Upper groundwater level threshold in Yushen coal mining area

Reducing the water loss during coal mining is the key task for water resource management in the ecologically fragile coal mining areas.Quantifying the relationship between the buried depth of groundwater table and phreatic evaporation rate can provide some scientific thoughts for mining in arid areas with water protection in western China.According to the difference between the thickness of the impermeable layer and the height of the water conducting fissure zone,this paper divided the groundwater level variation trend in Yushen mining area into different parts.A series of tests were conducted to analyze the process of groundwater evaporation and the law of phreatic evaporation at different groundwater levels.By constructing two tangent lines at the turning point of the water characteristic curve,it deduced the analytic formula about the upper limitation threshold of the buried depth of groundwater table.The soil and water characteristic curve of aeolian sand was measured with the funnel method in Yushen mining areas,based on which,the limitation threshold of groundwater buried depth in Yushen mining area was obtained.The results show that the trend of groundwater table changes in Yushen mining area can be divided into three parts,which include shallower region,the transition region and deeper region.The most of groundwater shallower region locates at the Phase III and Phase IV planning areas in Yushen.The areas with the depth of groundwater level less than 4 m accounts for 59.1% of the mining areas,the mining subsidence easily causes groundwater to be buried shallow or exposed.The evaporation process of aeolian sand in Yushen mining areas can be divided into two stages,namely,the steady evaporation stage and the water vapor diffusion stage.The water transport mechanism changes when the groundwater level is about 0.5 m,and the evaporation turns to the water vapor diffusion stage.The equation of soil water transport was established under the condition of steady evaporation,and then the calculation formula of upper limitation threshold of groundwater depth was deduced.The upper limitation threshold of groundwater depth is related to the height of capillary rise and the air-entry pressure,which is equal to the matrix potential of topsoil when the hydraulic connection of capillary between topsoil and groundwater is broken.Furthermore,the relationship between the upper limitation threshold of groundwater depth and the fitting parameters α,n of van Genuchten equation was established.Based on the parameters of soil and water characteristic curve of aeolian sand,the upper threshold of groundwater depth in Yushen mining areas was confirmed to be 50 cm,which is in common with the experiment results.It is a common problem that the buried depth of groundwater table become shallower during the mining of mid-deep coal seams.For Yushen mining areas,being located in the arid and semi arid areas,controlling the upper limitation of buried depth of groundwater table has become a new scientific problem to be solved in the mining of coal seams.