Research On The Mechanism Of Surrounding Rock Large Deformation And Its Prevention In Longxi Tunnel Of Duwen Road

In the tunnel and underground engineering, the weak surrounding rock under high geostress, in conjunction with underground water and self-swelling effect, the rocks began to lose its self-bearing totally or partially, causing progressive and apparent deformation which cannot be bonded effectively. This is known as the Great Wall Rock Distortion. In this case, it is different from brittle failure (rock burst), collapse and slip which is confined to certain structure plane in the surrounding rock loose circle.The Great Wall Rock Distortion is a kind of hazardous geology casualty in underground engineering, which is capable of destroying supporting structure, causing clearance-intrusion , collapsing, even stop tunnel, damage equipment and cause casualty, if processed not appropriate. By now, the Duwen highway Longxi Tunnel has happened several times large deformation hazards. Finding out the mechanism of large deformation , lauch forcast for large deformation and taking effective control measurements is very important to assure the smoothly construction of Longxi Tunnel. At the sametime, have important academic value on theory of large deformation research.The dissertation detailedly explains the engineering geology conditions of Logxi Tunnel area, and the distributions feature and cause of geostress field were researched.On the basis of mounts of site tracing investigations, the formation mechanism for large deformation of Longxi Tunnel has researched. On the base of engineer geology analysis, applying the modern support theory and construction conception of underground engineer, and combining with modern monitor technique and numerical simulation, has researched and discussed about support, forecast and informationdesign-construct etc, two phase on large deformation' prevention. Obtain these chief research results.(1) Adopt stress relieving method on spot, the 3D geostress was obtained. Combine analysis of structural features and Seismic Geology, think it is reliable that geostress stress get from relieving method, that is the direction of of maximum principal stress is NE , intersecting tunnel axis with large-angle which is unfavorable to stability of surrounding rock. The maximum principal stress value is 24～29MPa, and 21～24MPa in cross section .According to "Standard for engineering classification of rock masses" (GB 50218-94), In tunnel line, the wall rock was placed in high stress state comparatively. By analysising on conformation characteristics and survey on the occurrence of surrounding rock, obtain that tectonic stress amount was consistent in certain province by geostress point back and forth, and proposed a method of estimating geostress base on Superposition Principle.(2) According to tracing research of tunnel surrounding rock geological characteristic, deformation and fracture characteristic, profoundly researching on formation mechanism and destroy mode of Longxi tunnel great wall rock distortion was done.Considered the Longxi tunnel large deformation mechanism include soft rock plastic flow, plastic-shear slip and lamellate soft rock bending. Among them, further, the plastic-shear slip was divided to shear-plastic slip of wedges, plastic-shear slip and unloading-spring back shear slip.(3) Theoretically, several predicted basic problem, such as the purpose and meaning of prediction, the prediction of contents, stage of prediction, principle, method, parameter, standard and process, and establish of prediction system were expounded, According to actual condition of Longxi tunnel, the prediction principle was proposed, which is the rock mass structure and geostress must be considered, and follow dynamic prediction and integrated analysis. As a result a suite of scheme for predicting large deformation in Longxi tunnel was established .By apply to typical section of tunnel,the applicability and effectivity were proved.(4) After studying on traditional sress-strength ratio, the modificative sress-strength ratio was proposed, which parameters adopt the maximum principal stress of cross section and rock mass strength, propound estimate rock mass strength by completeness coefficient and empirical formula basing on Hoek-Brown rule, which is more correspond to actual condition.By taking existing large deformation tunnel for probating, the feasibility was proved preliminary.(5) According to thoughts of NATM and "Technical Specifications for Construction of Highway Tunnel" evaluating the condition of NATM application in Longxi tunnel, adopt FEM method to proceed numerical simulation analysis to various support structural parameters under actual geostress .Aim at peculiar geological characteristic of Longxi tunnel, a suit of surrounding rock classification schedule which service the support design was established, and on base of above achievements, the support principle and schedule for large deformation were established too.