Study On Calculation Method Of Surrounding Rock Pressure Of Loess Tunnel Based On Loose Circle Theory

With the implementation of the policy of the development of the western region and the Belt and Road,the construction of the tunnel project is playing an increasingly important role in the infrastructure construction and energy resources development in the western region,especially the Loess Tunnel with special surrounding rock properties.Therefore,based on the study of loosening zone in Loess tunnel,a method of calculating surrounding rock pressure based on loosening zone theory is proposed.The relevant research contents and conclusions are as follows:(1)Based on the Zaosheng No.3 tunnel of Yinxi Railway,the physical mechanics and deformation parameters of loess are measured through field sampling and laboratory tests.The physical mechanics parameters of loess are provided for the following numerical simulation,and the relationship between the parameters and the change of water content is statistically analyzed.The parameters are very vulnerable to the influence of water content,with the increase of water content.Through the field surrounding rock pressure test,the surrounding rock pressure of deep-buried large cross-section loess tunnel is obtained,and the variation law of surrounding rock pressure during the construction of loess tunnel is analyzed,which provides a basis for the theoretical calculation of loosening zone and the verification of the calculation method of surrounding rock pressure.(2)Analyzing the main supporting objects of Loess tunnel,it is considered that the self-weight of soil in loose zone is the main supporting object of Loess tunnel.Based on Mohr-Coulomb criterion,the elastic-plastic analysis of surrounding rock of tunnel is carried out,and the theoretical calculation formula of loose zone with and without support is obtained.Through numerical simulation of loose zone of loess tunnel,the thickness of loose zone in numerical simulation is put forward.Stress monitoring method is used to determine the distribution characteristics of tunnel loosening zone,which is mainly cat's ear,and the thickness of loosening zone at arch waist is generally larger than that at arch top and side wall.Through field test of loosening zone in Loess tunnel,the size of loosening zone in deep-buried loess tunnel with support is obtained,and the test results are analyzed to verify the theoretical calculation and number.The accuracy and feasibility of numerical simulation.(3)Through FLAC3 D,many factors affecting the loosening zone of loess tunnel are simulated and analyzed,including surrounding rock factors(cohesion,internal friction angle,gravity,etc.)and support parameters(support stiffness,timing,advance support).The sensitivity of these factors is also analyzed.Among them,the loosening zone of loess tunnel is more sensitive to the change of physical and mechanical indexes of surrounding rock,especially the change of the loosening zone of Loess tunnel.It is internal friction angle and cohesion,and the change of support stiffness and support timing in support parameters has a greater impact on the thickness of loosening zone.(4)Based on the formation of tunnel loosening zone,a new method is proposed to determine the deep and shallow buried loess tunnel and calculate the surrounding rock pressure of deep buried loess tunnel.The calculation method of surrounding rock pressure of shallow-buried tunnel is compared and analyzed,and the calculation method of surrounding rock pressure suitable for shallow-buried loess tunnel is determined.Based on the measured surrounding rock pressure,the new calculation method of surrounding rock pressure of deep-buried loess tunnel is compared with other commonly used methods,and the feasibility of this calculation method is verified.