Research On Support Mechanism And Construction Technology Of Tunnel In Soil-Rock Composite Strata

Soil-rock composite strata that mechanical properties are quite different is very common in shallow buried section of mountain tunnel,and the.deformation force of tunnel excavation and support in soil-rock composite strata is quite different from that of homogeneous strata.There are still many problems in this soil-rock composite strata with support parameters and construction methods designed according to homogeneous strata.In order to clarify the law of tunnel deformation and stress in soil-rock composite strata and solve the problems in construction.Based on the Nankunshan Tunnel of Shanzhan Expressway,this paper uses numerical simulation,on-site monitoring and indoor test methods to study the support forces and problems in the construction tunnels in soil-rock composite strata.The main work and results are as follows:(1)Research on construction method of tunnel in soil-rock composite strata.This paper expounds the problems of the original design double side drift method,introduces the common construction methods and existing problems of soil-rock composite strata in this project,and proposes double-sided upper guide pit method.Comparing with the design scheme,double-sided upper guide pit method isn't superior in controlling the surrounding rock deformation and surrounding rock stress;upper double side drift method and lower step method is superior in the initial support force uniformity,construction schedule and construction cost.In the actual project,the two schemes should be selected according to the comprehensive situation of the site.(2)Research on the mechanical characteristics of tunnel support in soil-rock composite stratum.Based on the project of the South Kunshan Tunnel,the on-site monitoring was carried out to study the supporting deformation force of the soil-rock composite strata tunnel during construction.The results show that the soil layer is the main reason and main area for tunnel settlement and horizontal displacement production,and that the horizontal convergence of the arch shoulder in soil layer is greater than the horizontal convergence of the arch in rock layer,and that the measured value of surrounding rock pressure accounts for about 25%to 45%of the theoretical value,and steel arches are mainly distributed under pressure,and that the axial force of the system anchor in the soil layer is small,and its effect is not obvious.(3)Research on second lining anti-floating technology of tunnel in soil and rock composite stratum.This paper discusses the existing problems of lining anti-floating technology.The indoor compatibility test,non-pressure grouting time test,early strength test of concrete and grouting time test under pressure show that the micro-expansion cement slurry has good compatibility with the second-line concrete of Nankun Mountain,and it can be used as a grouting material.The combination between the grouting material and the concrete is much more affected by the pressure factor than the time factor.That the early strength curve of the second lining concrete,and that the grouting pressure used under different grouting time can't exceed the early compressive strength of the second lining concrete.The minimum grouting pressure used for grouting material and two-line concrete at intervals of 4h and 6h is 0.2MPa,and the minimum grouting pressure used for grouting material and second-lined concrete is 8h.The minimum grouting pressure is 0.4MPa.