Statistical Characteristics Of Surrounding Rock Pressure Of Shallow-buried Long-span Metro Tunnels In Upper Soft And Lower Hard Rock Stratum

With the development and utilization of urban metro tunnels and underground space,a number of shallow underground tunnels have been developed.In view of the influence of randomly distributed joints in the upper soft and lower hard rock mass on the loose pressure and distribution characteristics of the shallow large-span tunnel,this paper uses the research method of the combination of theoretical research,numerical simulation and empirical analysis to carry out systematic research,summarize and analyze the statistical distribution characteristics of the surrounding rock pressure,and lay the foundation for the reliability design of the tunnel.The following innovative results are obtained:(1)On the background of the above soft lower hard composite stratum,a set of random joint network program which can solve the problem of multiple groups of different density is developed.The program can solve the problem of any multiple stratum combinations and each stratum contains different joint number,which overcomes the insufficiency of the original homogeneous stratum.The case that the formation can be reduced to homogeneous stratum under certain conditions.(2)Through orthogonal experimental design,the effects of burial depth,weathering layer thickness and geometric parameters of rock mass structural plane,such as joint direction angle,trace length and joint density,are mainly considered.All the models are numerically calculated and analyzed.The characteristics of tunnel loosening and failure in rock composite strata and the influence of various influencing parameters on tunnel loosening and failure are discussed in detail.It is concluded that the failure modes of the tunnel are symmetrical.The failure modes are from local tension loosening at the beginning to shear force shearing non-penetrating rock bridge developing into slip plane,and shear sliding failure of integral staggered platform traction.(3)After numerical calculation and analysis of all stochastic joint network tunnel models,the boundary line shapes of all models are predicted,and the quadraticparabola type is obtained,and the boundary shape is fitted,and the specific expression is given.Subsequently,the statistical analysis of the failure and loosening scope of all models is carried out,and the multiple linear regression analysis of the loosening and failure scope is carried out,and the relationship between the various influencing factors and the loosening and failure scope is obtained.(4)In view of the limitations of the existing traditional theory of shallow surrounding rock pressure,it can not be directly applied to the existing complex geological engineering.Based on the principle of stress transfer and the shape of failure boundary predicted and fitted,the analytical formulas for calculating the surrounding rock pressure of shallow-buried long-span tunnels in complex geological conditions is derived in this paper.The effect of joints fissures is fully considered in analytical formula.The validity and rationality of the formula are verified by an engineering example.Comparing with the traditional calculation method of surrounding rock pressure,the accuracy of the proposed method is demonstrated,which provides a theoretical basis for solving similar strata combination of rock strata engineering.(5)According to the proposed formula of surrounding rock pressure,Monte Carlo random sampling method is adopted to determine the critical sampling times when the surrounding rock pressure converges and is stable.Through random sampling of various parameters in the formula subject to a certain probability distribution,the statistical results show that the surrounding rock pressure subject to a certain normal distribution characteristics,and gives a certain explanation from the aspect of probability reliability,which makes the uncertainty of surrounding rock pressure caused by the uncertainty and discreteness of geotechnical parameters more meaningful.(6)The lining effect is used to reflect the statistical distribution characteristics of surrounding rock pressure.Through the stress characteristics of the lining,the stress effect of the typical section of the structure is obtained.Through the Monte Carlorandom sampling,the stress distribution characteristics of the structure effect are obtained.(7)This paper discusses and analyzes the influence of the excavation method on the statistical distribution characteristics of the surrounding rock pressure.Taking the common CRD method as an example,the statistical distribution characteristics of the surrounding rock pressure obtained from the full section excavation are compared with the distribution characteristics of the surrounding rock pressure obtained from the CRD method excavation process,and the latter is smaller.Under the condition of single factor distribution,when each factor obeys normal distribution,the pressure of loose surrounding rock obeys certain normal distribution characteristics.When the joint spacing obeys the negative exponential distribution,the surrounding rock pressure obeys the negative exponential distribution.In the case of the combination of various factors,whether the joint spacing is lognormal distribution or negative exponential distribution,the distribution characteristics of surrounding rock pressure are normal distribution.