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Excavation‑induced structural deterioration of rock masses at different depths

Zhang R., Xie H. P., Ren Li, Deng H. I., Gao M. Z., Feng G., Zhang Z. T., LI X. P., Tan Q.

Archives of Civil and Mechanical Engineering

2022

Vol. 22, no. 2

art. no. e81, 1--15

The excavation-induced deterioration of rock mass quality and integrity may significantly affect the stability of deep underground spaces. However, the influence of the burial depth on excavation-induced rock mass structural deterioration remains unclear. To address this issue, in the Jinping II auxiliary tunnels, borehole acoustic wave and digital panoramic borehole imaging tests were conducted at five depths, and the surrounding rock mass structure deterioration was comprehensively quantified at these depths. The results show that the wave curve significantly varies with increasing depth, and the deeper the tunnel section is, the more severe the rock damage is, the larger the excavation damage zone is, and the more complex the generated fracture network is. An excavation-damaged zone and excavation-disturbed zone (EdZ) were found in the rock masses at shallow depths, while a highly damaged zone (HDZ) and EdZ were observed at greater depths. Further investigation demonstrates that the formation pattern of excavation-damaged zones (EDZs) at deep depths follows a stress-concentration controlled mode, and at very shallow depths, horizontal unloading plays a dominant role in the EDZ formation process (yielding a pure unloading-controlled mode), while at intermediate depths, a mixed EDZ formation mode is observed.

Energy characteristics and micro cracking behaviors of deep slate rock under triaxial loadings

Zhang Anlin, Feng Tao, Jiang Liangwen, Wang Dong, Wang Zhewei, Zhang Ru, Feng Gan, Zhang Zhilong, Deng Jianhui, Ren Li

Acta Geophysica

2022

Vol. 70, no 4

1457--1472

Sichuan-Tibet railway tunnels are generally long and deep, and some of them pass through layered slate, which is prone to failure under high in situ stress. Insight into the mechanical behaviors of deep slate under different stress environments is critical to the safety of engineering practices in this area. To this end, we studied the energy characteristics and microcracking behaviors of slate samples under different triaxial loadings. The results showed that the confining pressure increased the strength, elastic modulus, Poisson’s ratio and crack characteristic stresses of the studied slate. As the confining pressure increased, the storage of elastic strain energy during the prepeak stress stage increased, and its release during the postpeak stress stage was inhibited. Additionally, we obtained the slate failure precursors by analyzing the acoustic emission (AE) evolution characteristics, i.e., “a sudden increase in AE metrics, followed by a quiet period, and then a sudden increase again” in terms of the AE count rate and AE energy rate after obvious expansion of the sample. Then, we further observed that with an increase in confining pressure, a gradual transition of the failure pattern from compressive shear to tensile shear occurred. Finally, we discussed the influence mechanisms of confining pressure on the slate failure behaviors and concluded that the change in the effective shear stress along the foliation planes played an important role in the failure mode transition.

Infuence of CO2–water–rock interactions on the fracture properties of sandstone from the Triassic Xujiahe Formation, Sichuan Basin

Li Hongfei, Xie Lingzhi, Ren Li, He Bo, Liu Yang, Liu Jun

Acta Geophysica

2021

Vol. 69, no. 1

135--147

Carbon dioxide (CO2) storage in deep saline aquifers has been lauded as one of the most efective techniques to mitigate greenhouse efects globally. Nevertheless, despite many investigations, clarifying the infuence of CO2–water–rock inter actions on the fracture characteristics of sandstone remains a challenge. In this work, the fracture properties of sandstone collected from the Triassic Xujiahe Formation are systematically studied in tests simulating CO2 sequestration. The results indicate that the water–rock interactions occur in a system of sandstone and CO2 solution. Due to the interactions, the poros ity of sandstone specimens slightly increases from 8.24 to 8.45% when immersed in CO2 solution and from 8.20 to 8.40% in pure water after 28 days. In addition, the parameters of fracture toughness, tensile strength, uniaxial compressive strength and elastic modulus are reduced by 24.12%, 27.16%, 31.78% and 33.21% after immersion in pure water, while they are reduced by 24.05%, 29.72%, 30.75% and 25.79% after immersion in CO2 solution, respectively. These results suggest that the mechanical properties of the Xujiahe sandstone deteriorate after soaking. The results also show that the critical fracture energy of sandstone specimens after immersion in the CO2 solution is 10.4% lower than that in pure water and 24.1% lower than that under natural drying conditions. These research results have great signifcance for understanding the dissolution processes during CO2 sequestration and their infuence on the fracture properties of sandstone, which may be theoretically instructive for CO2 storage in the Xujiahe Formation in the Sichuan Basin.