Abstract
During excavation of deep rock, the release of strain energy plays an important role in geologic hazards caused by excavation. However, in the previous studies, the influence of transient unloading of in situ stress caused by blasting excavation has been ignored, and the blasting excavation of a tunnel is regarded as a single blast process. In this paper, the dynamic adjustment process of strain energy and the energy storage limit of surrounding rocks caused by transient unloading of in situ stress under elastic conditions were firstly analyzed. Then, the brittle-ductile-plastic transition model based on the Hoek–Brown strength criterion was simulated in FLAC3D. Finally, the dynamic release process of strain energy of surrounding rocks caused by multiple unloading disturbances was analyzed during the excavation of 2# Underground Laboratory of Jinping II Hydropower Station employing the newly proposed index, energy release coefficient (ERC). Results show that the strain energy of surrounding rock masses firstly decreases, then increases, next reduces, and finally stabilizes under the transient unloading of in situ stress. In the process of dynamic change of strain energy, when the strain energy exceeds its storage limit, a large amount of strain energy will be released and thus will lead to damage of the surrounding rock masses. Because the cut holes and the first circle of breaking holes are far away from the final excavation boundary, the unloading disturbance to the strain energy of surrounding rock masses is small. Furthermore, the energy release of surrounding rock masses is mainly caused by the unloading of the last circle of breaking holes and perimeter holes, and the closer to the final excavation boundary, the more intense the energy release.
Highlights
In the western region of China, the high in situ stress induced by the large buried depth becomes one of the typical geological features of underground engineering, which is a threat to the safety and stability of surrounding rocks of a deep tunnel after excavation. e environment of high in situ stress will lead to the storage of high strain energy in rock masses
At 1.2 m away from the final excavation boundary, the energy release is mainly caused by the excavation unloading of ms7, ms9, and ms11, among which the energy release of surrounding rock masses caused by the unloading of ms9 is the largest and the fastest. e energy release of the rock mass unit at 2.2 m away from the final excavation boundary is mainly caused by the unloading of ms9 and ms11, among which the unloading of ms11 causes the largest and the fastest energy release of the surrounding rock masses
When the accumulated strain energy exceeds the energy storage limit, it will cause a large amount of strain energy release and damage to the rock masses
Summary
In the western region of China, the high in situ stress induced by the large buried depth becomes one of the typical geological features of underground engineering, which is a threat to the safety and stability of surrounding rocks of a deep tunnel after excavation. e environment of high in situ stress will lead to the storage of high strain energy in rock masses. After successfully describing the postpeak mechanical properties of brittle-ductile-plastic transformation of Jinping marble, the parameters and constitutive model mentioned above were employed to simulate the energy variation process of surrounding rock masses induced by transient unloading of in situ stress during the blasting excavation of central pilot tunnel (7 m × 7 m, gate shape) in the Jinping Underground Laboratory. At 1.2 m away from the final excavation boundary, the energy release is mainly caused by the excavation unloading of ms, ms9, and ms, among which the energy release of surrounding rock masses caused by the unloading of ms is the largest and the fastest. e energy release of the rock mass unit at 2.2 m away from the final excavation boundary is mainly caused by the unloading of ms and ms, among which the unloading of ms causes the largest and the fastest energy release of the surrounding rock masses
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