Soil and water conservation in mining area based on ground surface subsidence control: development of a high-water swelling material and its application in backfilling mining
It has always been a crucial issue in the mining industry that the surface subsidence induced by underground coal mining can lead to the severe deterioration or loss of soil and water resources in mining area. Backfilling has become one of the most frequently used mining method to solve this puzzle, as it can effectively control the movement of overlying strata and therefore soil and water resources will not be destroyed by mining activities. A new type of high-water swelling material (HWSM) was developed to address the key issues of current backfilling materials used in collieries, such as the high cost, inadequate strength and low expansion rate. Based on the principle that the strength of backfilling body satisfies the requirement, a critical ratio of water to solid material (a mixture of fly ash which was used as an aggregate, lime, anhydrite, cement and additive ingredients) was determined which varied from 0.8:1 to 0.9:1. Meanwhile, the mass ratio of fly ash, anhydrite, lime, cement and additives was determined to be 24.6:0.8:5.1:1.8:1. Laboratory tests showed that strength of samples that met the required setting time for practical use could reach 5.39-5.52 MPa after 63 days. The expansion rate after 3 h varied between 9.9 and 10.8 % and the bleeding rate ranged from 0.3 to 0.33 %. Industrial application has been conducted in Wangtaipu colliery and results showed that the proposed HWSM had an outstanding mechanical performance with good expansion rates and fluidity. These properties indicate that the developed material is suitable for use as a good backfilling material in mining so as to effectively reduce the ground surface subsidence and to conserve the soil and water.