Selection of alternative soil for filling reclamation with Yellow River sediment in coal-mining subsidence areas

Abstract: Underground coal mining results in large-area subsidence and severely disrupts agricultural production in eastern China, especially in Jining City, due to high underground water table and flat terrain. The most common method of reclaiming the subsidence areas in Jining City is filling reclamation. However, filling materials (such as fly ash, coal waste) have some disadvantages, such as heavy metal pollution and insufficient quantity. Jining City is located along the Yellow River, which has the largest sediment concentration in the world. Due to this, some researchers have proposed a method of filling subsidence areas with Yellow River sediment. However, in some subsidence areas, insufficient soil cover on sediment layer is a critical problem; in addition, the soil texture of some areas is clay which is not good for crop growth. For these reasons, we proposed a method of alternative soil over Yellow River sediment layer. The alternative soil was composed of topsoil, subsoil and Yellow River sediment, and the proportions were 1:1:0, 1:1:0.86, 1:1:1.33, 1:1:2, 1:1:3, 1:1:4.67 and 0:0:1, respectively. The research results showed that, alternative soil favored maize growth, especially root system, and the dry biomass of maize planting in alternative soil with the proportions of 1:1:1.33-1:1:2 was significantly larger than the control treatment (P<0.05). The texture of alternative soil with the proportions of 1:1:0.86-1:1:2 was loam, which is good for crop growth. The bulk density and the density of alternative soil with the proportions of 1:1:0.86-1:1:2 were 1.38-1.41 and 2.64-2.68 g/cm3 respectively, which were suitable for crop growth. Compared with the control treatment, the alternative soil had a considerable reduction in pH value and electrical conductivity (P<0.05), which was not harmful to crop growth. The infiltration rate of alternative soil was increased, which favored water storage capacity, and the alternative soil with the proportions of 1:1:0.86-1:1:2 had the similar infiltration rate to natural farmland. As for soil-water characteristic curves, water content of the alternative soil decreased with the increasing of sediment content under the same soil water suction. Furthermore, capillary porosity, available water porosity and water constants of alternative soil were also roughly decreased with the increasing of sediment content, which meant that soil available water supply capacity and water-holding capacity were decreased. Field capacity, moisture content at capillary rupture, wilting coefficient and hygroscopic coefficient of alternative soil with the proportion of 0:0:1 were the minimum among all treatments, which were 6.15%, 3.70%, 1.59% and 0.72%, respectively. In addition, with the principle that the better the more alternative soil over Yellow River sediment, we recommend that the alternative soil with 50% sediment (1:1:2 proportion) is the suitable material in filling reclamation with the Yellow River sediment.