Study On Mechanical Characteristics And Stability Of Surrounding Rock Of Water-rich Soft Rock Tunnel

Compared with common rocks,the strength of soft rocks is lower,and it is usually accompanied by the problems of large porosity and loose cementation.When the soft rock is in the soaking condition,these shortcomings of soft rocks will be more magnified.When the tunnel passes through the soft rock stratum,the surrounding rock is easy to lose stability in the process of tunnel excavation.There are often engineering problems such as large deformation of soft rock or even tunnel collapse,which will seriously threaten the people’s lives and property.Therefore,it is necessary to study the mechanical characteristics and stability of the surrounding rock of water-rich soft rock tunnel.Based on the laboratory experimental study of water softening of the soft rocks,this paper studies the mechanical characteristics of the soft rocks under water softening condition.After that,the advantages and disadvantages of strength criterions which are widely used at present are analysed.The peak value stress of soft rocks under different confining pressure is calculated by using different strength criterion.It is determined that the Hoek-Brown strength criteria is more accurate for soft rock by error analysis with the test results.Meanwhile,considering the groundwater,the quantitative determination method of H-B strength criterion parameters under the water-rich condition is proposed.On this basic,the elastic-plasticity analysis of the surrounding rock is carried out.Based on the analysis of the bolt support principle and the bolt support form and combined with the engineering practice,a scheme of adding feet-lock bolt in the primary support of surrounding rock of water-rich soft rock tunnel is provided.Finally,taking a certain expressway tunnel in Ankang City of Shaanxi Province as an example,the mechanical characteristics and stability of the surrounding rock of the water-rich soft rock tunnel are comprehensively evaluated by theoretical analysis,numerical calculation and field test.The conclusions are as follows:(1)Phyllite samples were taken from the certain tunnel.Water immersion test,P-wave velocity test,nuclear magnetic resonance test and uniaxial compression test are carried out respectively.The results show that with the increasing of the dipping time in water,the porosity and pore radius of rock samples expand,the compressive strength,elastic modulus and longitudinal wave velocity of rock samples decrease,and the Poisson’s ratio changes little which is almost negligible.It can be concluded from the change of mechanical parameters of rock samples that water has obvious softening effect on soft rocks.The 180 days is the inflection point of softening degree for phyllite.The softening rate is gradually accelerated within 180 days.After 180 days,the softening rate of soft rocks gradually slows down and the mechanical parameters tend to the extremum value.So far,the softening of phyllite is basically completed.(2)The advantages and disadvantages of four strength criterions which are widely used at present are analysed.The peak value stress in the conventional triaxial compression test of soft rocks is calculated by using four strength criterions.By comparing with the error of test results and combining with the advantages and disadvantages of various strength theories,it is determined that the Hoek-Brown strength criteria is most suitable for soft rock.In order to take the influence of groundwater into account,the water-rich influence factor(D_ω)is put forward.Considering the soaking time(t),the quantitative relationship between t and mechanical parameters is determined.(3)Based on the selected strength criteria and the elastic-plastic theory,the calculation method of the surrounding rock displacement of the tunnel is obtained.The method is applied to the the certain tunnel in Ankang City of Shaanxi Province,and the vault subsidence and peripheral convergence of the surrounding rock of the tunnel are calculated.(4)Through the analysis of bolt support principle,it is found that the essence of bolt reinforcement for surrounding rock is to restore the excavated surrounding rock to three-dimensional stress state.Based on the analysis of the effect of different bolt support forms and the characteristics of the actual engineering,a scheme is proposed to enhance the stability of the surrounding rock of the tunnel by adding the feet-lock bolts in the primary support of the water-rich soft rock tunnel.(5)Taking a certain expressway tunnel in Ankang City as an example,three tunnel excavation models under different soaking time are established by MIDAS/GTS numerical analysis software,and three kinds of bolt support forms of systematic bolts support,feet-lock bolts support and combination support of systematic bolts and feet-lock bolts are respectively used to simulate tunnel construction.Through the numerical simulation results,it can be found that with the increase of soaking time,the deformation and stress of surrounding rock and the stress of primary support are increased,but all the stress values are within the safe allowable range.When adopting systematic bolts or feet-lock bolts support,the vault settlement value is greater than 1/3 of the allowable displacement value.According to the regulations of construction management,reinforcement measures should be taken.When the combined support of systematic bolts and feet-lock bolts is adopted,the deformation value of the tunnel is less than 1/3 of the allowable displacement value,which means the tunnel deformation is effectively controlled and the surrounding rock is stable.(6)In the certain expressway tunnel in Ankang City,the typical sections are selected,and the bolt support forms of systematic bolts,feet-lock bolts and combination of systematic bolts and feet-lock bolts are adopted respectively.At the same time,the selected sections are monitored and measured.Through the monitoring data,it is found that the vault subsidence and peripheral convergence values of each section are the closest to the numerical simulation results when t=365days,that is,the surrounding rock condition of the tunnel is the closest to the surrounding rock condition when t=365 days,which indicates that the surrounding rock condition of the tunnel is under the water-rich condition for a long time.The monitoring results are basically consistent with the theoretical analysis and numerical simulation results,which verifies the reliability of the combined support of systematic bolts and feet-lock bolts.