| In recent years,there has been an increasing number of issues resulted from rockburst incidents in the construction of large-scale deep underground projects in China,which poses a severe threat to the safety of construction personnel and machinery.The prevention and control of rockburst disasters have always been the issues that plague the engineering industry.Many scholars and engineers have proposed a variety of countermeasures and methods,but the effects are still limited.In traditional rockburst prevention methods,as long as the rockburst occurs,the rockburst stones may directly pose a threat to the safety of construction personnel and equipment.Therefore,based on the structure,function,and protection principle of the traditional SNS protection net,this paper proposes a flexible protection net system for preventing tunnel rockburst.Taking rockburst-specific flexible protection net(RFPN)as the research object,through theoretical calculation,numerical simulation,physical experiment,and intelligent analysis,etc.,this study explored the protection principles,structural characteristics,design theory,impact dynamics characteristics,kinetic energy characteristics of rockbursts,design methods and practical applications,etc.of RFPN.The main research achievements are as follows:(1)The theoretical calculation formula of rockburst ejection velocity was deduced,and the theoretical in macroscopical view calculation method of rockburst rock ejection velocity based on the theory of dynamic Mohr-coulomb and Revised Fracture Angle was clarified.Through true triaxial rockburst tests,numerical calculations by CDEM,and on-site investigations,the theoretical calculation methods were tested from the test scale to the engineering scale.The research results show that except for the rocks of #1and #2,the average Fracture Angle of the other rocks in the test are range from 24.6 °to 39.9°,which is basically in the range of 24.7° to 40.4° that calculated by theory.The average coincidence degree of the ejection velocity and kinetic energy of the rockburst is 92.7% and 83.8%,respectively.In the indoor test,the theoretical calculation results can reflect the real status and basic characteristics of the speed and kinetic energy of the rock ejected by the rockburst to a certain extent;in practical engineering,the theoretical calculation method of rockburst ejection velocity is still an effective way to obtain rockburst ejection velocity.(2)The design prototype of RFPN and its construction method was proposed,and the work principle and structural characteristics of RFPN was clarified.Through the analysis of the attributes of the protection net components,it was made clear that the protection net exists in the three states from the design and production to the usage,i.e.,the zero state,initial state,and working state,thus providing ideas for the research on the design principles,analysis methods,and mechanical characteristics of the protection net.Meanwhile,by Force Density Method,the initial state of the protection net was studied,that is,the optimized fixing method and initial state were determined through the form-finding analysis of the protection net.After optimized the fixed method,the vertical deformation of the protection net reduces to 2.14 mm,and it is greatly improved that compared with 10.64 mm without fixed plate.(3)Research on the dynamic characteristics of RFPN when impacted by rocks,was carried out.Numerical simulation analysis was adopted to analyze the force and displacement of each component in detail.The research results show the following.Generally,the greater the initial kinetic energy of the rock,the greater the deformation of the protection net;means of adjusting the spacing of anchor bolts can be adopted to control the kinetic energy absorption of a single protection net;the wire-rope net is the main energy-absorbing component of the protection net;wire-rope nets and anchor bolts are the main load-bearing components of the protection net;Under the condition of strong rockburst,the safety factor of wire-rope net corresponding to impact position one is 0.91,and the local failure will occur;Under the condition of slight to moderate rockburst,the safety factors of each components of RFPN are greater than 1,which are in a safe state.It means RFPN can effectively intercept the projectile stones caused by slight or moderate rockbursts.(4)An impact test for a full-size RFPN was carried out to verify the deformation characteristics and mechanical properties of its function in practice under working state.Based on the test analysis of the components in the RFPN,the force characteristics,deformation characteristics,and failure modes of each component were summarized.By comparing the test result with the numerical simulation calculation result,it was found that the numerical simulation result of RFPN was slightly larger than the test result in tensile force(About 9.54～18.75 k N),the deformation was smaller than the experiment result(About 6.5～11.3cm),and the reason was analyzed;the internal force change law and displacement deformation trend of the protection net under various working conditions were consistent with the test results;it is still feasible to use numerical analysis methods to study the mechanical mechanism and movement characteristics of the new protection net.(5)The design method and process of RPFN were proposed and applied in practical engineering.Starting from the structural characteristics of RFPN,based on the theoretical calculation method and numerical calculation results of rockburst ejection rock velocity,the design method and process of the protection net based on energy matching were established.Taking the Micangshan Tunnel as an example,and different protection net design schemes were designed for different grades of rockburst in the Micangshan Tunnel by using this design method.Meanwhile,the corresponding CSVC support vector machine program was compiled,and the rockburst grade prediction model was established.The prediction accuracy of rockburst in the test section of Micangshan tunnel is 75%,and the prediction results are slight to moderate rockburst and achieve a good result.By corresponding and combining the prediction results of rockburst grades during tunnel construction with different grades of rockburst protection net design schemes,the rapid design of RFPN was completed.Subsequently,the safety of RFPN and the actual effect of usage in the tunnel were evaluated through numerical simulation and the establishment of a field test section of the protection net.It is found that the primary lining of the tunnel is not broken down by stones or show the cracked obviously,and the average monthly construction speed is increased by23.8%,which indicates that RFPN has achieved a good effect in protecting Micangshan tunnel from slight to moderate rockburst.Finally,according to some shortcomings of the current protection net scheme technology,the structure of the RFPN was improved and upgraded to improve the effect of RFPN in terms of safety,construction convenience,and cost-effectiveness. |
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