Research On The Mechanical Model And Stability Control Technology Of Anchor Injection In Fractured Surrounding Rock Of Large-span Tunne

In order to reveal the control effect of the anchor support structure on the stability of the surrounding rock and the invert during the excavation of a tunnel,this paper takes the Tongzi tunnel as an example.The mechanical parameters of the surrounding rock are obtained through in-situ load tests and shear tests.Based on the similarity theory,similar materials and optimal proportions for simulating the tunnel surrounding rock are selected to produce anchor-grouting fractured rock samples.Through uniaxial compression and variable angle shear tests on the similar fractured rock samples,the deformation characteristics,strength characteristics,and variation laws of the rock mass under different anchor layout conditions are studied.Combining the composite material mechanics theory,an elastoplastic constitutive model of the anchor-grouting fractured rock mass is established,and the expression of mechanical parameters is derived.The constitutive model is further developed using Visual Studio 2015,and the rationality of the model is verified through numerical simulation experiments on uniaxial compression.The model is then applied to numerical simulation of tunnel excavation,and the following main research results are obtained:（1）Anchor-grouting fractured rock samples were made using quartz sand,fine sand,barite powder,gypsum,and cement as raw materials.Uniaxial compression and variable angle shear tests were carried out on the samples.The results showed that the mechanical parameters of the anchor-grouting fractured rock mass,such as compressive strength,elastic modulus,and Poisson’s ratio,changed correspondingly with the anchor angle and the number of anchor bolts.Specifically,under the same number of anchor bolts,the average compressive strength gradually increased with the increase of the anchor angle;under the same anchor angle,the compressive strength gradually increased with the increase of the number of anchor bolts.In addition,the compressive strength was relatively close when the anchor angle was-30°and 30°,indicating that the angle between the anchor bolt and the inclined fracture surface had a small influence on the mechanical properties in the vertical direction.Moreover,the increase of the anchor angle had a more significant effect on the cohesion.（2）According to the theory of composite mechanics,the mechanical characteristics of the anchor-grouting fractured rock mass in a certain direction depend on the relative volume fractions of anchor bolts,grouting material,and matrix in each direction.The overall mechanical properties can be equivalent to orthotropic anisotropy,and nine elastic parameters with three symmetric planes can be derived.The results are in good agreement with the experiments.In addition,when the anchor-grouting fractured rock mass enters the plastic yield stage,its failure modes can be roughly divided into:overall compression-shear failure,pressure-induced tensile fracture failure,sliding shear failure between rock blocks and fracture surfaces,opening fracture failure of the fracture surfaces,and anchor bolt failure.（3）The customized elastic-plastic constitutive model of the anchored grouted fractured rock mass was developed by utilizing Visual Studio 2015 compiler and C++programming language.The model was based on the orthogonal anisotropic elastic equation and the Mohr-Coulomb yield criterion.The model was developed through the rewriting of the base class,member functions,model registration,custom model,information exchange between FLAC^3Dand the model,and the model state indicator.The grouted fractured rock mass model was built in FLAC^3D,and the structural elements were used to simulate the anchor bars.The basic mechanical parameters and the custom constitutive model were called upon to conduct the uniaxial compression tests in three directions.The stress-strain curves of the model were compared with the results of indoor compression tests in the Z direction,and the high fitting degree of the stress-strain curves demonstrated the rationality and accuracy of the custom constitutive model.（4）The coal-bearing strata section of the Tongzi Tunnel from ZK40+090 to ZK40+260 was taken as the research object.A tunnel model was established using Midas GTX NX 2019 and FLAC^3Dsoftware to simulate the new Austrian tunneling method（NATM）advance core soil support and excavation.Customized constitutive model"agr"and conventional Mohr-Coulomb model were used for calculation and analysis.The results showed that the deformations of the surrounding rock and the tunnel face calculated by the customized"agr"model were larger than those calculated by the conventional Mohr-Coulomb model in the direction along the structural plane,indicating that the conventional Mohr-Coulomb model cannot well reflect the anisotropy of rock mechanics properties controlled by the structural plane.In addition,there is a certain rule for the deformation of the tunnel vault after excavation:the deformation in the center of the vault is larger than that in the surrounding area.Therefore,during the construction process,it is recommended to use anchor rods with large diameters and lengths as much as possible for support within a radius of 5m from the center point of the vault.The anchor spacing should be1.2-1.5m and the anchors should be arranged in a horizontal and staggered pattern along the excavation direction.For the area beyond the radius of 5m from the center point of the vault,smaller diameter and length anchor rods can be selected,with an anchor spacing of 1.5m and an inclination angle of±10°-15°with respect to the excavation direction.（5）In summary,this paper establishes a mechanical model for anchor-grouted fractured rock mass based on similarity material tests and composite material mechanics theory.The model is applied to simulate the new Austrian tunneling method（NATM）full-section advance core soil pre-reinforcement and excavation in large-span tunnels with weak and fragmented surrounding rock.The deformation patterns of the surrounding rock in different sections are obtained,and support parameter recommendations to maintain the stability of the surrounding rock are given.This research provides useful information for the efficient construction and safe operation of tunnels.