Abstract
Excavation damaged zones (EDZs) in deeply buried underground powerhouse have become major obstacles to design and support, which potentially threaten safety and stability and increase construction and support costs. In this study, investigations of the EDZs were performed by applying an acoustic velocity detecting method in Houziyan hydropower project, southwest of China. A total of 38 testing boreholes distributed in high sidewalls of the main powerhouse were carried out, and corresponding 153 curves were obtained and analyzed. Then, EDZs were divided into highly damaged zone (HDZ), slightly damaged zone (SDZ), and excavation influence zone (EIZ), respectively. Furthermore, we classified the wave velocity curves into four categories: type I, type II, type III, and type IV. EDZs were qualitatively assessed based on the curve categories; in addition, we used a qualitative assessment method, which mainly involved an index of damage degree named D. The assessment results show that HDZ, but not SDZ, was significantly asymmetrically distributed in the upstream (average depth of 4.1 m) and downstream (average depth of 7.5 m) high sidewalls; in partial areas, depth of HDZ exceeded the length of designed rock bolts, which indicates that rock bolts cannot restrain crack development and EDZs evolution. Generally, EDZs distribution was consistent with deformation and failure phenomena distribution; compared to the field failure phenomena, the assessment results were reliable and reasonable. Finally, EDZs formation mechanism was discussed, and it can be concluded that the relatively large intermediate principal stresses σ2 were a critical driving factor of the EDZs evolution.
Highlights
With rapid economic development and increasing huge demand for energy, more and more underground structures such as deep transportation tunnels and mining roadways are being constructed or planned
Most of these data were gathered, and qualitative and quantitative analysis were promoted. e depth values of Excavation damaged zones (EDZs) of the main section are listed in Velocity (m/s)
To obtain the extent of EDZs, a total of 38 testing boreholes were made, and 153 corresponding wave velocity curves were gathered. e depths of EDZs were assessed by the proposed quantitative method, and formation mechanism and evolution of EDZs were discussed. e following conclusions can be drawn: Firstly, according to the in situ testing results, failure phenomena occurred in the testing boreholes and were exposed after excavation such as irregular lamps cake core, splitting, and spalling, showing that geostresses in the Houziyan underground powerhouse belong to the high in situ stress. e relatively large intermediate principal stresses σ2 were subvertical to the axis of the main powerhouse, which directly caused the concentration of tension stress and intensive damage of the surrounding rock mass
Summary
With rapid economic development and increasing huge demand for energy, more and more underground structures such as deep transportation tunnels and mining roadways are being constructed or planned. The excavation dimensions of the main underground powerhouse at the Dagangshan project located in the Dadu River are 226.6 m in length, 30.8 m in width, and 74.3 m in height [4]. E excavation dimensions of the main underground powerhouse at the Lianghekou project constructed on the Yalong River are 275.9 m × 28.4 m × 66.8 m (length × width × height, respectively) [6]. E powerhouse of the Jinping I hydropower station located in the Yalong River was positioned in the high in situ stress zone; the magnitudes of maximum principal stress (σ1) were approximately 20.0–35.7 MPa, the second principal stress (σ2) magnitudes were approximately 10.0–25.0 MPa, and Advances in Civil Engineering the minimum principal stress (σ3) magnitudes were approximately 4.0–12.0 MPa [5]. Recovery after collapse in the regions of β80 diabase dikes in Dagangshan projects took 18 months, which delayed construction and resulted in substantial economic losses [4, 6]. us, the stability of underground caverns subjected to excavation plays a critical role in engineering safety
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