Mechanism and prevention of water-sand inrush in soft rock with weakly abundant water:A case study in Shanghai temple mining area

There are many water-sand inrush incidents with intermittent characteristics in soft rocks with weakly abun- dant water in the Jurassic coalfield in recent years. Large amount of water (sand) in a short time,and lack of effective technical preventive measures seriously threaten a safe coal production. In order to obtain the mechanism of water-sand inrush and to formulate effective preventive and control measures,a case study was conducted at New Shanghai No. 1 Mine. The parameters such as rock mechanical strength and softening coefficient were tested in the laboratory and the rock was synthetically identified as a typical soft rock formation. Orthogonal polarizers show a low maturity of sand- stone,and a clay mineral cement. Hydrological tests show that the mud cement of sandstone particles are prone to ero- sion,can form a solid-liquid bipolar mixture,and shows quicksand properties under hydrodynamic conditions. Mud- stone easily absorbs water,expands and becomes muddy. Based on the long-term observations of aquifer water level and surface rock movement,it is assumed that delamination can occur in both low and high overlying rocks. In addi- tion,the delamination fissure has the function of collecting water source. Considering the overlap of the water length curve and the water length curve of the aquifer in the process of sand breaking in 111084 working surface,there is ob- vious correlation between the water burst and the change of water level in the aquifer. At the beginning,there is a large amount of water and the water level drops significantly. There are several jumps in the process of water volume decrea- sing,meanwhile several corresponding jumps in the water level of the aquifer during the process of rising. The ampli- tude of jumps (drops) decreases. The interval between the two jumps is extended. Based on the above information,it is proposed that the effect of water collection in delamination fissure can strengthen the short-term water burst strength of weak aquifer,and weak cemented sandstone provides a source of mixed water inrush. Water dynamics change during the release of delamination water. With the release of water,the power of water release decreases,and a large amount of sediment seals the water burst channels. It shows a self-sealing effect of sediment. As the overburden continues to com- press,the water storage capacity of delamination layer is gradually weakened,showing a sudden decrease in water lev- el. The progressive expansion of delamination layer water recharge routes,showing a terraced extension of the water burst rest period. The process of recharging-flooding-plugging-recharging-replugging is alternating. The water burst mechanism of intermittent water-sand mixture is revealed. The occurrence of delamination layer water damage requires five necessary conditions such as water source ( water-rich),time,space,channel,and pressure,emphasizing the im- portance of water diversion channel for water diversion fissures. The application practice shows that pre-drying of roof water can change the water-rich condition of the aquifer. Changing the water inrush channel and pressure condition with prearranging guide tube,this kind of accident can be prevented effectively,ensuring the safety of mine production. At the same time,it can realize mining without water dripping from the roof and in the goaf area,optimize the op- erating environment of the mining face,weaken the soft rock effect,and promote the efficient production of the mine. Key words: weakly water-abundant soft rock; water inrush with sand; bed separation water; mud-sand self-sealing