Study On Optimization Of Tunnel Bottom Structure In Upper Soft And Lower Hard Strata

In recent years,with more and more investment in road tunnel construction in China,road tunnel construction has been developing rapidly,with longer and longer construction mileage and more and more numbers.China’s vast territory and complex geological environment have led to the emergence of many tunnel construction projects built in the upper soft and lower hard composite strata.This special stratum is characterized by a weak upper stratum,but the lower stratum has good integrity and hard rock,with high bearing capacity and strength parameters.The back arch is a reverse arch structure set at the bottom of the tunnel when the geological conditions are poor,which has the characteristics of complex construction process and high capital cost.The construction of tunnels in soft and hard strata with high strength in the upper and lower strata can lead to longer construction time and economic waste if the design of the elevation arch is unreasonable or conservative.In order to improve the above-mentioned problems,this paper optimizes and designs the support structure of the tunnel bottom in the special stratum of soft and hard strata,and proposes a lining structure support scheme with expanded arch footings.The main research contents and conclusions are as follows.(1)Based on ABAQUS finite element software,the excavation and support of the tunnel were simulated separately for the lining structure with the conventional application of the elevated arch and the lining structure support scheme with the expanded arch footing.The support effects of the two different tunnel bottom structure forms were compared in terms of surface deformation,surrounding rock displacement and stress,lining deformation and force,and second lining safety.The results show that:the support scheme of lining structure with expanded footing reduces the excavation area and the disturbance to the surrounding rock and structure due to the elimination of the elevated arch,which has certain advantages in controlling the surface deformation,settlement of the surrounding rock arch,settlement of the lining arch,axial force and bending moment of the lining arch,etc.,and has a certain degree of reduction in value compared with the lining structure with elevated arch;due to the process of expanding the footing,the section of the second lining side wall and the footing position In the process of expanding the foot of the arch,the cross section of the second lining side wall and the foot of the arch are increased,and the safety coefficient of the lining at the two positions is significantly improved compared with that of the lining with elevated arch,and the structural safety is stronger.However,in terms of control of the rise of the bottom of the arch,the bottom of the arch,the axial force and bending moment of the arch waist,and the axial force and bending moment of the sidewall,the expanded foot lining structure is inferior to the lining structure with elevated arch.However,due to the hard condition of the basement rock of the tunnel studied in this paper,its own condition is better,with better bearing capacity and strength,the arch bottom surrounding rock itself is very small uplift value,only within 1.5mm,and the lining in the sidewall position,although the axial moment is larger,but here the expanded arch footing support scheme due to the expansion of the arch footing,but here the lining safety is better.From the numerical simulation point of view,it can be considered to adopt the expanded foot of arch lining structure support scheme instead of the traditional application of the back arch tunnel support scheme.(2)An indoor model test with a similar ratio of 1:50 was conducted for the lining structure with the elevated arch and the expanded footing support scheme proposed in this paper,and the feasibility of the lining structure support scheme with the expanded footing was further studied in terms of lining forces,lining safety,and lining-rock contact pressure.The results show that the lining structure with expanded footing is inferior to the lining structure with elevated arch in terms of footing axial force,side wall axial force,arch shoulder axial force and bending moment,and lining-rock contact pressure at the arch shoulder and arch top position because the elevated arch is eliminated and no closed support system is formed.Compared with the lining structure with elevated arch,there is a certain degree of increase in the values,but the increase is small and controlled within8%.The safety coefficients of arch waist,arch shoulder and arch top lining are basically the same for both lining structures,and the difference in values is not much,within 5%.The lining safety coefficient at the two locations is significantly improved due to the expansion of the lining section at the side wall and foot of the arch,and the lining safety coefficient is increased by 89.84%and 36.05%respectively compared with that of the lining structure with elevated arch.From these aspects,the expanded footwall lining structure is better than the lining structure with elevated arches.(3)For an engineering example of a tunnel under construction in Shandong Province with the expanded arch lining structure support scheme proposed in this paper,the main quantities of works per linear meter were counted separately from those of the traditionally applied superelevation arch support scheme.The results show that the expanded footing lining structure support scheme can reduce 32.45 m~3 of excavation,3.56m~3 of primary support,9.35 m~3 of second lining support and 9.92 m~3 of concrete backfill per linear meter due to the elimination of the back arch design,which brings great economic benefits.From the above numerical simulations,model tests and economic aspects,it is possible to consider eliminating the traditional design of elevated arches and adopting the expanded footing lining structure support scheme proposed in this paper for the tunnel in the special composite stratum of soft upper and hard lower strata studied in this paper.It can reduce excavation and support,reduce construction disturbance,shorten the construction period,and bring great economic benefits,solving the problems of difficult construction and high capital cost in the construction of the traditional elevated arch,which can provide reference for the design and construction of similar projects in the future,and has important theoretical significance and practical value.