Research On Deformation Characteristics And Control Measures Of The Large-Section Loess Tunnel

The characteristics of loess are source of tunnel mult-class problems. The deformation characteristics and control measures of large-section loess tunnel have been researched deeply by using theoretical analysis, laboratory model test, numerical simulation and field measurement methods. According to field application and feedback of typical loess tunnel, the surrounding rock deformation characteristics, failure mechanism, spatial displacement and deformation control have been solved. Specific findings are highlighted below:(1)Loess tunnels were usually built in Q2or Q3loess strata which are low moisture content strata. The engineering properties of loess are significant structural and vertical joints. The loss engineering characters was analysis laboratory triaxial test, and the potential of integrated structure which is from uniaxial stress state is applied to triaxial stress state, the loess structural quantitative has been realized by applying the uniaxial stress state to the triaxial stress state. According to the stress concentration of the elastic hole, the vertical joints are caused by level tension. Comprehensive analyzed of the engineering properties of loess, Bilinear Strain-Hardening/Softening Ubiquitous-Joint Model is selected to consider the type of structural and vertical joints.(2)Based on the characteristics of loess, the deformation of wall rock is shows concretely as follows:concrete manifests of mutagenicity, the large deformation, long duration, easily formation of surface cracks; The stress adjustment experiences a complex loading and unloading process during the excavation progress and the range of tunnel face space effect is2times diameters; The mechanics respond caused by the excavation is most strongly at cavern perimeter and develops along tunnel radius; The influencing factors of deformation in loess mainly include the engineering properties, environmental stress, tunnel depth, tunnel cross-section shape and size, and groundwater and construction management, etc.(3)According to the investigation and numerical analysis of tunnel collapse in large section of loess tunnel, shear slip and weak tensile strength become the main reason for tunnel failure case. Under no-support conditions of the loess surrounding rock failure process performance:local fracture produced→local fracture extended→facture rapid transfixed→residual strength. The wedge slip body is firstly performed on the side-wall parts along direction of the minimum principal stress, then spread for vault and arch bottom and the integrity of surrounding rock is undermined. As the weak tensile strength, the lossening collapse is occurred on the vault. The tangential stress increased area is existed in the deep rock, the part rock bears weight with external load soil pressure arch formed a significant effect, since the rock wall can be divided into loose zone→pressure-arch→initial stress area.(4)According to the model test, the steel and spray net support is the effect way which can control the development of loess surrounding rock deformation and stress relaxation. The surrounding rock and support characteristic curves in vault show that Steel and spray net support provides the maximum support reaction force for70%of rock pressure and corresponding vault settlement5.1mm. The combined support structure of the steel and spray net plays the expected limit and reduces the occurrence of harmful loose.(5)Due to loess engineering properties, the deformation model of large-section loess tunnel show that the vertical displacement of crown slowly weakening and the rapid weakening of the horizontal displacement. The horizontal constringency is generally less than the crown settlement, and the law is in accordance with the Boltzmann function. The support of the upper bench plays an important role in the control of crown settlement, and the supporting structure reasonably adjusts the stress distribution and improves the stress concentration effective; The support of the upper bench plays an important role in the control of crown settlement, it should be pay more attention in the construction; The ring closure of support produces obviously restrain effect on the tunnel deformation and is beneficial to the stability of displacement field.(6)The longitudinal deformation of loess surrounding rock is analysis by site numerical analysis; the extrusion deformation trend is significant for the existence of free face. On the whole, The longitudinal deformation of the crown is smaller than the wall; For the forecast of longitudinal deformation, the empirical formula of Panet is25%, Hoek is30%and3D calculation is33%; The range of free face are effectively reduced and beneficial for the tunnel face stability; the suitable reserved length of core soil is2R/3.(7)The ground and support reaction curve of large section of loess tunne is drawn by the numerical calculation and theoretical analysis. The initial support adaptation is studied by convergence-confinement method. The maximum reaction force is larger than the stress values of the equilibrium point, so it meet the requirements for adaptability. (8)Based on orthogonal design for the optimization of supporting opportunity and stiffness, a reasonable optimal indicator is selected and the combination is calculated. It shows that better adaptability to optimize the combination. The displacement control benchmark of large section loess tunnel deep and shallow has been established based on in-situ monitoring and numerical analysis.