Research On Calculation Method And Application Of Loose Surrounding Rock Pressure Of Shallow Tunnel In Loess Area

In the process of railway and highway transportation network construction,tunnels have the advantages of overcoming elevation barriers,improving alignment and protecting the environment,and often become an important choice for traversing mountains and ridges.However,due to its unique macroporous structure and obvious vertical joint development characteristics,the earth pressure calculation of shallow loess tunnels is different from ordinary tunnels.Therefore,based on the Kole Bay Tunnel,this paper focuses on the mechanical characteristics of surrounding rock in the process of shallow loess tunnel excavation through the combination of laboratory test,field test and theoretical analysis.The main work of this paper is as follows:(1)The basic physical and mechanical parameters and stress ratio structural parameters of the loess were obtained through laboratory triaxial test and basic soil property test of the loess in the Cola Bay Tunnel.The analysis results of stress-strain curve and structural parameter curve of loess under different confining pressure and water content show that the increase of water content has a certain weakening effect on the strength and structure of soil,while the increase of confining pressure has a certain strengthening effect.Based on the analysis of the strength characteristics and structure of the tested loess,the concept of structural exertion was put forward,and the new relationship between the loess strength and structural exertion was obtained through the analysis of the variation and development law of loess strength under different structural exertion.The relationship between cohesion and structure performance is positively proportional,and the internal friction Angle is almost not affected by structural changes.(2)Based on the analysis of the existing shallow-buried loess tunnel engineering field survey data,a rigid translational failure mode of shallow-buried loess tunnels controlled by six angles is proposed,which is derived by combining the structural Moore Coulomb criterion and the upper limit method of limit analysis The corresponding upper limit solution of surrounding rock pressure is obtained,and the optimal upper limit solution of surrounding rock pressure and the corresponding failure mode are obtained by using MATLAB to compile the optimization algorithm.On this basis,the influence of structural performance,internal friction angle,lateral pressure coefficient,tunnel depth,tunnel span and surrounding rock gravity on the surrounding rock pressure and failure modes of shallow loess tunnels is analyzed.The results show that the pressure of surrounding rock decreases with the increase of internal friction angle,structural performance and lateral pressure coefficient,and increases with the increase of tunnel buried depth,tunnel span and surrounding rock gravity;the failure mode increases with the increase of internal friction angle,structural performance and lateral pressure coefficient.The increase in friction angle,structural performance and lateral pressure coefficient shows a trend of inward contraction,and a trend of outward expansion as the buried depth of the tunnel increases.(3)The field monitoring test of the surrounding rock pressure in the shallow buried area of the loess tunnel was carried out.Through the analysis of the monitoring results,the spatial and temporal distribution law of the surrounding rock pressure between the initial support and surrounding rock during the excavation of the shallow buried loess tunnel was obtained.The test results show that the spatial distribution of surrounding rock pressure is not uniform,and some testing points show obvious bias characteristics.The change of surrounding rock pressure over time will experience three stages: rapid growth,slow growth and leveling off.Finally,the applicability of the proposed method is verified by comparing the field measured values,the theoretical values of surrounding rock pressure in common shallow buried tunnels and the calculated results.The general rule of structural performance is summarized,that is,for shallow buried loess tunnel,the reference range of structural performance is 0.4～0.5.