Discovery And Significance Of Water-Gas Pressure Fluctuations Within Karst Cavity

Natural or artificial action leads to a rapid decline of groundwater level,causing changes in water and gas pressure in karst cavities.In order to explore the law of water pressure changes,this paper takes Dachengqiao subsidence area in Ningxiang,Hunan Province,and Jili area in Laibin City,Guangxi as the research areas.Based on the analysis of karst subsidence characteristics,and the monitoring data of water pressure in these areas,the geological model was generalized.The physical experimental model was designed to simulate the underground karst cavity with good ventilation conditions,relative closeness,closeness and communication.Pressure sensors were set up in different parts to monitor water and air pressure in simulated karst caverns.The experiments show that:(1)When the water level of karst cavern decreases in a good ventilation,the water pressure in the cavern changes steadily and has little influence on the cavern.(2)When the water level of the karst cavity decreases in a relatively closed environment,the water pressure in the cavity is always in a negative pressure state and fluctuates regularly and frequently.The higher the initial water level,the greater the negative pressure of water in the cavity,and the greater the fluctuation frequency.The larger the volume of karst cavity is,the larger the negative pressure of water can reach.But when the initial water level is the same,the smaller the negative pressure of air is,the larger the negative pressure of water is.(3)When the water level of karst cavity decreases under the condition of closed-open-closed,the water pressure in the closed stage still shows regular high-frequency fluctuation characteristics.When the closed and open states are switched,the abrupt change of water pressure is much larger than that of the closed situation.(4)During the process of karst cavity water level lowering,the gas pressure is lagging behind the "water&gas" pressure fluctuation,but the fluctuation frequency and trend of the two are consistent.(5)The change of water-gas pressure caused by the decrease of water level in karst cavity under closed condition can be described by a linear function of time variable.The slope of the function is affected by the water outlet and the cross-sectional area of karst cavity.According to the literature review,it is found that the phenomenon of high frequency vibration of water pressure under closed condition belongs to pressure pulsation,and the influence of pressure pulsation on karst collapse is analyzed.It is also proposed that the karst cavity in the relatively closed state water level decline process,the water vapor pressure in the cavity through the karst pore fracture acting on the overlying soil layer is not only the static pressure of the negative pressure,but also caused by the pressure pulsation of the dynamic pressure.the combination of the two causes the creation and expansion of the soil hole,and eventually causes collapse.In the prevention and treatment of karst collapse,by setting different size ventilation holes in different volume simulated karst caverns,and referring to the design principle of impedance pore size in impedance surge chamber of hydropower station,this paper explores the appropriate pore size value of borehole ventilation method to prevent and treat karst collapse,and finally gets the pore size evaluation method to eliminate pressure fluctuation of karst cavern and effectively balance water pressure in the cavern.