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The Dagangshan high arch dam , located in the area with complicated geological conditions and extremely high seismic intensity in Southwest China, has received substantial attention worldwide. Before the impoundment of the reservoir, continuous deep transverse cracks were observed in the low-elevation galleries of the arch dam. To investigate crack formation and propagation in the galleries of the high arch dam, microseismic monitoring and 3D finite element analysis of the galleries were performed during the filling process. The microseismic monitoring system was installed in the foundation gallery and the drainage gallery to detect microcracks around the galleries of the arch dam. The background stress field for crack formation and propagation in the galleries was studied based on 3D finite element modelling . The results indicate that the self-weight of high arch dam produces excessive circumferential tensile stresses at the arch crown of the galleries near the dam heel before impoundment, which is the cause of crack formation in the galleries of the Dagangshan high arch dam. As the impounded water level rises, the tensile stresses and the microseismicity in the galleries near the arch dam heel transfer to the galleries close to the arch dam toe, and the cracks in the galleries near the dam heel would not propagate further but be closed. However, cracking may occur in the gallery close to the dam toe if its microseismicity continues increasing. The conclusions have significant implications for reinforcement and operation of the galleries in similar high concrete arch dams.

Fast and efficient determination of the mechanical parameters of surrounding rock masses is vitally important to the calculation and evaluation of the stability of surrounding rock masses in tunnel engineering. In this paper, a displacement back-analysis (DBA) model is proposed to identify the mechanical parameters based on support vector regression (SVR) optimized by multi-strategy artificial fish swarm algorithm (MAFSA). The MAFSA adopts the differential evolution strategy, the particle swarm optimization strategy, the adaptive step size and phased vision strategy on the basis of artificial fish swarm algorithm (AFSA) to enhance the global search capability and improve convergence speed and optimization accuracy. Then, the kernel width and the penalty parameter of SVR are optimized by MAFSA, forming into MAFSA-SVR. Meanwhile, the training and testing samples for MAFSA-SVR are constructed by orthogonal design and forward calculation by FLAC3D code. Finally, the DBA model is established based on MAFSA-SVR and applied to the mechanical parameter inversion of surrounding rock masses in the Heshi tunnel with the following conclusion: the relative errors of all the mechanical parameters are less than 8% between the inversed values of the DBA model based on MAFSA-SVR and the actual values. The method proposed in this paper could provide an efficient tool for the mechanical parameter inversion of the tunnel surrounding rock masses.

The stability of excavated high-steep rock slopes is most significant in the long-term safety of large-scale hydropower projects. Dagangshan rock slope had an unloading fractured zone (XL-316) and a fault (f231) during excavation, which resulted in a high risk of slope instability. Anti-shear tunnels were constructed for enhancing the overall shear resistance and reinforcing the slope at six different elevations. The microseismic technique was employed to investigate microseismic activities during excavation and after reinforcement. An integrated method of microseismic energy density and the magnitude-frequency relation based on microseismic monitoring was proposed to analyse the stability and reinforcement effect of the slope. During excavation, a large number of microseismic events took place with few low-magnitude events and numerous high-magnitude events. The aggravated damage occurred in the major discontinuities during excavation. The number of low-magnitude microseismic events increased after reinforcement, and the b value obviously increased, which means the slope stability was substantially enhanced. Microseismic activities were evidently reduced, and the accumulated microseismic energy increased slowly. The stresses were transferred towards the edge of the anti-shear tunnel, and a small number of low-energy events occurred. The targeted reinforcement has served well for its purpose and improves the mechanical performance of the discontinuities. The application of the proposed method was verified by 3D RFPA-Centrifuge software. The proposed method can be considered as an effective way to evaluate the reinforcement effect and stability of high-steep rock slopes.

Underground storage in unlined caverns is of great significance for storing energy resources. Construction of underground storage caverns is an extremely complex process, involving extensive multi-bench excavation and strong unloading. Excavation-induced damage of surrounding rock masses may lead to instability of underground storage caverns. The aim of this paper is to put forward a method by integrating numerical simulation and microseismic monitoring for evaluation of cavern stability. A novel numerical method called Continuous-Discontinuous Element Method (CDEM) is applied to simulate micro-cracks under excavation-induced unloading conditions. Meanwhile, a microseismic (MS) monitoring system is employed to monitor real-time MS events during construction of storage caverns. Numerical results are validated using the monitoring data from the MS monitoring system. The integrated method is proved to be successful in capturing micro-cracks in underground storage caverns. Local instability, potential unstable zones and micro-crack evolution are analyzed, and cracking mechanisms are also discussed. (C) 2016 Elsevier Ltd. All rights reserved.

The right bank slope of the Dagangshan hydropower station has undergone large deformation during excavation, which may continue to increase and cause slope instability and failure, threatening dam construction and safe operation. This study analysed the deformation and instability of this discontinuity-controlled rock slope by three-dimensional discontinuous deformation analysis (3D-DDA). A microseismic monitoring technique was adopted to capture the spatial and temporal distributions of microseismic events. The unknown geological discontinuities and the potential slip surface inside the slope were determined. By combining monitoring data with the geological information, the 3D-DDA model was established to analyse the 3D deformation characteristics of the slope. In addition, the slope deformation and failure under various conditions, such as over excavations, as well as various joint penetrations, joint dip directions, and friction angles were further investigated. It was found that the simulation of the critical sliding state of blocks by 3D-DDA is effective after improvement of the critical sliding model of joint contact between blocks, and the computational results showed good agreement with monitoring results. Slope deformation is caused by the accumulation of micro-crack damage in deep rock masses, and it is closely related to its internal and external conditions. Furthermore, the failure modes and mechanisms of the slope with 3D topography are presented, which may be helpful for the design of a slope reinforcement scheme.

The TBM tunneling at the Jinping II hydropower station in Southwest China has received extensive concerns around the world because of its large engineering scale and the high rockburst risks faced in the tunnel advancement. The associated energy changes of rockbursts and control method for safe TBM tunneling are to be further investigated. A movable microseismic (MS) monitoring system was established to capture the MS events and rockbursts when the TBM excavated the headrace tunnel #1 at the Jinping II hydropower station. The spatial and temporal patterns of the energy changes in the tunnel rock masses were studied. Meanwhile, the evolution of a rockburst encountered in front of the TBM excavation face was revealed, and the performance of the top pilot tunnel method on the reduction of the rockburst risks in the headrace tunnel #1 was evaluated based on the energy changes of the surrounding rock masses. It can be concluded that energy accumulation and energy release firstly occurred in the surrounding rock masses at the southern end of the top pilot tunnel section of the headrace tunnel #1. Then, energy transference of the rock masses took place from the southern end to northwest of the top pilot tunnel giving rise to the occurrence of a moderate rockburst about 30 m in front of the tunnel. However, no rockbursts appeared when the TBM excavated through the top pilot tunnel section of the headrace tunnel #1. Therefore, the top pilot tunnel method really works in reducing the risks of rockbursts during the TBM tunneling in deep tunnels.

Rock mass is known to exchange energy with the external environment when it is under load until it fails, a destabilization phenomenon driven by energy. In order to investigate the energy evolution of high rock slope and its relation with rock mass failure of the slope during slope excavation and after reinforcement, a parameter called microseismic energy density was proposed and the relationship between microseismic energy density and the stress and strain of rock mass was discussed. Taking right bank slope of the Dagangshan Hydropower station as an example, the microseismic energy release and energy transference of rock mass during slope excavation and after reinforcement were revealed. These lead to conclusions: Firstly, not only the distribution and intensity of microseismicity but also the potential dangerous area of the slope can be determined synthetically by microseismic energy density. Secondly, the distribution of high energy release zones is closely related to the locations of the deep unloading fracture zones XL316-1 and the fault f(231) moderately dipping out of the slope. Finally, the microseismic event rate and energy density decreased significantly after reinforcing, and the relative high energy release zones transferred to the edge of anti-shear tunnels and low-threat faults, dikes in the deeper area of the slope, so its mechanical behavior was obviously improved. Conclusions can provide references for developing energy criterion and the prediction method of high rock slope failure.