| Stability of surrounding rock around tunnel portal is very important to the entire tunnel construction. Tunnel portals are shallow and unsymmetrical loading in most cases, and the surrounding rock likely in poor performance, easily weathered and affected by rainfall. In order to ensure the stability of the excavation process, rheological properties and creep under different water content should be considered. At present, research in this area is still very limited,especially creep of cataclastic low-grade metamorphic rock fragments under different water status.In view of this, based on the support projects of typical tunnel entrances of Xiamen-Chengdu expressway in Guizhou province, combined with the existing research, more systematic and in-depth study on creep of low-grade metamorphic rock fragments around tunnel portal and application in the tunnel entrance engineering were conducted. The main contents of this paper as following:①Study on the basic mechanical properties and compression creep of low-grade metamorphic slate from project region. Several basic mechanical parameters under natural air-dry and saturation were determined by conventional uniaxial tests and triaxial tests. The relationship between saturation and soaking time were obtained by water immersion tests. Strain-time curves under different moisture content were gained by creep tests, and the creep law of low-grade metamorphic slate under different stress and moisture content were analyzed.②Development and verification of creep constitutive model of low-grade metamorphic slate considering water damage. Identifying the model by test results, introducing concept of water damage of rock and defining the factors, and the damage evolution equation is derived from the test results. Creep equation of low-grade metamorphic slate considering water damage was established, and extended to three-dimensional stress state. Then the derivation of finite difference equations was played in the basis of finite difference theory, and combined with the environment of FLAC3D to complete model of the secondary development, which is verified by examples.③Research on the mechanical behaviors of dynamic construction and excavation process optimization of the shallow tunnel under unsymmetrical loadings. Large-scale three-dimensional simulation geological model were built to study the optimization of excavation process comparing the annular pilot heading conservation core soil method and CRD method using developed model, and some basic laws obtained. The analysis results of stress characteristics of surrounding rock and supporting structures can provide technical guidance for the design and construction of tunnel which has the similar topographical and geologic conditions.④The relations between stability of the shallow and unsymmetrical loadings tunnel and topographic factors. Most of the existing research focuses on practical engineering, lacking of general applicability. So, topographic factors such as thickness of side covering soil, transversal gradient, maximum burial depth and so on were considered to design orthogonal tests to study the relationship of factors affecting the stability of the shallow and unsymmetrical loading tunnel.⑤Research on the pre-reinforcement technology of portal section of shallow tunnel with unsymmetrical loadings. For different types of portal section, calculation and analysis were performed using FLAC3D software to comparing the stress and deformation characteristics of tunnel under different pre-reinforcement technologies. Principles of selecting compatible pre-reinforcement technology for different types of portal section were obtained. On this basis, several pre-reinforcement technical points were proposed.⑥Research on the calculation method of lining loads of shallow tunnel with unsymmetrical loadings. There are many theoretical calculations of rock pressure, while few for shallow tunnel with unsymmetrical loadings. The highway tunnel specification gives the calculation method of shallow tunnel and unsymmetrical loaded tunnel respectively. The calculation theory of unsymmetrical loaded tunnel is similar with the shallow tunnel, and the assumptions are different from the actual situation. Therefore, a series of corresponding formulas are deduced based on limit equilibrium theory for calculation lining loads of shallow tunnel under unsymmetrical loadings through engineering case analysis and the three-dimensional numerical analysis results of the tunnel... |
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