Water-preserved coal mining technology in open pit based on cutoff wall with high fly ash content

The water inflow of an open pit mine in eastern Inner Mongolia is as high as 100 000 m3 / d. The main sup- ply source is Hailar River,which penetrates into the coal seam along the hidden layer of coal seam through the deep exposed sandstone and then flows into the pit. The drainage shaft is used to discharge the water,however,the method wastes a lot of water resources,damages ecological environment and increases production cost. A curtain wall can be built not only to cut off the water conduit route and reduce the mine water inflow,but also preserve water resources and mining environment. Therefore,the strength,impermeability,viscosity and the effect of high-content fly ash curtain wall building material are studied through experiments and theoretical analysis,then applied in the field for water cut. The results show that the compressive strength and impermeability of the filling material are positively correlated with the cement content when the water-solid ratio is 0. 7 ∶ 1. 0. The strength of the filling material with 80% fly ash content grows slowly in the early stage,but increases rapidly in the later stage. The compressive strength at 60 d and 90 d is 1.90 times and 3. 25 times of that at 28 d respectively. CaCl2 and NaCl could accelerate the activity of fly ash,promote hydration reaction,reduce inter-particle porosity,and significantly improve the early strength and impermeability. As the shear rate increases,the viscosity of the paste backfilling material has an obvious phenomenon of shear thinning. When the shear rate is 30 s-1 ,an inflection point appears showing shear thickening phenomenon,during which the bentonite could increase the viscosity and improve the impermeability by eliminating the phenomenon of shear thicken- ing. The water cutoff test on the high content fly ash paste backfilling material is conducted in the field of north side recharge area of the mine. It is found that water inflow is reduced by 24 000 m3 / d. The water level difference between inside and outside the curtain wall gradually increases,and the level outside the wall gradually recovers to normal with a water level buried depth of -6 ~ -7 m,which means it has recovered to ecological level required by vegetation. In conclusion,the notable effects have been achieved in cutting off water and preserving water resources and environ- ment,offering some ideas and references for green and water conservation coal mining in open pit mines.