Study On Construction Stability Of Large Section Loess Tunnel

With the development of national construction,more and more projects are facing the problem of passing through the loess stratum,which has special properties.With the increase of loess tunnel section,the difficulty of tunnel excavation increases sharply.In order to ensure that the construction process of large section loess tunnel can be carried out safely and the construction quality can meet the requirements.The paper based on the engineering of No.1 Tunnel in Huanxian County of Yinxi railway,determined the limit value of tunnel depth and shallow burying.Studied the mechanical characteristics and deformation law of loess tunnel surrounding rock and supporting structure in the construction process under different excavation methods and process parameters by the combination of numerical simulation and field monitoring.Then deduced the constitutive law of Q₂ loess based on the damage statistical theory.Based on the model,a method for judging the surrounding rock stability of Q2 loess tunnel was proposed.The main work and research results are as follows:?1?To determine wether the loess tunnel is deep or shallow,this paper studied the deep and shallow buried determination method which is suitable for large section loess tunnel.Then calculated the boundary value of Huanxian County No.1 Tunnel of Yinxi railway.Determined the depth and shallow burial limits of different surrounding rock sections of the tunnel.And considered the section of DK372+862?DK372+922 as the key and difficult point for the construction of Huanxian County No.1 Tunnel.?2?To obtain the advantages and disadvantages of different construction schemes this paper studied the construction of large cross-section tunnel passing through the ultra shallow loess stratum by using the finite element software and analyzed the law of vault settlement,clearance convergence,surface settlement,support structure stress and surrounding rock stress.Obtained the most dangerous time of surrounding rock by introducing the concept of"three-dimensional stress degree"and analyzing the surrounding rock stress in the process of tunnel excavation.?3?Through the finite element software,this paper optimized the construction scheme of three-bench seven-step excavation method of loess tunnel,and compared it with the field monitoring data.Analyzed the stress and deformation laws of surrounding rock and supporting structure under different process parameters.Studied he causes of errors of field monitoring data and numerical simulation results.On this basis,considering the results of numerical simulation and field monitoring,a parameter optimization scheme of three-bench seven-step excavation method is given.?4?Based on the analysis shortcomings of the existing loess constitutive model,assumed that the micro element strength of loess obeys the distribution of power function,a constitutive model which can reflect the stress-strain relationship of loess under different water content and confining pressure conditions was established by using statistical method.According to the mechanical properties of Q₂ loess and the results of previous tests,the model parameters of Q2 loess were determined.And the results of previous experiments are used to verify the constitutive model,which proves the rationality and correctness of the constitutive model this paper established.?5?Based on the statistical constitutive model of Q2 loess damage established in this paper,combined with the limit strain judgment method of surrounding rock,a qualitative judgment method of tunnel surrounding rock stability suitable for Q₂ loess stratum was proposed.