Abstract:

[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.

Key words: in-situ stress, water conservancy hub, hydraulic fracturing method, regression inversion, deep valley, groundwater dissolution