| The Hanjiang-to-Weihe River Diversion Project,the South-to-North Water Diversion Project in the Shaanxi Province,aims at solving the water-shortage problem of the cities located in the Weihe river coast in central Shaanxi area.The Qinling water conveyance tunnel is one of the three main components of the water diversion project,and is the first one to penetrate the Qinling Mountains with the maximum burial depth of nearly 2000 m.The tunnel is 81.77 km in length,and nearly 39 km of which passes through the main ridge zone of the Qinling Mountains.Rockburst hazards are frequently encountered during excavation of the Qinling water conveyance tunnel,characterized by high in-situ stresses,strong excavation disturbance and complex geological conditions.Rockbursts pose serious threat to the safety of personnel and equipment,and have become the bottleneck problem restricting the safe and efficient excavation of the Qinling water conveyance tunnel.Rockburst is a progressive failure of rock mass,and the microseismic monitoring technique can not only capture the microcracks within rock mass,but also calculate the occurrence time,location and released energy of these microseismic events,then,damage of rock mass can be evaluated by analyzing the source parameters of a large number of microseismic events,therefore,the rockburst may be predicted successfully.The microseismic monitoring technique and numerical simulation based on the RFPA code were mainly used in this study.First,the characteristics of rockburst in the Qinling water conveyance tunnel were determined,and the spatial-temporal distribution,seismic sequence and released energy of microseismic events during the evolution of rockbursts are studied.Second,variation of the frequency spectrum of microseismic events during the evolution of rockbursts was acquired,additionally,time-frequency analysis and energy distribution of the rockburst waveforms were studied.Then,failure of structural plane at different scales,which may induce rockbursts was simulated by the RFPA code.Next,the identification and verification method for the unexposed structural plane around tunnel was proposed,and the precursor for rockbursts along a structural plane was found based on the statistical parameter in quantitative seismology.Finally,results of using energy release technologies to control rockbursts in the Qinling water conveyance tunnel were evaluated.This study provides a reference for monitoring,analysis,warning,and control of rockbursts in deep-buried tunnels of the Hanjiang-to-Weihe River Diversion Project or other projects,and the main achievements of this study are listed as follows:(1)A moveable microseismic monitoring system was built considering the layout,geological conditions and construction procedure of the Qinling water conveyance tunnel,and the excavation unloading induced microcracks can be captured by the system in real-time and continuously.The velocity of stress wave was calibrated by the artificial knocking tests,additionally,some common seismic signals were analyzed in both time and frequency domains using Fast Fourier Transform(FFT)to recognize the microseismic events precisely,and improve the accuracy for evaluating the stability of surrounding rock mass and warning of rockbursts.(2)Characteristics of the 298 rockbursts located in K33+873.3 to K36+979.6 of the#3 Qinling water conveyance tunnel were revealed,and the reasons for formation of the high-density rockburst zones were analyzed.The spatial-temporal distribution and evolution of microseismic events,and seismicity sequences during the development of rockbursts were studied based on a large number of microseismicity and rockburst data.The released energy of microseismic events corresponding to rockbursts of different intensities was analyzed qualitatively,which provides a reference for predicting the intensity of rockburst in situ.(3)Variation of the amplitude-frequency and time-frequency of microseismic events during the evolution of two successive rockbursts in the#3 Qinling water conveyance tunnel was studied using FFT and S Transform(ST)to explore the precursor for rockburst in the frequency domain.The 89 rockburst waveforms in the#3 Qinling water conveyance tunnel were classified into time-sustained rockburst waveforms,single-shock rockburst waveforms and multi-shocks waveforms based on their time-frequency characteristics.Energy of the rockburst waveforms at different frequency bands was studied based on the waveforms reconstructed by the Inverse S Transform(1ST).(4)Slip of large-scale structural plane of different mechanical properties and failure of the structural plane consisted of lockers were simulated by the RFPA code,and the associated energy releases were analyzed to study the mechanism,which may induce rockbursts.Failure of the surrounding rock mass was considered as the result caused by the static load and dynamic disturbance,and structural planes with various kinds of attitudes,which located in different parts around tunnel,were modelled by the RFPA dynamic-static version to investigate their influences on the failure of rock mass.(5)Occurrence and intensity of rockburst are heavily influenced by the structural plane,and the evolution and precursor of rockburst along a structural plane are different from that of the strainburst.The unexposed structural plane at the left sidewall of the#4 sub-tunnel of the Qinling water conveyance tunnel was inferred from the abnormal clustering of microseismic events,and was verified by the in-situ photos and ratio of S-wave energy to P-wave energy(ES/EP).The daily event counts and average energy level(A EL)accompanied by the energy index(EI),and density contour of microseismic events were used in company with the SSS theory(stress buildup,stress shadow and stress transfer)to evaluate the characteristics of the three successive intense rockbursts.The b value,the statistical parameter in quantitative seismology,was used as the warning indicator,and the precursor for rockbursts along a structural plane was acquired by analyzing the variation of b value during the evolution of the three successive intense rockbursts.(6)The stress-relief blast and radial stress-relief boreholes were conducted to control rockbursts in the#4 sub-tunnel and#4 Qinling water conveyance tunnel,respectively,and results of using energy release technologies to control rockbursts actively were evaluated through analyzing the distribution of microseismic events and variation of the released energy during the in-situ tests. |
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