Construction of isolation layers for preventing spontaneous combustion of coal gangue dump and its effects

Abstract: Spontaneous combustion occurs easily in open-air stacking coal gangue dumps, which leads to serious geological disasters and environmental pollution. To improve the traditional direct soil cover methods of coal gangue dump treatments, a new isolation layer with fly ash and clay soil for preventing spontaneous combustion of a coal gangue dump was developed, which was placed as a cover on the surface of the coal gangue dump and before the topsoil covers. The study area of this research was a slope in the east part of the coal gangue dump of NO.3 Colliery, Yangquan Group. The area was divided to four plots. Each plot was more than 200 m2. Different kinds of isolation layers were applied in each plot. The isolation layers were designed as fly ash (FF), clay (NN), layered fly ash, and clay (FNC) and a mixture of fly ash and clay (FNH). FNC was composed of fly ash first and then clay, and its thickness was 10cm and 20cm respectively. FNH was designed with the proportion of fly ash: clay =1:2, and its thickness was 30cm after being ground twice by 4t rolling.One year after the isolation layers were constructed, the samples were taken from the study area at regular intervals. The result of a dynamic test on the permeability of the samples showed that the utilization of a isolation layer of fly ash and clay before the soil covering could block the air effectively. The permeability of the FF or NN layer was lower than the critical value (0.2×10-9 m2) of the spontaneous combustion at the beginning, however, as time went by, the value of permeability increased, and finally could not effectively block the air. The effect of the mixture of the two materials was better than only one material. The experiment showed that the mixture with the fly ash: clay =1:2 (volume ratio) proportion had the best blocking effect, and its permeability was below 50×10-12 m2. By testing the surface temperature and compaction after one year of revegetation, the result showed that the temperature of the FNC and FNH applied area was more stable with increasing depth which was between 10-15°in the depth of 10~30cm, indicating the good effect of blocking oxygen. The values of compaction of the different layers were increasing sharply with increasing depth, which formed an ideal profile for the vegetation. FF had the highest compaction value. FNC was the second, and the top value of its compaction was about 2 800 kPa in the depth of 30.48-38.12 cm. The compaction value of FNH was changed slightly due to the different depth. This research provides reference data and a method for the configuration of isolation layers for preventing spontaneous combustion of coal gangue dumps.