Study On Mechanical Response And Support Design Method Of Surrounding Rock For Underground Tunnel Construction With Rock Stratum Mining Method

Urban subway construction is in the ascendant in China.By the end of 2021,there are 253 lines under construction in 55 cities in Chinese Mainland,of which a large number of rock formations have appeared,such as Qingdao,Chongqing,Dalian,etc.It is roughly estimated that the construction scale of such formations exceeds 1300 km.However,the interval tunnels generally account for more than 60% of the total metro civil engineering investment,which has a great impact on the project cost.For rock stratum,mining method is widely used because of its flexible section layout,convenient construction organization and many other advantages.However,at present,the theory of mine method tunnel design and surrounding rock response is relatively vague,and there is a lack of scientific design methods,which leads to a great difference in the understanding of rock metro interval tunnels by engineering technicians,and there are problems such as too strong subjectivity and conservative support parameters.In this paper,the comprehensive research methods of statistical investigation,theoretical analysis,numerical simulation,field measurement and engineering practice verification are used to study the mechanical response of surrounding rock and the design method of initial support in the construction of subway tunnels in rock areas using mining method.A systematic theory and optimization design method are formed.The main research contents and innovative achievements are as follows:(1)The theoretical system and calculation method of surrounding rock damage zone of rock mining tunnel are established.The study on the mechanical response of surrounding rock in the construction of rock stratum mining method tunnel is carried out,and the mechanical behavior of surrounding rock in the elastic and elastic-plastic states after tunnel excavation is analyzed.It is pointed out that the existing calculation methods only consider the loose ring caused by the ground stress release,ignoring the impact of blasting,resulting in a large difference between the theoretical value and the measured value.The theoretical calculation method of the loose zone range of surrounding rock based on Fennervsolution is proposed by adopting the stress legislation method and the functional relationship method respectively.It is revealed that the blasting expansion loose circle formed in the process of explosive explosion has a great influence on the surrounding rock,that is,the blasting stress wave has a crack propagation effect on the ground stress loose circle,and the final surrounding rock loose circle is formed under the joint action of the ground stress and blasting load.The applicability and effectiveness of the calculation method are verified by the field measurement of Qingdao Metro Line M1.At the same time,the coupling coefficients m,n of the method are discussed,and the range of values applicable to the calculation of surrounding rock loose zone in typical rock areas is given.(2)The contribution of various supporting structures to the engineering stiffness of the interval tunnel with mining method is revealed.Based on the tunnel project of Qingdao Metro Line M1,this paper analyzes the sensitivity of the initial support structure parameters to the project stiffness(including ground settlement and vault settlement)under various working conditions of the interval tunnel.The sensitivity of the construction method,anchor length,initial support thickness and initial support stiffness parameters of the tunnel under different sections and geological conditions are analyzed;Special comparative tests and field tests were carried out for the role of the system anchor bolt,which deeply revealed the role of the system anchor bolt: in small section tunnels,the role of the anchor bolt is very small,and its sensitivity to the project settlement deformation is the lowest,and some working conditions even show that the anchor bolt is under pressure,so the system anchor bolt can be completely canceled in the single line interval tunnel of the subway mining method.In the study,the optimal combination of tunnel initial support parameters is also proposed for different working conditions.(3)A new calculation method of tunnel surrounding rock load based on the calculation theory of surrounding rock loose zone is proposed.In view of the problem that the current calculation method of tunnel surrounding rock load is outdated and lacks theoretical support,relying on the Qingdao Metro Project,the tunnel surrounding rock pressure under various geological conditions wasvitested on site,revealing that the measured tunnel surrounding rock pressure is quite different from the calculated value in the railway tunnel design code,and the code value is 2-3 times the actual value.Therefore,based on the research results of the tunnel surrounding rock loose zone calculation theory,and on the basis of the detailed analysis of the measured data,a new method for the calculation of the surrounding rock pressure of the mine method tunnel is proposed.Compared with the standard method,this method considers comprehensive factors,and adds two parameters,namely the rock compressive strength and the tunnel burial depth.Through the calculation and comparison with the measured data,the applicability and effectiveness of the new theoretical method are verified.(4)The surrounding rock evaluation system and supporting structure design method based on engineering object oriented are developed.Surrounding rock property is a multi parameter system.This study breaks the traditional surrounding rock classification method and attempts to completely describe the complex surrounding rock property of tunnel with a single index.Starting from the actual engineering environment where the tunnel is located,a new surrounding rock property evaluation system oriented to engineering objects has been established,including five personality indexes.The corresponding connection mechanism between the support structure and the surrounding rock properties has been further established.Instead of formulating parameters for the surrounding rock classification,it directly designs the functional relationship between the five surrounding rock property indicators and the support structure,which changes the old design method of determining a set of support system by a single surrounding rock classification,thus establishing an optimal design method of organically combining the support structure and the surrounding rock properties.Through the application analysis of practical engineering cases,it is verified that the new method is not only scientific and effective,but also the overall support parameters have been greatly optimized compared with the traditional methods.(5)The single-layer shotcrete anchor lining structure has been innovatively practiced in the field of subway engineering.In view of the unique hard rock geological conditions of Qingdao Metro,a singlelayer shotcrete anchor lining tunnel was developed and implemented in the surrounding rock environment of Class A chamber in the deep buried section.The "first layer+surface layer" layered and interval construction method is proposed to repair the cracks in the first layer by spraying pressure filling,fully ensuring the durability and compactness of shotcrete.In view of the problem that the drainage system cannot be set for the single layer lining,the "internal modeling networked drainage system based on the principle of biodegradation" is invented.A networked drainage system composed of flexible degradable materials is set between the surrounding rock and the shotcrete layer.After degradation,hidden drainage channels are naturally formed in the shotcrete layer structure,which becomes a permanent networked drainage system.The application of this design method fully reflects the green and low-carbon environmental design concept.