Study On Surrounding Rock Mechanical Response Mechanism Of In-situ Enlarged Tunnel

With the rapid development of economic construction, construction of traffic tunnel growths rapidly , and the demand for volume of traffic on the highway tunnel is also getting higher and higher.In some economically developed regions, two-way four-lane or even two-way six-lane highway tunnel is unable to meet the growing demand for traffic.The construction of builting two-way eight-lane highway tunnel or converting the existing highway tunnel to two-way eight-lane is specially urgent.Tunnel expansion in situ is expangding the tunnel sections on the basis of the original tunnel to form a larger span highway tunnel.The most significant feature of tunnel expansion in situ is the need to remove the supporting structure, construction process is complex. The removal of supports of the original tunnel and expansion digging will disturbe tunnel rock largerly, and rock stress path is complicated. The equilibrium state of rock body and the original structure supports are destroyed, and try to reach a new equilibrium and steady-state.,the stress and strain start a new process of change. These changes led to a stress redistribution in surrounding rock and spatial deformation towards tunnel digging direction.Relying on a tunnel project in Fujian, combined with theoretical analysis, numerical analysis and monitoring trial on-site together , the deformation characteristics and mechanical mechanism during tunnel expansion in-situ construction are explored. The results provide a theoretical basis for tunnel design, construction,and monitoring in tunnel expansion in situ,also can guide the implementation of highway tunnel extension project . The main research work contains:①The factors that affect rock classification include: rock saturated uniaxial compressive strength,rock integrity coefficient,weak structural surface occurrence,and the initial stress state, but to the tunnel for extension, the tunnel span also should be considered. Through taking depth analysis of various qualitative and quantitative factors, complete rock hierarchical fuzzy comprehensive evaluation system which considers the tunnel span factor is established.②The tunnel for expantion is non-circular,and to non-circular tunnel, looking for a single-valued analytic function, establish a one to one function between the infinite plane where the non-circular tunnel located and the infinite plane with a round hole in it .Through the function, a mathematical model for veranda shape of non-circular tunnel can be solved,which is called outland mapping function. Then using the plane elastic-plastic method to solve the stress and displacement in non-circular tunnel. Last,through the measurement of axial force , relevant state of surrounding rock in tunnel expansion in-situ can be analysised.③Tunnel expansion in-situ is divided into layers of peeling method and step method, using the equivalent circle method to simplify the calculation of tunnel expansion In-situ,then use the complex variable function theory and the Schwarz alternating method to solve the tunnel expansion In-situ problems.Finally,by analysing of rock stress and displacement characteristics , the width of a single extended dig can be determined.④Using the planar elastic-plastic finite element method and large-scale general- purpose finite element software ANSYS, highway tunnel excavation works will be numerically simulated. By establishing Isotropic elastic-plastic tunnel expansion in-situ construction mechanical model, displacement,stress state in surrounding rock,internal forces in lining structure,bolt axial force and other acts of construction mechanics in both expansion modes can be comprehensivelly analysed.⑤Combined with theoretical analysis, excavation sequence, controlled blasting and indexes of construction quality controlling system in the two kinds of expansion types will be studied.