Cracking Mechanisms Of Surrounding Rock Mass Induced By Transient Unloading During Excavation Of Deep Buried Caverns

The high-intensity drill-and-blast excavation of deep underground caverns has been need in a large number of great water conservancy and hydropower engineering projects built or being built in alpine valleys of west China region, with the implementation of "West-east Electricity Transmission Project", such as Jinping I, Jinping ?, Xiaowan, Xiluodu and Baihetan etc. Large numbers of stress controlled cracking failures such as spalling, slab cracking, zonal disintegration (layered fracture), and rockburst appear in the excavation processes of deep buried tunnels and powerhouses, for the excavation of underground tunnels changes the original shape of the rock mass. Especially excavation unloading induced sudden rockburst could cause engineering disasters. Cracking mechanism in surrounding rocks of tunnels induced by excavation unloading is investigated via theoretical and numerical analyses, and cracking mechanisms of surrounding rock and their influence factors induced by the quasi-static and transient unloading are discussed, respectively. This study is of great significance for further understanding of the cracking failure mechanism of surrounding rocks, and offers references for the pertinent engineering constructions and disaster prevention and mitigation in underground engineering.The cracking mode and cracking criterion, cracking distribution characteristics and their influencing factors of surrounding rock induced by coupling effect of excavation blasting and transient excavation unloading were studied systemically and deeply. The combined method of theoretical analysis, numerical calculation and engineering verification ware adopted in this paper, the main content and the research results of the thesis are as follows:Cracking criterion and failure criteria of the deep surrounding rockmass are discussed based on stress intensity factor, the fracture toughness and composite crack initiation criterion analysis. Cracking mode and cracking criterion of deep buried surrounding rock under the quasi static excavation condition are analyzed and summarized, stress redistribution of surrounding rock under quasi static condition of deep cavern excavation shows that deep surrounding rock cracking mechanisms is under the compression-shear condition cracking and extension.The cracking mechanism and mode of surrounding rock induced by coupling of blasting load and excavation unloading were studied, cracking calculation model under the coupled action of explosion load and excavation unloading of surrounding rock was established. Using dynamic finite element method (ANSYS/LS—DYNA) equivalent simulation stress change process of surrounding rock during deep underground cavern excavation unloading, the results show that deep underground cavern excavation surrounding rock stress change is a dynamic adjustment process, the local stress state in this process may be in tension or tension-shear rather than merely under compression or shear stress. The cracking mechanism of surrounding rock induced by the transient unloading of excavation under the high in-situ stress state is also not the same with that under the quasi-static state. There are mainly two different cracking mechanisms induced by the transient unloading of excavation:the one is the opening-mode fracture induced by the tensile stress, the other one is tensile-shear or compression-shear mode fracture induced by the transient adjustment of the stress in the surrounding rock.Numerical simulation of cracking evolution has been conducted to compare the fracture processes between loading and unloading.This was done by using the dynamic Rock Failure Process Analysis (RFPA2D) code, and simulate cracking of surrounding rock in circular tunnel under dynamic loading, reveal the cracking parallel to the excavation surface phenomenon caused by deep underground cavern excavation.The cracking mechanisms of surrounding rock and their influence factors induced by the quasi-static and transient unloading are discussed, respectively. The results indicate that the excavation unloading is one of most important reason for the cracking of surrounding rock in the deep buried tunnel. The compression-shear mode is the main failure mode for the quasi-static unloading. Whether the surrounding rock cracks is mainly determined by the initial condition and the parameters of excavation. Compared with the quasi-static release of stress, a dynamic additional stress in surrounding rocks is generated by the transient release of in-situ stress, which leads to enlarging the radial unloading and the circumferential loading effect. And the cracking effect and range of surrounding rocks are further aggravated and increased during this transient process, respectively. Additionally, the tensile stress induced by transient unloading is the main cause for the cracking of surrounding rock. The less the duration of excavation unloading is, the larger the tensile zone and cracking range and range will be. The cracking depth and range increase with increasing the lateral pressure coefficient, and the shape of cracking region appears to the'V notch. In addition, based on the main factors influencing the cracking distribution characteristics of surrounding rock such as initial stress state of the rock, the initial crack and excavation method (unloading duration) is analyzed, further understand the cracking mechanism of surrounding rock and crack distribution, from the standpoint of fracture mechanics and rock dynamic mechanics to provide theory basis for the cause of the damage of the surrounding rock during excavation process of underground caverns.By numerical computing, cracking of surrounding rock caused by the transient unloading excavation under high geos-tress condition is studied, and surrounding rock cracking effects of transient unloading stress induced by the excavation for arch crown, anchor beam and straight wall zone of main powerhouse is analyzed. By different excavation sequence on the influence of the surrounding rock cracking, especially further explore the mechanics of large scale rock falling of upper arch and surrounding rock cracking of rock anchor beam area of main powerhouse. By analysis the influence of different excavation sequence for the surrounding rock cracking, determine the cracking area and the distribution characteristics of surrounding rock induced by excavation.Finally, through analyses of the Jinping I, Jinping II and Pubugou power station typical deep buried underground caverns excavation damage phenomenon, reveals the mechanics of damage. According to the engineering example illustrate bolting shotcrete support and consolidation grouting is the most common and effective method to keep the stability of surrounding rock, especially the prestressed anchor is an effective measure to prevent type tension cracking.