| Rockburst is a dynamic failure phenomenon of deep buried underground engineering induced by the under construction disturbance and other external loads.The mechanism of rockburst has not been unified till now just for the complex influencing factors.When the intensity of rockburst reaches a certain level,it directly threatens the safety of workers in site and restricts the construction progress.Therefore,monitoring and early warning for the rockburst is one of the direct and effective measures to ensure the safety of construction.To study the complex rockburst phenomenon that occurred in the deep buried underground caverns of Huanggou Pumped-storage Power Station in Heilongjiang province,it is focused on the identification of precursors,rockburst mechanism,monitoring and early warning of the evolution process of rockburst based on the comprehensive methods including geological survey,field monitoring,laboratory experiment,theoretical analysis,numerical simulation and engineering application verification.It is resulted in the basic geological and mechanical characteristics of rockburst of the deep buried underground caverns,the field failure characteristics of rockburst,the microseismic and electromagnetic radiation activities during the evolution of rockburst,the numerical simulation method of surrounding rock damage process based on RFPA strength reduction method,and the rockburst mechanism.The multi-index early warning method based on microseismic monitoring method and electromagnetic radiation monitoring method is proposed and the effect is verified.The research results can provide reference for the study of rockburst mechanism,monitoring and early warning method during the excavation of deep buried underground caverns.The main results are as follows:(1)Based on the field investigation of the basic geological conditions and the statistical analysis of rockburst characteristics,it is analyzed from the perspective of engineering geology that the included angle between the dominant trend of the surrounding rock structural plane and the axial direction of the caverns is small,resulting in an unfavorable combination for the surrounding rock stability in space.At the same time,the local faults in the powerhouse and the main transformer room cut each other and the stress concentration area gradually formed during the construction disturbance.The weak geological structure,the unfavorable spatial combination of the surrounding rock of the caverns,and the local stress concentration conditions provide the geological and mechanical basis for the occurrence of rockburst.The spatiotemporal distribution of the rockburst is mainly characterized by the delayed occurrence time and the wide occurrence concentrated in the vault or arch shoulder.The rockburst intensity is mainly mild to moderate.(2)The law of microseismic activities during the evolution of rockburst has been carried out for 4 months based on the microseismic monitoring method.The occurrence of microseismic events is greatly affected by the construction disturbance and is positively correlated.The spatiotemporal evolution of microseismic events is consistent with the influence area of the excavation.The clustering area of the microseismic events and the area of energy loss are all controlled by the influence of weak geological structures such as faults.The microseismic events appear also along local faults.The combination of external loads such as construction disturbance and weak geological structure are the fundamental reasons for the occurrence of microseismic events during the evolution of rockburst.The energy and frequency of microseismic events increase suddenly before the rockburst,while the cumulative apparent volume and energy index increases and drops suddenly,respectively.The phenomenon can be regarded as the rockburst precursors.(3)The research of the law of electromagnetic radiation activities during the evolution of rockburst was carried out based on the electromagnetic radiation method.Firstly,through the census of the background electromagnetic radiation distribution of the main caverns such as the upper drainage gallery,the middle drainage gallery,the lower drainage gallery and the underground powerhouse,it is revealed that the values of the electromagnetic radiation in the project area is lower in the upper/lower drainage gallery and relatively higher in the middle drainage gallery.The dynamic-continuous monitoring method is adopted for the frequent occurrence area of rockburst and the results show that the spatiotemporal evolution of the electromagnetic radiation intensity is basically consistent with the number of electromagnetic pulses,but there are distinctive differences in the surrounding rock of geological weak structure,that is,the electromagnetic radiation intensity is obviously divided to different zones in space.Therefore,it can be considered that the value of electromagnetic radiation intensity in the fault structure or joint position,the intersection position of the caverns and the influent area under the construction disturbance is higher,which is mainly affected by the stress gradient anomaly of surrounding rock.At the same time,the electromagnetic radiation and microseismic monitoring results jointly reveal the stress migration and evolution law of the surrounding rock,and the electromagnetic radiation monitoring information can enrich the information of rockburst precursors.(4)The numerical simulation of surrounding rock damage process based on RFPA strength reduction method is focused on the surrounding rock damage evolution process of underground powerhouse under the influence of different lateral pressure coefficients and fault structures,and is also compared with the microseismic monitoring results.Comparing and verifying,the damage distribution revealed by the two methods is basically consistent.The numerical simulation results show that when the main powerhouse is excavated to(?)154m,with the continuous reduction of strength,the surrounding rock damage areas at the upstream side wall and arch shoulder of the main powerhouse are continuously accumulated and expanded to the nearest direction of fault f34,and obvious V-shaped damage areas are formed at the arch shoulder.An obvious V-shaped damage area is formed in the powerhouse area.Considering the influence of the main fault structure in the powerhouse area,different lateral pressure coefficients are set for the two working conditions of the main powerhouse excavation to(?)154 m and(?)126 m,respectively.Under the influence of the fault,the stress concentration area is significantly changed,mainly concentrated in the area where the caverns’structure is closest to the fault position.The surrounding rock damage first occurs near the the fault,and then extends from the arch shoulder and the bottom corner to the nearest direction from the fault.The upstream fault of the main powerhouse and the caverns are the main areas of the damage and deformation,which is consistent with the practice.(5)The mechanism of rockburst is studied comprehensively from the basic factors of rockburst including the initial geostress conditions,basic mechanical properties of rock and focal mechanism of rockburst evolution process.Firstly,the numerical simulation method is carried out for the regression inversion of the background stress field and it is revealed that the initial stress magnitude of the underground caverns is medium.The critical buried depth of rockburst is nearly 450 m.Based on the strength to stress theory,the intensity of rockburst is predicted to be slight.Secondly,the tendency of rockburst is experimented and the possibility of slight rockburst is also predicted from the basic mechanical characteristics of surrounding rock,which was confirmed by the characteristics of widespread slight rockburst in the early stage of construction.At the same time,the indoor triaxial creep test results show that there are three stages of active-quiet-surge for the acoustic emission activity during unloading,which is consistent with the delayed rockburst.The three-stage activities of acoustic emission are also corresponding to the stage of initial creep-constant creep-accelerated creep during the unloading deformation.Finally,by studying the distribution of b value,S wave and P wave energy ratio closely related to the source mechanism in the monitoring information,it shows that the b value is generally less than 0.8,that is,the fault type microseismic events are the main events.The energy ratio of S wave to P wave is mainly between 3 and 10,followed by less than or equal to 3.The former accounts for about 50%of the total microseismic events,and the latter accounts for about 40%.The micro-fractures during the evolution of rockburst are mainly tensile shear,followed by tensile failure.At the same time,some shear failure is accompanied,especially the tensile and tensile shear failure along the fault.The weak geological structure is in a state of limit equilibrium in site.When the local stress is continuously adjusted and subjected to external disturbance,it is easy to induce rockburst.The main mechanism of rockburst is the elastic-plastic deformation caused by tension and shear.(6)On the basis of the above research,the multi-index early warning method based on microseismic and electromagnetic radiation monitoring is carried out.An early warning index system including five parameters including microseismic energy,apparent stress change gradient,cumulative apparent volume change rate,electromagnetic radiation intensity change rate and pulse number change rate is established.The early warning thresholds are quantified,namely,the daily cumulative energy of microseismic events(230J),the apparent stress decrease gradient(9.0k Pa/d),the cumulative apparent volume increase rate(1230m~3/d),and the change rate of electromagnetic radiation intensity(40%)and the change rate of pulse number(50%).In order to verify the early warning effect,the engineering verification results show that the early warning time of this method is basically the same,and the potential rockburst location can be delineated in the range of tens of m in space. |
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