Research on river dike failure of short-distance coal seams mining under Hunchun River

Mining under river can cause dike subsidence and damage,which leads to the river overflowing and river dike loses stability. It reduces flood discharge capacity,therefore, flooding incidents happens. Coal mining under river and river dike protection should be considered and carried out together. Taking the mining of short-distance coal seams under Hunchun River as an example,quaternary gravel aquifer,weathering rock aquifer and Hunchun River have close hydraulic connections by analyzing the relationship of aquifers and surface water in the mining field. The bedrock on coal seams are medium hard and is easily soften. The mudstone is easily swelled and disintegrated, which can restrain the fracture extending. The maximum fissure zone height is 56.95 m in the working face of short-distance coal seams and the fracture cannot reach to Hunchun River. In order to not influence the quaternary gravel aquifer,weathering rock aquifer and Hunchun River after mining activities,water-proof safety coal and rock pillar was designed under the weathering belt. The calculation showed the upper limits of mining elevation of 19_b and 20 coal seams were-110 m and-134 m, respectively. The influence of mining on river dike was evaluated through probability integral method,which indicated the mining influence area and settlement extent of river dike were enlarged. The maximum horizontal deformation of the dike with 3.4 mm/m is allowed. However,a certain subsidence occurred and the maximum subsidence reached to 2.61 m which is lower than the highest flood level of past years,so it is needed to put forward safeguard procedures for the river bank. The technology proposed has significant meaning for river bank protection with short-distance coal seams mining under Hunchun River.