Structural optimization of reclaimed subsidence land interlayers filling with the Yellow River sediments using a Hydrus-1D model

Abstract: Land subsidence is the gradual settling or rapid sinking of the land surface, due mainly to the subsurface movement caused by the high intensity of human activities, such as underground coal mining. A large area of cultivated land has suffered serious land subsidence in recent years in China. The subsided farmlands need to be recovered with the locally available unconsolidated materials in the existing reclamation strategies. In a new and effective reclamation technology, the Yellow River sediments can be used to reclaim the subsided land, further to cope with the channel sedimentation, as the sediments are removed from the river by dredging. However, the Yellow River sediment is characterized by coarse-textured material with limited soil water-holding capacity. The reclaimed soil also presents the low soil water-holding capacity, in which the soil directly spreads over a backfilled layer of Yellow River sediments during conventional soil reconstruction. Alternatively, a clay interlayer can be emplaced into the backfilled layer of Yellow River sediments for the higher water-holding capacity. The Yellow River interlayer filling reclamation can be widely expected to effectively recover the damaged cultivated land in the condition of limited soil resources, particularly on the reclamation of coal mining subsidence land. The purpose of this study was to explore the soil water movement in the reclamation soil with different sandwich structures of Yellow River sediments. An optimal interlayers filling was also proposed to enhance the water-holding capacity during soil reconstruction. A Hydrus-1D software was selected to clarify the effects of different thicknesses, locations, and quantities of interlayers on the infiltration and evaporation characteristics of reclaimed soil with the interlayer filling of Yellow River sediments under a certain total thickness of reclaimed soil. Two control (CK1 and CK2) and eight experimental treatments (T1-T8) were set, where the CK1 represented the native undisturbed farmland, and the CK2 represented a conventional reconstructed soil profile. Treatments T1-T8 consisted of various combinations of subsoil and sediment overlain by 20 cm of topsoil. The results showed that the transfer of the subsoil overlying within the sediment layers significantly improved the water movement and water retention of the reconstructed soil profile. Specifically, the water-holding capacity during infiltration improved by 12.79%, the cumulative evaporation decreased by 10.09%, and the water retention coefficient increased by 21.51%, compared with the CK2. There was the greatest effect from the thickness of the interlayer, whereas, the position of the interlayer presented the least effect. There was also the strongest interaction between the position and number of interlayers. An orthogonal test demonstrated that an optimal profile was achieved with the highest water-holding capacity, where two interlayers were added in the sediment, when the 30 cm thickness of interlayer, and the interlayer was located the filled layer at 20 cm. Overall, the numerical simulation can provide the groundwork to design the field-scale experiments for the specific quantity of native soil. The finding can greatly contribute to the interlayer filling reclamation, further moving forward the recovery of subsided farmlands.