Abstract
Based on the existing Canadian ESG microseismic monitoring system, a mobile microseismic monitoring system for a soft rock tunnel has been successfully constructed through continuous exploration and improvement to study the large-scale nucleation and development of microfractures in the soft rock of the Yangshan Tunnel. All-weather, continuous real-time monitoring is conducted while the tunnel is excavated through drilling and blasting, and the waveform characteristics of microseismic events are analysed. Through the recorded microseismic monitoring data, the variation characteristics of various parameters (e.g., the temporal, spatial, and magnitude distributions of the microseismic events, the frequency of microseismic events, and the microseismic event density and energy) are separately studied during the process of large-scale deformation instability and failure of the soft rock tunnel. The relationship between the deterioration of the rock mass and the microseismic activity during this failure process is consequently discussed. The research results show that a microseismic monitoring system can be used to detect precursors; namely, the microseismic event frequency and energy both will appear “lull” and “active” periods during the whole failure process of soft rock tunnel. Two peaks are observed during the evolution of failure. When the second peak occurs, it is accompanied by the destruction of the surrounding rock. The extent and strength of the damage within the surrounding rock can be delineated by the spatial, temporal, and magnitude distributions of the microseismic events and a microseismic event density nephogram. The results of microseismic analysis confirm that a microseismic monitoring system can be used to monitor the large-scale deformation and failure processes of a soft rock tunnel and provide early warning for on-site construction workers to ensure the smooth development of the project.
Highlights
Rock is an anisotropic and nonhomogeneous material
Based on a combination of the temporal, spatial, and magnitude distributions of microseismic events and the microseismic event density nephogram, the timing, intensity, and extent of large-scale deformation and failure within the soft rock tunnel can be noted with the temporal variations in the microseismic events and microseismic energy. e findings can provide strong guarantees for safe tunnel construction and production management
On November 11-12, the number of microseismic events increased rapidly, a total of 19 microseisms occurred within the surrounding rock within the two-day period, the number of daily microseismic events was greater than that of the previous period, the stress concentration in the rock mass was correspondingly obvious, and an area of large-scale deformation appeared within the rock mass, and this indicated the occurrence of soft rock failure. e number of microseismic events on November 13 showed a slight decline, but it was still relatively high, indicating that the stresses in the surrounding rock stress were not fully released and that the possibility of surrounding rock failure was still high
Summary
Rock is an anisotropic and nonhomogeneous material. When cracks within rock germinate, expand, and penetrate, the internal accumulation strain energy is released in the form of waves, resulting in microseismic events [1,2,3,4,5]. Microseismic monitoring technology can be employed to collect and analyse microseismic events, the data from which can be used to invert the time, location, and magnitude of microseismic event and predict the possibility, energy, and position of large-scale deformation and failure of soft rock tunnels [6,7,8]. Based on a combination of the temporal, spatial, and magnitude distributions of microseismic events and the microseismic event density nephogram, the timing, intensity, and extent of large-scale deformation and failure within the soft rock tunnel can be noted with the temporal variations in the microseismic events and microseismic energy. It exhibits two major characteristics when the loess tunnel is constructed. e first characteristic is that, following the excavation of the loess tunnel, the concentration of stress on the vault and the part is excessively large. ese stresses resulted in substantial settling of the vault and the instability of the tunnel structure, leading to the occurrence of landslides. e second characteristic is that the collapsible loess basement hosting the tunnel is destroyed by water, leading to a decline in the foundation bearing capacity
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