Study On Bottom Void And Mechanical Performance Of Utility Tunnel Structure Under Ground Fissure Dislocation

Xi’ an is a city with very developed ground fissures,and the urban underground utility tunnel project is a typical linear project.The construction of utility tunnel in Xi’an inevitably crosses ground fissures,and the structure of utility tunnel is affected by ground fissure dislocation for a long time in the process of construction and operation.Under the ground fissure dislocation,the utility tunnel structure will produce tension extrusion,tension torsion shear,shear leakage and other failure forms and bottom void.The bottom void often occurs together with other failure forms,which is the important factor for the failure forms.Therefore,it is particularly important to carry out the study on the bottom void of utility tunnel structure and its mechanical performance.Based on the research background of an underground utility tunnel crossing ground fissures in Xi’an,this paper uses the method of combining theoretical calculation clarify the failure mechanism of the bottom void area,obtains the calculation method of the range of the bottom void area,and puts forward the prevention measures for the bottom void area.The main research results are as follows:(1)According to the basic characteristics of ground fissures in Xi’an and based on the structure-soil interaction theory of utility tunnel under ground fissure dislocation,it is concluded that the cause of the void area at the bottom of the utility tunnel structure is that the utility tunnel structure is inconsistent with the soil deformation under ground fissure dislocation.Its action process can be divided into three stages:joint action,local voiding and complete voiding.The mechanical characteristics and deformation mechanism of the utility tunnel structure in the bottom void area are clarified.(2)Based on the stress characteristics and deformation mechanism of the utility tunnel structure,the semi-infinite elastic foundation and Euler-Bemoulli beam are selected to construct the model by using the link method in the elastic foundation beam theory.Assuming that the relative displacement between the utility tunnel structure and soil caused by the overlying load,the end bending moment and the foundation interaction force is zero,the nonlinear equations for calculating the range of the bottom void area of the utility tunnel structure are established.The MATLAB is used to solve the equations,and the range of the bottom void area of the utility tunnel structure is obtained.(3)Using a self-designed model test device,considering the starting conditions of the model test,the bottom void model test of the utility tunnel structure with different angles under ground fissure dislocation is carried out.The variation laws of surface strain,bottom displacement,contact pressure between utility tunnel structure and soil,and internal force of utility tunnel structure under orthogonal(90°)and oblique(45°)conditions are obtained.According to the test results,the range of the void area at the bottom of the utility tunnel structure was determined,which was 2.87 to 3 times the diameter under the orthogonal condition and 3.125 to 3.5 times the diameter under the oblique condition.(4)The calculation method of the bottom void area based on the relative displacement of zero is compared with the model test results,and the results show that the calculation method is reasonable and effective.The method is used to calculate the bottom void area of a utility tunnel under the dislocation of ground fissure f3 in Xi’an,and the prevention range of the utility tunnel structure in the ground fissure area is optimized.The targeted prevention and control measures are put forward from the four aspects of structural strengthening,waterproof treatment,foundation reinforcement and field monitoring,which provides experimental and theoretical support for the structural design,construction and safe operation of utility tunnel in ground fissure area.