| With the rapid development in our economy,the scale of infrastructure construction continues to expand.Especially,in the southwest region where geological environment is complex,its development level and construction scale are constantly increasing,and the excavation of many large-scale underground caverns is inevitable.The excavation of the rock mass will destroy the original mechanical equilibrium state of the rock mass,and the stress of the disturbed rock mass will be redistributed,which will form a local stress concentration area,causing large deformation of the surrounding rock or inducing a series of geological disasters,such as collapse,rockburst,etc.Rockburst is a special failure mode in the process of excavation under high stress environment in brittle rock area.The strong energy release and surrounding rock damage caused by the rockburst pose a serious threat to the construction safety and seriously affect the construction progress on site.The existence of structural plane has a great influence on the redistribution of stress in surrounding rock.Strain-structure rockburst process has certain characteristics and prediction methods have their own unique laws.At present,the monitoring and early warning of rockbursts has become the most concerned topic in the engineering construction in the southwest region.Successfully solving this problem or in-depth research on this problem is very important for the implementation of the national strategy in the southwest region.The construction area of Shuangjiangkou hydropower station has complex geological structure,high in-situ stress,and frequent rockbursts in the deep-buried section,which seriously affects the construction safety and construction schedule.Therefore,it is of great theoretical value and practical significance to study the rockburst mechanism and early warning of the access tunnel in the main powerhouse of the Shuangjiangkou hydropower station under complex geological conditions.This article focuses on the key scientific problem of "Deep-buried tunnel strain-structure rockburst process and early warning methods under the influence of excavation disturbance",and the access tunnel in the main powerhouse of the Shuangjiangkou hydropower station is taken as the engineering background to carry out rockburst monitoring early warning and numerical simulation test in the process of excavation.Through the microseismic monitoring results,the occurrence law of strain-structure rockburst disaster and the corresponding microseismic activity characteristics were revealed,and the type of rockburst sequence of the tunnel was determined.Combining microseismic monitoring and earthquake prediction theory,a new rockburst early warning index was proposed.Wavelet packet and HHT analysis technology were applied to rockburst early warning.The evolution law of the frequency spectrum of micro-seismic activity during the rockburst occurrence of the access tunnel in the main powerhouse was studied,and the frequency precursor information of the rockburst was determined.Through RFPA3D numerical simulation software,in-depth study of the distribution and transfer law of the maximum principal stress of the surrounding rock of the tunnel under the effect of gradual excavation and unloading,and the characteristic of the rockburst sequence of the access tunnel in the main powerhouse was explained.The research results revealed the mechanism of strain-structure rockburst.The formation and occurrence conditions of strain-structure rockburst during the tunnel excavation process were summarized.The research results had a certain scientific theoretical value and practical engineering significance for underground engineering rockburst disaster early warning and prevention under the high stress and complex geological conditions in the southwest region.The major achievements are listed as follows:(1)Based on the access tunnel in the main powerhouse of the Shuangjiangkou hydropower station,combining the mechanism of microseismic early warning and technical feasibility,the complex construction environment and engineering geological conditions on site were analyzed,a suitable microseismic monitoring scheme was formulated and a unique and applicable 6-channel microseismic monitoring system was constructed for the tunnel.For the complex on-site construction environment,the fast Fourier transform was used to convert and identify the on-site signal waveform.Through the real-time collection of microcrack information data,the tunnel rockbursts can be monitored continuously for 24 hours and the dynamic information related to the activities within surrounding rock can be obtained.The construction of the microseismic monitoring system for the access tunnel in the main powerhouse of the Shuangjiangkou hydropower station had laid the foundation for real-time monitoring and accurate prediction of rockburst.(2)Based on the microseismic activity rate and released energy time series curves,it was determined that the rockburst sequence type of the access tunnel in the main powerhouse was foreshock,main shock and aftershock.The change laws of source parameters including apparent stress,apparent volume,energy index and Es/Ep values with time were analyzed,the activity characteristics within surrounding rock during rockburst were revealed and the precursor information of rockbursts was determined.Based on the daily event number N and b,a new rockburst early-warning index was proposed,which can better predict the rockburst risk and qualitatively analyze the internal activity law within surrounding rock.The results showed that the sharp decrease of energy index and the rapid increase of cumulative apparent volume indicated the deterioration of surrounding rock,which was the early warning period of rockbursts.The change characteristics of Es/Ep values revealed that the rockburst process undergoed the transformation process of compression shear damage,tension and shear mixed damage,and tensile damage.When the lgN/b value was relatively large,the risk of rockbursts was greater,and the size of the lgN/b value had a good corresponding relationship with the rockburst intensity.(3)The main frequency values of 241 microseismic events were extracted by Fast Fourier transform.Then,the change characteristics of the main frequency values over time during the occurrence of rockbursts were analyzed to reveal the activity characteristics within the surrounding rock.The wavelet packet analysis technology was used to convert the waveforms of 241 microseismic events to extract the main frequency band information.An early warning model of rockburst frequency spectrum of the access tunnel in the main powerhouse was first established.The model can reflect the change characteristics of the energy distribution of each frequency band in the entire signal frequency band over time,and can more accurately predict rockburst.The HHT instantaneous frequency conversion technology was introduced to convert and analyze the waveforms monitored during the time period near the rockburst,revealing that the instantaneous frequency distribution range of the microseismic event waveform before the rockburst has changed from wide to narrow and the frequency has a significant decreasing trend.By using the spectrum information of a single signal,the effective precursor information of rockburst was extracted,which can be used for early warning model and early warning analysis,and provided an important reference for the judgment of the microseismic activity state witnin the surrounding rock.(4)In this paper,by using the real failure process analysis software RFPA3D,the process of the initiation,development and expansion of the cracks in the surrounding rock of the tunnel section under high stress environment was simulated.According to the damage characteristics of the surrounding rock excavated in the early stage of the tunnel,the law that can predict the characteristics of the structural plane was revealed.The rockburst mechanism of the strain-structure rockburst was explored.Based on the stress field distribution characteristics and evolution law of tunnel surrounding rock under the combined influence of excavation and unloading and structural plane,the prone rockburst areas were identified and delineated.The numerical simulation results,field damage characteristics and microseismic monitoring results were compared and analyzed,which well explained the reason why the field rockburst type was foreshock,main shock and aftershock.It can provide certain scientific basis for the prediction of stronger rockburst. |
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