Abstract: In order to explore the seepage failure of thick loose layer bottom soil, the ultra-thick loose layer in the fourth shallow mining area of Xinglongzhuang Coal Mine is set as an example. Based on the particle grading, microscopic testing, anti- seepage test and the theoretical calculation of the safe critical head height in two groups soils. the critical conditions of water inrush and sand collapse in the overlying aquifer of No.3 coal seam in Shanxi formation are investigated. It is found that the particle grading and color changes of the two groups soil samples have significant differences. The percentage of particle size (less than 0.075 mm), the upper group soil is lower than 30%, while the lower group soil is up to more than 70%. The upper group soil falls into the type of feldspar, with feldspar accounting for more than 90%. The structure is loose and the content of SiO₂ is high, thus belonging to the typical sandy soil type. The lower group soil is mainly kaolinite, accounting for more than 70%. With compact structure and high Al₂O₃ content, they belong to typical clay soil. The quantized values of critical water pressure and critical hydraulic gradient at different crack widths are obtained. With the increasing of crack width, both critical water pressure and critical hydraulic gradient show a gradual decline trend, and an obvious inflection point appears at 20 mm crack. The safe critical water head height was calculated theoretically. The allowable safe water head height of the bottom aquifer of the Quaternary is preliminarily obtained between 54 m and 70 m. Compared with the actual water head value, the analysis showed that the sand collapse and penetration failure would not occur in this area under normal mining conditions.