Study On The Mechanism Of Water And Sand Inrush In Water-rich Sandy Dolomite Tunne

Sandy dolomite is a special geological phenomenon which is caused by weathering and dissolution in the complex environment of multi-stage tectonic movement,and it is widely distributed in southwest China.Sandy dolomite of the Water Diversion Project in Central Yunnan(WDPCY)has the characteristics of "wide distribution,long water-rich tunnel section and high water inrush frequency".The disaster of water and sand inrush frequently occurs in the tunnel passing through the water-rich sandy dolomite stratum,which poses a severe challenge to the safe and efficient construction of the tunnel.Based on the water-rich sandy dolomite tunnel in Yuxi Section of WDPCY,this paper systematically studies the process of water inrush,disaster characteristics and conditions,influencing factors,types of disaster-causing system and disaster-pregnant model of sandy dolomite in the water-rich section by means of theoretical analysis,construction tracking,geological survey and numerical simulation.The disaster mechanism of water and sand inrush in water-rich sandy dolomite tunnel is revealed from macro and micro scale.The research results may provide guidance for the prevention and control of water and sand inrush disaster of water-rich sandy dolomite and similar tunnel projects.The main conclusions are as follows:(1)Based on the case analysis of water and sand inrush in water-rich sandy dolomite tunnel,the disaster conditions,characteristics and influencing factors of water and sand inrush in water-rich sandy dolomite tunnel have been studied,and the disaster conditions also are obtained,which including energy storage degree,energy release condition and surrounding rock stability.Meanwhile,the temporal and spatial distribution characteristics(geological occurrence and time sequence characteristics)and precursor characteristics of the disaster of water-rich sanded dolomite tunnel are revealed.Finally,the influence factors of engineering geological,hydrogeological and engineering on water and sand inrush of tunnel is analyzed.(2)The disaster-induced water inrush system of water-rich sand-dolomite tunnel can be divided into fault type(water-blocking type,water-guiding type)and structural fracture type(monocline type,syncline type and anticline type),and the disaster-induced model of water-rich sandy dolomite tunnel can be divided into three types: direct exposure type,seepage instability type and intermittent damage type.(3)A three-dimensional numerical model was established by FLAC3 D to simulate the fluid-solid coupling of water and sand inrush in the sanded dolomite tunnel through the fault zone.The stress,displacement,plastic zone and seepage field in the process of tunnel excavation under the influence of factors such as the degree of sanding and fault were analyzed.The results show that there are four stages in which water inrush occurs near the excavation of water-rich sandy dolomite: the beginning of seepage flow,the development period,the water inrush and the steady period.The pore water pressure decreases abruptly after the continuous decrease.From the point of view of sandification degree and fault,the variation law of stress,displacement,plastic zone and seepage field during tunnel excavation is analyzed,and the mesoscopic mechanism of disaster caused by water and sand inrush in tunnel is revealed.The stress increases continuously and then drops sharply when the fault is encountered.The displacement increases slowly and then suddenly during the excavation.In the process of excavation of sandy dolomite tunnel,the plastic zone is mainly shear failure distributed above the arch waist and floor.The increase of sandy degree and the addition of groundwater enlarge the scope of shear failure of tunnel.In the process of tunnel crossing the fault,the plastic zone gradually shifts from the arch and the arch to the floor.The arch and the arch are the weak surface in the process of tunnel excavation.(4)The disaster mechanism and evolution law of water and sand inrush in the water-rich sandy dolomite tunnel are applied to the analysis of water and sand inrush disaster at position YX6+595.2 in the No.2 branch tunnel of Xiaopu tunnel,and the measures and suggestions are put forward for the prevention and control of water and sand inrush disaster.The research results can provide theoretical reference for the design and construction of similar projects.