Study On The Stress Yielding Support Structure With Foamed Concrete In Extrusion Large Deformation Tunnel

With the implementation of China’s western development strategy and the promotion of the construction of the Sichuan-Tibet Railway,the construction of regional high ground stress tunnels faces the challenges of ground stress release and control of large extrusion deformations.The tunnel lining structure,as the main bearing support structure in addition to the natural bearing arch of the ground,plays an important role in controlling the deformation of the tunnel surrounding rock.Traditional tunnel support structure mainly includes the initial support and secondary lining structure.High extrusion level of ground stress and tectonic stress often leads to extrusion of cracking of the lining structure,or even failure.Based on this,the concept of stress yielding support is gradually replacing the idea of hard support in traditional support system.The stress environment of the tunnel lining structure is improved by improving the force mechanism and deformation constraint conditions of the lining structure,allowing the tunnel surrounding rock extrusion creep to produce small deformation and absorb deformation energy.This paper takes the stress release and deformation control of extruded large deformation tunnel in high ground stress soft fractured stratum as the research background,referring to existing research results and technical theories,for the tunnel lining structure force characteristics,proposed a new composite lining structure-foam concrete lining,Theoretical analysis and research on stress yielding support,research on mechanical test and microstructure analysis of foam concrete,mechanical test research on contact surface of heterogeneous material of composite lining structure,simulation study on optimization of foam concrete lining parameters and evaluation of pressure letting effect has been carried out.Comprehensive theoretical analysis,mechanical tests and numerical simulation results have led to the following main conclusions.(1)The main reason for the failure of the support structure of extruded large deformation tunnel is the non-uniform dynamic extrusion of soft and broken strata acting directly on the lining structure.The tunnel stress yielding support system is a tunnel support method that weakens the stress state of the surrounding rock by releasing a quantitative amount of the surrounding rock extrusion deformation,so that the main stress traces of the surrounding rock are bent and deflected favorably,and the peak concentrated stress is guided to the deeper part of the surrounding rock.The characteristic curve of stress yielding support can be roughly divided into linear elastic deformation stage,let compression deformation stage and dense enhancement stage.(2)The raw materials and mass ratio of foam concrete used in this paper are:cement: fly ash: water: fine sand = 1.77 : 0.5 : 1.1 : 0.93,where the foam volume admixture ratio is about 1.25,the additive admixture mass is about 2%,and the polyvinyl alcohol fiber admixture mass is about 2.06‰.The modulus of elasticity of foam concrete in uniaxial compression is about 1.01 GPa,and the compressive strength without lateral limit is up to 1.89 MPa,while the yield strength increases significantly with the increase of enclosing pressure when the foam concrete is in triaxial stress state.In addition,the splitting tensile test of foam concrete showed significant plastic deformation,and the tensile strength was much less than its compressive strength,which was about 650 k Pa.The microscopic electron microscopy results of foam concrete in normal and compressed states show that the internal closed spherical pore size range has a normal random distribution with a maximum size radius of about 20 μm and a honeycomb thin-walled structure with a thickness varying from 5-10 μm.(3)The uniaxial compressive strength of PM-C25 and PM-C30 composite cubic specimens was about 25.2 MPa and 30.3 MPa,respectively,and normal compression behaviors were basically the same,with a total of three obvious peak points in the stress-deformation curves.The shear misalignment occurred on the structural surfaces of the composites,and all of them showed brittle shear damage,and the shear test curves were basically the same.The compressive stiffness and shear stiffness of the composite structure surface usually depend on the normal stress and cannot be kept constant,but the friction coefficient of the structure surface is basically constant,which is about 0.85 and 0.9 respectively.(4)The constant bearing and release-pressure behavior of foam concrete lining mainly originates from the pressure energy absorption of foam concrete buffer layer.Numerical calculation results show that: extrusion deformation tunnel initial support structure and secondary lining structure filled with foam concrete buffer layer can play the effect of pressure support,and the foam concrete buffer layer is set to 20 cm thick to let the best effect of pressure support.The buffer layer coordinates the deformation of the surrounding rock and the lining structure through its own compression,and induces the lining structure to normalize the stress state.Under the action of strata extrusion structure,the foot of the arch of foam concrete lining structure often appears stress concentration phenomenon,the size of the maximum main stress of the lining structure is roughly the initial support structure > secondary lining structure > foam concrete buffer layer,and the secondary lining structure is mainly compressive stress,the initial support structure is mainly tensile stress.(5)The horizontal convergence deformation of the tunnel is influenced by the extrusion of the strata,and when the lateral pressure coefficient <1.25,the horizontal internal intrusion deformation of the unilateral surrounding rock is smaller than the vertical internal intrusion deformation of the surrounding rock at the vault and supine arch positions.When the lateral pressure coefficient >1.25,the internal intrusion deformation in the horizontal direction of unilateral surrounding rock is larger than the internal intrusion deformation in the vertical direction of the surrounding rock at the top of the arch and the elevated arch position.When the lateral pressure coefficient = 1.25,the internal intrusion deformation in the horizontal direction of the unilateral surrounding rock is basically the same as the internal intrusion deformation in the vertical direction of the surrounding rock at the top of the arch and the elevated arch position.In addition,the stress levels in the tunnel surrounding rock all increase in the extrusion level.When the lateral pressure coefficient is ≤1,the stress level at the top of the arch and the elevated arch position is higher than that at the arch waist position;when the lateral pressure coefficient is ≥1.25,the stress level at the top of the arch and the elevated arch position is lower than that at the arch waist position.Comparing with the theoretical analysis,indoor test,microscopic inspection and numerical simulation results,it can be seen that the compression support structure can meet the long-term support demand of the extruded tunnel with large deformation caused by high ground stress and weak stratum,and shows good constant resistance to compression and coordinated deformation compared with the traditional support structure,which can absorb the appropriate amount of creep in the surrounding rock and provide stable support reaction force.In addition,the buffer layer in the pressure-allowing support system can weaken and balance the stress environment of the second lining structure,avoiding the occurrence of stress concentration and bias pressure,and ensuring the long-term stability of the surrounding rock and main structure of the extruded tunnel.