| Design and construction of crossing-mountain tunnel often faces considerable safety problems due to high external hydraulic pressure and water inrush. To avoid such problems and protect groundwater resources in the area of tunnel through mountain,the principle of water blockage combined with water drainage is put forward. For a deep-lying mountain tunnel, how to evaluate the regularities of distribution and the intensity of the external pressure is identified as a vital issue to ensure the construction safety and protect environment.This paper discusses the controlling mechanism of the hydrogeological structure on groundwater seepage flow. Based on 61 case studies of tunnels with water inrush hazard, geological environmental conditions and the relationships between water stress and water yield are analyzed. According to the investigation of geological factors and permeability in the area of crossing-mountain tunnel, the hydrogeological structure is classified ashomogeneous hydrogeological structure, nearly horizontal stratified hydrogeological structure, monoclonal stratified hydrogeological structure and complex hydrogeological structure.And then, the calculation method and regularities of distribution of water pressure in the homogeneous hydrogeological structure and nearly horizontal stratified hydrogeological structure are presented, at the same time, regularities of distribution of external water pressure in monoclin alstratified hydrogeological structure is also analyzed. The main conclusions are highlighted as follows:(1)Geological environmental conditions, including formation lithology, geologic structure and landform, in the area of crossing-mountain tunnel with water inrush hazard are analyzed. The results show that, in the area of a crossing-mountain tunnelwith water inrush hazard, the formation lithology mainly includes solubility rock strata and sandstone strata and the dangerousgeological structures are identified as drape and fault.(2)Based on the former research about hydrogeological structure of mine, landslide and valley, the hydrogeological structure system of crossing-mountain tunnel is established.The permeability spatial structure characteristics of main medium, rock output state and void type are regarded as dividing basis. The hydrogeological structure of crossing-mountain tunnel is divided into six classifications, namely homogeneoushydrogeological structure, stratified hydrogeological structure, veinhydrogeologicalstructure, fractured composite structure, karst composite structure and mixed composite structure.(3)In homogeneous hydrogeologicalstructure, groundwater stress decreases during the tunneling process. Generally, there is a positive relationship between the permeability of homogeneous structure and the decrease of groundwater stress in a no-support tunnel.In the case of a support tunnel, the groundwater stress nearby the lining should decline quickly and a negative relationship between the permeability of homogeneous structure and the decrease of groundwater stress is unveiled.(4)The groundwater stress is influenced by the distance between the tunnel and the permeability-change boundary in both support and no-support tunnels. When the permeability of the upper layer is greater than that of the lower one, the groundwater stress nearby the free surface of tunnel gradually increases with the decrease of distance between the tunnel and the permeability-change boundary and a threshold of the water stress is found. The calculation of tunnel external water stress is classified based on the relationship of the distance between the tunnel and the permeability-change boundary and the impact distance of the groundwater stress.(5)In a nearly horizontal stratified structure system, if the permeability of the upper layer is smaller than that of the lower one, seepage field is dominated by stronger permeability rock. The groundwater flows mainly in stronger permeability rock in the horizontal direction.The groundwater stress nearby the free surface of tunnel generally decreases with the decrease of distance between the tunnel and the permeability-change boundary, and gradually stabilizes. External water pressure can be calculated with the axisymmetrical analytical solution formula.(6)In upright rock stratum with the development of monoclinal stratified hydrogeological structure, in a no-support tunnel,upright rock stratum is classified to three models,namely“before strong, after weak”, “before weak, after strong”, “independence”.In a support tunnel,upright rock stratum is classified to four models, namely“before strong, after weak”, “before weak, middle strong, after weak”, “beforestrong, middle weak, afterstrong”, “beforeweak, afterstrong”. Each model has its own rules of external water pressure.(7)Ina systems with the development of monoclonal stratified hydrogeological structure with 45°dip angle terrane, in a no- support tunnel, Both in a no- support tunnel and in a support tunnel, in frontal rock stratum with smaller permeability, there is a negative relationship between external water pressure and distance of section and permeability-change boundary, to a maximum value at the permeability-change boundary. In a no- support tunnel, external water pressure increases suddenly at permeability-change boundary. In a support tunnel, external water pressure keeps a constant value in rock stratum with bigger permeability.(8)The analysis method of external water pressure in the area of crossing-mountain tunnelsis developed based on the case study of SANDU tunnel.. First of all, the area of the crossing-mountain tunnel can be divided into several sections with different hydrogeological structures, and then the values and the rules of distribution of the external water pressure in different hydrogeological structure systems are analyzed. |
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