Biao Li

Perturbation propagation and cumulative damage to the rock surrounding adjacent caverns pose major challenges to construction safety during the excavation of large underground powerhouses. This study investigates these processes at a hydropower project in southwestern China. An ESG high-precision microseismic monitoring system was deployed to examine the spatiotemporal evolution of microseismicity, fracture mechanisms, and their relationship with rock mass damage in adjacent busbar tunnels during excavation of Lifts VI and VII of the main powerhouse. Comparisons between microseismic and conventional monitoring revealed the spatiotemporal relationships among microseismic activity, accelerating deformation, and declining anchor cable loads around the busbar tunnels. Numerical simulations of cavern excavation, considering configurations with and without busbar tunnels, were conducted to assess how the free surfaces of the busbar tunnels affect the surrounding stress and displacement fields, and the numerical results were compared with both monitoring datasets to corroborate the findings. The results show that (1) microseismic activity evolves in response to excavation intensity through three stages: steady accumulation, accelerated activity, and decay; (2) microseismic events cluster at the intersections between the main powerhouse and busbar tunnels, and as excavation progresses the fracture mechanisms remain dominated by tensile and mixed tensile–shear events with no significant change, while the concurrent increase in energy and moment magnitude is confined to the stage of macroscopic damage; (3) temporal variations in microseismic activity rate track changes in rock deformation rate and anchor cable load, while microseismic clusters coincide spatially with zones of rock deformation and failure and declining anchor cable loads; and (4) the free surfaces of the busbar tunnels alter the stress and displacement distributions at cavern intersections. Predicted displacement concentrations, zones of high radiated seismic energy density, and observed damage locations agree well spatially, jointly indicating where rock deformation and failure are concentrated. These findings can provide a basis for excavation control and support design at similar cavern intersections.

Large deformation is one of the most critical hazards in tunnels excavated under complex geological conditions. It often causes significant economic losses and threatens construction safety. For layered soft rock tunnels subjected to high in situ stress, the deformation and failure mechanisms are largely governed by the bedding dip angle. To clarify these mechanisms and optimize the corresponding construction control measures, this study investigates a carbonaceous slate section of a railway tunnel in the Western Sichuan Plateau. Field monitoring and FLAC3D numerical modelling are coupled. The influence of the bedding dip angle on the plastic-zone evolution and the failure modes of the surrounding rock is analysed. The micro-bench, three-bench, and reserved core soil methods, together with the rock bolt length, are comparatively evaluated. On this basis, a differentiated reinforcement strategy is proposed for bedding-induced asymmetric loading. The results indicate that: (1) The bedding dip angle governs the failure mode of the surrounding rock. Under the micro-bench method, the plastic zone in subvertically bedded rock masses exhibits a quasi-symmetrical distribution along the normal direction of the bedding planes. The sidewalls predominantly undergo flexural failure. In contrast, under bedding-induced asymmetric loading, the plastic zone concentrates at the left springline and right shoulder. An asymmetric composite failure mode is formed, characterized by shallow flexural–tensile cracking and deep-seated interlayer shear. (2) Under the subvertical bedding condition (89°), the reserved core soil method mitigates the excavation-induced unloading disturbance most effectively. It achieves the lowest peak stress and the smallest tunnel convergence, which is 15.7% and 33.0% lower than those of the micro-bench and three-bench methods, respectively. Its plastic zone reaches full numerical convergence. The reserved core soil method is therefore identified as the optimal excavation Scenario under this condition. (3) The rock bolt length exhibits a threshold effect on deformation control. The most substantial improvement occurs when the bolt length is increased from 4 m to 6 m, beyond which the benefit tends to plateau. A bolt length of 6 m is therefore recommended as the best-performing Scenario among the tested values (4, 6, 8, and 10 m) for the investigated geological and support conditions. For surrounding rock subjected to bedding-induced asymmetric loading, a differentiated reinforcement strategy targeting the vulnerable zones reduces the maximum deformation by 18.8% and 28.3% compared with the uniform reinforcement Scenario and the baseline Scenario, respectively. These findings provide practical insights into excavation-method selection and support optimization for layered soft rock tunnels under similar conditions.

To address the safety concerns of vault surrounding rock damage induced by rock blocks under high-stress conditions of underground powerhouses, a microseismic (MS) monitoring system was established. This study investigates the spatiotemporal evolution of MS events in the vault, the associated fracture patterns, and the correlation between MS events and surrounding rock damage during the excavation of the underground powerhouse. The spatial evolution patterns of the stress field, displacement field, and fracture field during the process of vault surrounding rock damage were revealed. The research results indicate that: (1) The presence of fault and free-surface cut rock blocks at the vault of the main powerhouse enhances the disturbance effect of excavation on the surrounding rock. (2) MS events exhibit localized clustering characteristics in space, with certain clusters highly coinciding with the location of the vault rock blocks. The MS activity characteristics in the regional rock mass, can serve as a qualitative indicator for early warning of surrounding rock deformation. (3) The combination of MS monitoring and numerical simulation effectively identifies surrounding rock damage information in the rock block areas of underground powerhouses. The research findings can serve as a reference for analysing surrounding rock damage and stability in similar engineering projects.

Highly accurate microseismic (MS) localization is the basis for rock damage assessment and disaster warning. The engineering background noise mixed in the MS signal seriously affects the subsequent analysis of the MS signal. A noise reduction method of singular spectral analysis-complementary ensemble empirical mode decomposition-wavelet threshold (CEEMD-WT-SSA) is proposed. The complementary ensemble empirical mode decomposition (CEEMD), complementary ensemble empirical mode decomposition-wavelet threshold (CEEMD-WT) and the proposed method are respectively used for denoise the noisy Ricker wavelet. The signal-to-noise ratio of the denoised signal by the proposed method is 56.77% and 37.88% higher than that of CEEMD and CEEMD-WT methods. Moreover, an adaptive artificial bee colony (ABC) algorithm is applied for MS source location. The time to quantile difference is introduced as the objective function. The blast positioning test results prove that the proposed method improves the positioning accuracy by 44.12% and 47.64% particle swarm optimization (PSO) algorithm and simulated annealing particle swarm optimization (SA-PSO) algorithm, respectively. The MS positions of underground caverns reveal that the calculated cluster of MS events using the adaptive ABC algorithm are more concentrated at the structural plane and appearance deformation failure location and in good agreement with field survey and routine monitoring data.

A microseismic (MS) monitoring system was established, and numerical modeling was performed using Fast Lagrangian Analysis of Continua in 3 Dimensions (FLAC3D) to examine the evolution characteristics of MS events during the excavation of underground caverns under high geostress. Specifically, the spatial and temporal damage characteristics of the rock mass, the dynamic relationship between the evolution of MS events, the site construction conditions, and the geological structures under high geostress were also analyzed. In addition, a three-dimensional numerical model of underground caverns was built to demonstrate the deformation characteristics of the rock mass. As a result, the characteristics of a large number of MS events, multiple concentration zones of MS events, and long periods of stress adjustment were discovered in underground caverns under high geostress. It was found that the primary cause of MS events was site blasting construction. In particular, the distribution of the MS events was strongly correlated with the location of the site blasting construction, and the frequency of the MS events was positively correlated with it. The structural plane is a discontinuous plane with very low or no tensile strength, and its presence also increased the number of MS events and raised the possibility of the surrounding rock becoming unstable. Moreover, the MS monitoring data agreed with the numerical modeling results, which can be useful in estimating damage to underground caverns under high geostress and predicting disasters.

As the hydropower development strategies of China continue to be implemented, a host of large hydropower projects have been completed or are being constructed in southwest China. During construction of the Jinchuan hydropower station, this study examined the stability of the surrounding rock during the excavation and unloading of hydraulic tunnels under demanding geological conditions. Microseismic (MS) monitoring technology was employed to monitor the deformation and failure of the surrounding rock online and in real time, based on engineering geological data and site surveys. To analyze the stability of the surrounding rock in the spillway tunnel and to study the temporal and spatial evolution characteristics of MS events, source parameter analysis and numerical modeling were performed. The 3D finite-difference numerical modeling software FLAC3D was used to simulate the mechanical response of the surrounding rock during the excavation and unloading of the spillway tunnel and the diversion tunnel. The numerical modeling results were compared with the monitoring results and site surveys to determine the failure mechanisms of the surrounding rock during the construction and unloading of the hydraulic tunnels. The research results can serve as a guide for studying the stability of the surrounding rock in similar hydraulic tunnels.