Study On The Mechanism Of Yielding Support For Deep Tunnel In Weak Rock

With the promotion of “Belt and Road Initiative” strategy in China,major infrastructure projects have been implemented successively in the western area.Due to the special geographical conditions,the tunnel has become an optimal choice in the mountainous area.In the tunnel construction,the conventional support often fails owning to the large deformation of weak rock,the control of large deformation of weak rock in deep tunnels has become an urgent engineering problem to be solved.Therefore,it is of great significance for researchers and engineers to study the mechanical mechanism of deformation control in the project of deep tunnels in weak rock.The dissertation deeply analyzes the mechanical characteristics of the initial support failure when the tunnel undergoes large deformation based on the nonlinear large deformation mechanics theory and the geological characteristics of deep tunnels in weak rock tunnel.Combined with the tunnel project under construction,field tests are carried out to analyze the in-situ surrounding rock strength,relaxation zone,geo-stress,anti-pulling of anchor bar,and high-strength shotcrete,etc.On such basis,mechanical and numerical analysis of yielding support in deep tunnels in weak rock are implemented,the difference and relation between “large deformation” and “Large mechanical deformation” are dissected.Mechanical mechanism of deformation and failure of conventional bolt-shotcrete support is revealed,then the principles and methods of application of yielding support are proposed.This dissertation can be concluded as follows:(1)Firstly,the failure mechanism of conventional bolt-shotcrete support is studied.The initial support plays a more “structural” function within the deep tunnel in weak rock,the overall bending deformation is the main failure form.Normally,the tensile strength and ultimate strain of shotcrete are very low.After tensile cracking and crushing,the shotcrete will lose its restraint on the steel arches,then the steel arches partially undergoes bending and torsional instability.Among the conventional support,the supporting measures are chosen based on surrounding rock classification without carefully considering mechanical characteristics,which will lead to incompatibility between the initial support's capacity of deformation and the deformation of surrounding rock.This is one of the main reasons for deforming failure of deep tunnels in weak rock.(2)The applicability of material nonlinearity,geometrical nonlinearity and contact nonlinearity in the mechanical calculation of deep tunnels in weak rock is studied.Moreover,the external manifestation of “large deformation” and the mechanical nature of “large deformation” are analyzed.The essence of the deformation of weak surrounding rock is the brittle failure of the rock and the non-linearity of the structural surface contact.Furthermore,the number and properties of the contact surfaces are changing during the tunnel deformation process.Continuum model is often used for numerical analysis of weak rock tunnels,and the nonlinearity of contact between the structural surfaces of rock mass is approximately represented by the material nonlinearity of the rock mass.For the initial support,geometric nonlinearity is vital for the bearing capacity of the support structure,should be considered.And the large deformation mechanics theory should be adopted during mechanical calculation.(3)A new method of bending yielding support is proposed by regarding the steel arches and shotcrete as two independent supporting stages in the initial support.By enhancing the rigidity of the out-of-plane support of the steel arches and the stability of the arch springing,making the utmost of the ductility of the steel,flexibly bending and yielding support are achieved.Moreover,according to the deformation monitoring and the stress and strain of the steel arches,a rigid arch-shotcrete supporting structure is established.(4)“deforming largely and avoiding large mechanical deformation” is appointed as the object for the initial support design of deep tunnels in weak rock,and the method of circular yielding support is proposed.The mechanical mechanism of circular yielding support is also studied.The conventional support mode which mainly based on bending deformation is changed in the circular yielding support.The axial force of the initial support is used to shorten the support circumference,to reduce the adverse effects of geometric non-linearity.Additionally,its yielding deformation is controllable,which will protect surrounding rock and initial support.A metal buckling hoop yielding device with low compressive stiffness is developed.Based on the mechanical characteristics of the yielding device,the yielding device is suggested to set at a place with small bending moment(or bending deformation).Furthermore,the shear force at such location is often large,and the yielding device should behave “strong shear and weak pressure” to ensure the rigidity and bearing capacity of resisting shear stress.(5)Then the mechanical mechanism of yielding prestressed anchor cable in large deformation control of deep tunnel in weak rock is studied.A new anchor bearing plate is designed to provide low pre-stressing force in the flexible yielding stage,and to reconstruct the three-dimensional stress state of the surrounding rock.The coordinated deformation of the surrounding rock,initial support and anchor cable is realized.When the yielding process is over,the main function of prestressed anchor cables on the initial support is to reduce the span,thereby to increase the rigidity and bearing capacity of the supporting structure.Then the supporting system with high bearing capacity and rigidity is constructed following the rule of “firstly flexible and then rigid”.(6)Based on the basic theory of structural strength,stiffness and stability,it is clear that the premise of yielding support is the determination of rheological property and the shear resistance of the surrounding rock.The goal of yielding support is to achieve“rigid-flexible-rigid” stiffness-changeable initial support,and to achieve flexible support by reducing the local compressive stiffness of the initial support,to achieve rigid support by increasing bending stiffness.Using the method of structural concept analysis,the tunnel support is divided into two parts: surrounding rock structure and initial support.The method of stabilizing surrounding rock is inspired from the stabilizing theory of arch structure.For the initial support,it is an important principle to keep the bearing capacity during the deformation and yielding process.The combination of prestressed yielding anchor cable and circular yielding support,reducing the adverse effects of geometric non-linearity on the initial support,achieving the rigid-strong support by using the span-reducing effect of prestressed anchor cables.Based on overlapping structure theory,the properties of surrounding rock are improved,the shear strength of contact between surrounding rock and shotcrete is also reinforced to ensure the integration of initial support and surrounding rock.