| Initial support would be used as an important construction of tunnel.Themechanical analysis of initial support is one of key problems for tunnel andunderground engeering for long time. However, the engineering application of is farahead of theoretical research of initial support in tunnel. It is not completely clear for thesupporting mechanics, and exit many faults. The paper takes tunnel and undergroundengineering as studying background, to study the mechanics of tunnel initial supportfrom different aspects and different viewpoints, for obtaining the deeply levelsupporting mechanics, and providing theory proofs for tunnel design. Combined withSupported by Program for Changjiang Scholars and Innovative Research Team inUniversity (IRT1045), took Cuntan tunnel in Chongqing as an engineering example, onthe basis of previous research, the mechanical characteristics and action mechanism oftunnel initial support is systemic and thoroughly researched by theory analysis,numerical simulation and in-situ test. The main works and achievement of thesis can bedrawn as follows:①The bolt’s axial displacement differential equation is established according tothe stress equilibrium in a small section of a rock bolt and the sheart stress transfermechanism of anchorage body interfacial layer between rock bolt and surroundingrock.Then the axial load distribution function and the shear stress distribution functionon the interface of the fully grouted bolt are obtained by solving the differentialequation under the interaction between rock bolt and surrounding rock mass.Thereaction of the shear stress on interface of the fully grouted bolt on the surrounding rockis assumed to be symmetric radial action. The plastic zone radius of circular tunnel forbolt-shotcrete support can be solved by means of Mohr-Coulomb yield condition.On thebasis of the model and function above, the mechanical effect of system for surroundingrock and initial support is analyzed. The selection of supporting time has great influenceon anchoring effect and surrounding rock stability; It is suggested that increasing thethickness of the grout layer properly can decrease efficiently shear stress concentrationat bolt end and also can improve the reinforcement effect of fully grouted rock bolt.②According to the characteristics of interfacial stess between surrounding rockand shotcrete support,the article established mechanical model of composite curvedbeame for support system of tunnel surrounding rock and shotcrete lining, which considering the interfacial deformation coordination and the effects of bearing arch ofsurrounding rock. Then from static equilibrium of two differential elements, thedifferential equation for the radial displacement of composite curved beam are derivedout. All analytical formulas of both interfacial stress and internal forces betweensurrounding rock and shotcrete lining by arbitary distribution load are derived.And thusthe mechanical condition of interface zone and the stress concentration position areobtained,which is convenient to guideline for assessment of stability of surroundingrock and and prediction of safety. Finally，the analysis of tunnel project excavated bybenth method shows that shotcrete lining made the formation of compressive stresszone in surrounding rock by transferring stress from the interface between surroundingrock and shotcrete,which is beneficial to improve the stability of surrounding rock.③According to the characteristics of shotcrete layer in initial support and thetheory of circular beam on elastic foundation,based on touch stresses between shotcretelayer and surrounding rock,the analytical solution of internal force is deduced.The stressconcentration position is obtained,which is convenient to make relative safetycountermeasures in the following construction.The analysis of tunnel project excavatedby bench method shows that the analytical expression of solving internal force based ontouch stresses is a new and effective way,so as to provide a more reliable basis for thesafety assessment of tunnel structures.④Prompt analysis of the internal force of tunnel lining in weak-brokensurrounding rocks is one of key problems for tunnel construction,useful foridentification of construction safety state and evaluation of stability of the supportsystem.Steel arch support would be used as an important primary support of tunnelwhen the surrounding rock is weak and broken.According to its characteristics andmechanical properties in-situ,a mechanical model of steel arch and shotcrete isestablished.Analytical solutions of the internal force of the composite support arededuced based on the theory of circular beam on elastic foundation and steel archstresses measurement.As a result,stress concentrated parts of tunnel lining can bequickly identified.An example of a tunnel excavated by using the bench method verifiedthe method.⑤The combined support method of rock bolt and shotcrete mesh and steel arch ininitial support of tunnel is widely used soft and broken surrounding rock，but it is verydeficient in understanding on bearing behavior of the combined supporting structure；and it still lacks quantitative analytical formulae especially for bearing capacities of primary supporting and secondary supporting.In allusion to the united supportingcharacteristics of rockbolt，shotcrete layer and steel sets of wall rock，the mechanicalmodel of overlap arch bearing body were put forward based on rock mechanics theory，which are composed with the outer compression arch(bolt supporting)and the innercompression arch(shotcrete and steel arch supporting).By means of elastoplastic theory，formulas of the bearing body of combined support were deduced.Then following theNATM prinsiple to transfer fully the ability of surrounding rock so that the deformationpressure is minimum on the combined support. Through the medium of establishing therelation between support displacement and structure stiffness,by means of optimizationmethod,to adjust support stiffness, finally gives reasonable supporting parameters. |
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