| Compared to double-arch tunnels,triple-arch tunnels have a larger span,more complex construction,and more intense disturbances in adjacent excavations,resulting in a significant pressure arch effect and a more unfavorable tunnel safety condition.There is no systematic and mature method to calculate the pressure in the surrounding rock of triple-arch tunnels.Therefore,this paper integrates theoretical research and numerical simulation,focusing on the process of step-by-step construction of triple-arch tunnels,the method of calculating the pressure of the surrounding rock under deep and shallow burial conditions,and the dynamic evolution of the pressure arch based on the better construction process,to guide the design and construction of triple-arch tunnels.The main research contents and innovation points are as follows.(1)The combined arch formation process is analyzed under the step-by-step construction of a triple-arch tunnel,and the transverse and longitudinal surrounding rock pressure characterization equations are proposed.The basic methods for solving the rock pressure in deeply buried and shallowly buried triple-arch tunnels are compared and analyzed by reviewing typical rock pressure calculation theories.The mechanism and basic assumptions of the combined arch formation are described.A combined arch formation mechanics model is proposed for a triple-arch tunnel considering the step-by-step construction process.From both horizontal and vertical perspectives,a characterization of the surrounding rock pressure of the triple-arch tunnel is given.(2)Based on Protodyakonov’s theory and process load design concept,a method for calculating the surrounding rock pressure of a deeply buried triple-arch tunnel is proposed.By elaborating on the concepts of state load design and process load design methods,the differences and connections between them are given.The process load design method is combined with Protodyakonov’s theory to improve the formula for calculating the vertical rock pressure in deep-buried tunnels.Combined with the mechanism of combined arch formation,the load distribution model and the calculation method of surrounding rock pressure in the deep-buried triple-arch tunnel with different construction sections and construction processes are proposed.The influence law of construction method,section size,and rock parameters on the surrounding rock pressure is systematically analyzed.Finally,the reasonableness of the method is verified by comparing the theoretical calculation results and field measurement data with the actual engineering practice.(3)Based on the slip surface morphology and principal stress deflection characteristics,a shallow buried triple-arch tunnel surrounding rock pressure calculation method is proposed.By exploring the shortcomings of the traditional Terzaghi theory in calculating the surrounding rock pressure in shallow buried tunnels,the improvement direction of the theoretical method is established.It is proposed that when the tunnel burial depth is too shallow,the slip surface in the tunnel loosening area should be regarded as tower-shaped,combined with the development process of stratum arch of shallow buried tunnel.In general,it is still viewed as a vertical form.It is also suggested that the deflection form of the principal stress in the loosened area should be chosen as a parabolic shape.Based on the morphology of the slip surface and the deflection characteristics of the principal stress,improved formula for the vertical pressure between slip surfaces in shallow buried tunnels is derived.The shallow buried triple-arch tunnel load distribution model is given,and the correctness of the improved solution is verified by relying on actual engineering.(4)The construction process of the triple-arch tunnel is compared and analyzed.The dynamic evolution law of pressure arch in the triple-arch tunnel is studied based on the better process.The pressure arch area is refined,and the judgment methods of the inner boundary,core boundary,and outer boundary of the pressure arch are given.Key indicators such as vault settlement,horizontal convergence,rock displacement in the middle line of the diaphragm wall,and plastic zone are selected to compare the advantages and disadvantages of different construction processes in triple-arch tunnels and analyze their applicability.Based on the better process,the longitudinal and lateral dynamic evolution rules of the pressure arch of the triple-arch tunnel are analyzed.The intrinsic mechanism of backward tunnel construction increasing the surrounding rock load of the preceding tunnel is revealed.The influence laws of lateral pressure coefficient,surrounding rock grade,and tunnel buried depth on the shape of pressure arch are also analyzed. |
