Study On Stability Mechanism And Risk Assessment Of Tunnel Construction In Upper Soft And Lower Hard Composite Strata

This thesis is based on the National Natural Science Foundation of China(NSFC)"Research on progressive failure mechanism and design method of tunnel in upper soft and lower hard strata "(No.51478118)and Guangxi Science and Technology Project "Study on stability mechanism and quantitative evaluation of tunnel construction in composite strata"(No.AD18126011).The upper soft and lower hard strata as the research object,the failure mechanism of tunnel instability and the safety of tunnel construction are systematically studied through the physical and mechanical test,model test,numerical limit analysis,safety quantitative evaluation and so on.The main research work and achievements are as follows:(1)From the mechanical properties of surrounding rock test,geological factors,engineering factors and comprehensive influence of multiple dimensions,related studies were carried out on the factors affecting the stability of tunnel in the upper and lower soft composite strata.Firstly,the mechanical properties of the surrounding rock samples of the upper and lower hard composite strata were studied by means of artificial sample preparation.The results show that the mechanical properties of surrounding rock are mainly determined by rock strata distribution,and the position of soft and hard interface and the proportion of high strength material are the main factors affecting the mechanical properties and failure mode of surrounding rock.Secondly,the influence of geological factors on tunnel stability in upper and lower hard strata is analyzed.Based on the orthogonal tests of initial vertical stress,initial horizontal stress,surrounding rock characteristics,structural plane characteristics and position,and surrounding rock combination of soft and hard strata,it is found that the characteristics and position of structural plane and surrounding rock combination of soft and hard strata are sensitive to tunnel stability.Then,based on the numerical limit analysis theory and safety factor method,the influence of engineering factors on the stability of tunnel in upper soft and lower hard composite strata is analyzed,the evolution mode and development law of limit failure in plastic zone are revealed,and the stability evaluation system based on the safety factor of surrounding rock is established.Finally,a multi-factor analysis model with geological factors,engineering factors,artificial construction and tunnel size as key indexes is established,and the artificial construction factors and geological factors are determined as the main influencing factors.(2)The temporal and spatial stability of tunnel with upper soft and lower hard composite stratum is studied by blasting and non-blasting excavation methods,and a new dynamic partial excavation method of tunnel with upper soft and lower hard composite stratum is proposed with liuyanghe tunnel of Wuhan-Guangzhou high-speed railway as the engineering background.Firstly,the temporal and spatial stability of non-blasting excavation in the upper soft stratum and blasting excavation in the lower hard stratum is explored by orthogonal test.It is found that the order of factors affecting the stability of the tunnel in the upper soft stratum is whether temporary support is installed,the exposure time without support and the removal time of temporary support.Secondly,by establishing the blasting construction analysis model of composite stratum,the temporal and spatial stability of blasting excavation tunnel with hard rock stratum under section is studied,and the influence law of different stratification angles on blasting vibration and tunnel surrounding rock is revealed.Through field monitoring,numerical simulation and fitting analysis,the propagation characteristics of blasting vibration wave in special stratum are revealed,and the correction model of blasting vibration wave propagation law for upper soft and lower hard composite stratum is established.Finally,based on the actual liuyanghe tunnel project of Wuhan-Guangzhou high-speed railway,a new dynamic partial excavation method of tunnel with upper soft stratum and lower hard stratum is proposed,and its adaptability in different surrounding rock combinations is analyzed.(3)Through tunnel model test,strength reduction method and numerical limit analysis method,the instability failure mechanism and stability quantitative evaluation of tunnel in upper soft and lower hard composite stratum are studied.Firstly,the stability model test of tunnel excavation in upper soft stratum and lower hard stratum is carried out.The results show that the distribution of soft and hard interface has a significant effect on the deformation of tunnel excavation.Secondly,the strength reduction method and numerical limit analysis method are used to study the tunnel stability and collapse modes,quantitatively evaluate the tunnel safety of the upper and lower hard composite strata,and reveal the distribution of potential failure surfaces.Then,based on the model test and numerical limit analysis,the failure mode of surrounding rock is revealed.The failure area is mainly around the soft and hard interface and the vault,and the soft and hard interface determines the size of the failure area and the influence range.(4)The overall safety risk assessment model and application of the tunnel in the upper soft and lower hard composite strata during the construction period are established,and the typical risks of tunnel collapse and water inrush are established and specially assessed.Firstly,the tunnel construction risk of upper soft and lower hard composite stratum is investigated and the effective risk identification is carried out,and the quantitative evaluation index system of safety risk is established.Based on the fuzzy-entropy weight theory,a quantitative evaluation model is constructed and the overall safety risk of Jintai Railway Wufengshan tunnel project is evaluated,and the evaluation result is "possible risk".Secondly,the risk assessment model of tunnel collapse safety is constructed and applied by probabilistic risk analysis method.The assessment result is that the risk grade of "impact of blasting excavation on upper part" is the highest.Finally,the risk of seepage water inrush caused by soft and hard interface is dynamically evaluated by using interval mathematics theory,and the result is "high risk" within the evaluation interval.The evaluation results are consistent with the construction site,which verifies the rationality and feasibility of the evaluation model.