Mechanical Analysis Of A New Tunnel Structure Through Karst Area

The wide distribution and types of karst landforms in China are rare in the world.Karst,as a unique geological form and unfavorable geology,is one of the common geological disasters in the construction of underground engineering in karst area.The excavation and blasting of the tunnel broke the original stress balance of the surrounding rock mass.In addition,the surrounding karst cave makes the surrounding rock of the tunnel in a complex state of stress.The existence of karst cave has a great influence on the deformation and instability of surrounding rock,and the current treatment method of "sidestep,stride and fill" is not enough to deal with the complex karst geology.In order to better solve the problem of karst geological disaster in the construction of tunnel in karst area,a new tunnel structure through karst area is proposed.In this paper,the structural mechanical properties of a new tunnel structure through karst area are analyzed by combining theoretical analysis and numerical simulation.The specific research contents and conclusions are as follows:(1)Combined with the advantages of the existing tunnel support structure and the bridge structure with strong crossing ability,a new tunnel structure which integrates the shoring,crossing and foundation treatment functions to cross the karst area is developed.And the structure type,structure,construction technology,structure characteristics and applied principle are introduced.(2)Using the complex function and the Schwarz alternating method,the analytical solutions of the displacement field and the stress field at any point in the surrounding rock caused by the excavation of a karst cave with arbitrary shape in the semi-infinite plane are derived.In order to verify the correctness of the derivation process,the arbitrary shape karst cave is simplified into an elliptic karst cave,and the expression of the stress field and displacement field at any point in the surrounding rock after tunnel excavation under the action of the elliptic karst cave is given.At the same time,a numerical example is given to compare the analytical results with the finite element results,which verifies the reliability of the method.(3)According to the elastic center method,the formula for calculating the internal force of any section of hingeless arch under arbitrary radial load is derived.Using the two-stage analysis method,the analytical calculation method of the influence of tunnel excavation on the internal force of pile foundation under the combined action of pile top load and karst cave is proposed.Firstly,the settlement displacement control equation of a single pile under load under vertical displacement caused by tunnel excavation is established,and the additional axial force at any section of the pile is solved.On this basis,the governing equation of horizontal displacement of single pile subjected to load under the coupled effect of longitudinal and transverse deformation of pile is established.On this basis,the governing equation of horizontal displacement of single loaded pile under the coupled effect of longitudinal and transverse deformation is established.The difference method is used to solve the equation,and then the internal force of pile is obtained.At the same time,a specific example is given to compare the analytical solution with the finite element results,which shows that they fitting each other closely.(4)The influence on the structural stability of the new type of tunnel is analyzed from three aspects: the different distance between the circular center of the karst cave and the tunnel,the different distribution angle of the karst cave and the different diameter of the karst cave,and a two-dimensional numerical model is established by MIDAS GTS.The results show that with the increase of the circular distance,the maximum radial displacement is transferred from the arch foot to the vault,but the distribution is uniform;with the increase of rotation Angle,the radial displacement appears local concentration from uniform distribution;as the hole diameter increases,the radial displacement appears local concentration,and the difference of maximum and minimum increases;It is also found that pile foundation can block the displacement caused by tunnel excavation.Compared with the ordinary structure,the new structure is superior to the ordinary structure in controlling the uplift at the bottom of the tunnel and the stability of the karst cave.