Research On Calculation Method Of Loose Circle Of Surrounding Rock In In-Situ Expansion Of Tunnel Based On Damage Theory

As a type of road traffic,tunnels undertake the important function of crossing mountains,greatly reducing the cost of highways and railways.In addition,for cities with limited land,they are also an indispe nsable part of subways.However,due to the rapid economic development in recent years and the substantial increase in traffic volume,a large number of existing tunnels cannot meet the growing traffic demand.Among the many types of tunnel reconstruction and expansion,in-situ reconstruction and expansion has become the first choice for reasons such as making full use of existing tunnel sections and low economic costs.The in-situ reconstruction and expansion project of the tunnel has caused many disturbances to the surrounding rock,and the changes of the displacement field and the stress field are much more complicated than that of the initial excavation.Therefore,this paper starting from the Mohr-Coulomb strength criterion,combined with the Sidoroff model in the damage theory,to the in-situ expansion of the tunnel.The tunneling loose ring of surrounding rock is analyzed,the formulas of plastic zone and loose ring radius are obtained,and the relationship between internal friction angle,cohesion,tunnel burial depth and the thickness of the loose ring is established,and the influence of the above factors on the thickness of the loose ring is analyzed.The main work of this paper is as follows:(1)Using the damage theory in describing the damage deg ree of the internal structure of the material,the Sidoroff damage model,which uses the damage modulus to represent the damage of the material,is selected,and the damage and elasticity are coupled.On the basis of the principle of energy equivalence,th e damage variable and the The relationship between the strains,the radius when the damage variable D is 0.8is defined as the boundary between the complete damage area and the incomplete damage area of the surrounding rock,and this is used as the radius of the loose ring of the surrounding rock,and then the expression of the damage variable with respect to the arbitrary radius of the tunnel is obtained,Combined with the Mohr-Coulomb strength criterion,the explicit expression of the radius of the loose ring of the tunnel surrounding rock is derived by solving the first-order inhomogeneous linear differential equation.The effects of internal friction angle,cohesion,and tunnel excavation radius on radial and tangential stress distribution,and the relat ionship between tunnel radius and surrounding rock loose ring under different internal friction angles are analyzed.In addition,the theoretical calculation values were compared with the field measured values of the loose ring of surrounding rock in Huain an Dingji Tunnel and Partridge Mountain Tunnel,respectively,and the accuracy of the theoretical calculation formula of the radius of the loose ring of surrounding rock of the Sidoroff damage model was compared.(2)Considering the redistribution of the d isturbed original rock stress after the initial excavation of the tunnel,the surrounding rock stress changes from the initial stress state to the elastic secondary stress state,according to the stress expression of the elastic region after redistribution,the elastic region and plastic The boundary conditions of the plastic zone,the stress coordination equation of the complete damage zone and the incomplete damage zone in the plastic zone,the invisible expression of the radius of the loose ring of the surrounding rock during in-situ excavation is deduced.On the premise that the stress,the radius of the expanded tunnel and the radius of the original tunnel are known,the specific value of the radius of the loose ring can be obtained.The effects of internal friction angle,cohesion,tunnel burial depth and the thickness of the surrounding rock loose ring under the condition of in-situ expansion are analyzed.(3)Based on the actual cross-section of the in-situ single-side expansion project of the Jinzishan Tunnel of the Qiwan Road Express Project in Zhongshan City,Guangdong Province,the three-dimensional numerical modeling was completed.Based on the above model,the Flac3 D calculation software is used to simulate the excavation tunnel by the cross-median partition method(CRD method),and the Flac post-processing is used to check the plastic elements and the Moore-Coulomb constitutive yield criterion.The mesh size of each model is determined,and the range of the loose circle is judged.The numerical results show that the loose circle of the vault is in the range of 1.8～5.4m,and the loose circle of the side wall is 0.3～1.2m.The distribution law of the loose circle is that the arch is large and the side wall is small,and it decreases from the center of the pilot pit to the middle partition and side wall.,the distribution law of the stress field and displacement field of the surrounding rock is also obtained,and the supporting scheme of the supporting project is optimized.