stress characteristics of the engineering area. Based on the Dongzhuang Water Conservancy Hub Project, this study aims to reveal the stress distribution patterns of rock mass in deep valley areas. [Methods] In-situ stress testing using hydraulic fracturing method was conducted in boreholes drilled on the riverbed slope and within test adits of the underground powerhouse area. Based on the three-borehole intersection hydraulic fracturing method for in-situ stress testing in the underground powerhouse area, the 3D stress results at the borehole intersections were obtained. A comparison between 2D horizontal stress and 3D spatial stress showed that in mountainous areas affected by valley and slope topography, there was a significant angle between the spatial principal stress and the horizontal principal stress vectors. Therefore, when constructing underground caverns in shallow mountainous areas with valley slope topography, the 3D spatial stress was the true stress condition that needed to be considered, and 3D in-situ stress testing was more representative. Additionally, numerical inversion analysis of the stress field was performed. [Results] The stress magnitude above the valley elevation in the engineering area was significantly influenced by slope topography, with stress contour lines basically distributed along the direction of the slope gradient. In the powerhouse area, the maximum principal stress was primarily governed by self-weight stress. Under the fault zone of the deep valley on the northwestern side, the residual horizontal tectonic compressive stress in the mountainous areas above the riverbed elevation was minimal or had largely been released over geological time. At the riverbed bottom, pronounced stress concentration was observed due to horizontal tectonic compression and the subduction effect from adjacent valley slopes, forming a typical valley “stress concentration zone”. Below the valley elevation, as the rock mass burial depth increased, the stress magnitude increased, with diminishing influence from surface morphology. The stress contours exhibited a horizontal distribution, indicating that the deep stress field was mainly controlled by horizontal tectonic compression. Near the valley, the orientation of the maximum horizontal principal stress tended to be orthogonal to the valley trend. With increasing distance from the valley, the influence of valley topography on in-situ stress weakened, and the orientation of maximum horizontal principal stress gradually deflected toward the measured stress orientation in the powerhouse area. Furthermore, based on the differentiation characteristics of in-situ stress in the valley area of the Dongzhuang Water Conservancy Hub Project, the area could be roughly divided into four zones: stress relaxation zone, stress transition zone, stress concentration zone, and stress stabilization zone. The rock mass thickness for each zone was determined according to the characteristics of stress variation. [Conclusions] This study investigates the abnormal reduction in measured in-situ stress at the bottom of vertical boreholes in the powerhouse area. Based on the dissolution traces observed in drilling cores and borehole videos, along with hydrogeological surveys, it is demonstrated that the long-term groundwater flow connectivity at the riverbed elevation has caused dissolution, forming relatively concentrated dissolution pores and fissures. The presence of these fractures alters the continuity and integrity of the rock mass, providing pathways for in-situ stress release, affecting stress transmission, and further resulting in stress reduction and uneven distribution. Thus, the abnormal reduction in stress magnitude with depth at the bottom of vertical boreholes is reasonably explained.