Experimental Investigation On Water Transport Of Three-layer Landfill Cover System Subject To Differertial Settlement

In the process of degradation,the landfill refuse is liable to cause differential settlement of the upper soil cover system.The deterioration of the performance of the soil cover system aggravates the infiltration of rainwater,leads to the increase of leachate and increases the risk of environmental pollution.At present,the effect and mechanism of non-uniform settlement on water transport in soil cover system are not clear.It is difficult to accurately evaluate the deterioration of seepage control performance of covering system.Therefore,it is of great practical significance to carry out relevant research to improve the level of landfill treatment and pollution control.According to the characteristics of high temperature and rainy in South China,a three-layer capillary barrier cover system was proposed.A capillary barrier covering system consists of a layer of fine-grained soil and a layer of coarse-grained soil(sand).A low permeability layer(such as compacted clay layer)is added to the bottom of capillary barrier cover system to form a three-layer capillary barrier covering system.Its impermeability in wet climate has been verified,but its long-term performance is not yet clear.One main factor influencing long-term functionality of cover system is related to differential settlements of the underlying landfill.Therefore,it is an urgent task to study the mechanism of differential settlement affecting water migration.The lateral diversion of the three-layer capillary barrier cover system is important.At present,there is no research on the lateral diversion of the soil cover system under differential settlement.In this study,a model box was developed to produce differential settlement under the three-layer capillary barrier cover system.The model box is used to carry out rainfall experiments of three-layer capillary barrier cover system under differential settlement,and the effects of differential settlement,rainfall time and tilt angle on water transport were studied.The experimental results show that the three-layer capillary barrier cover system will produce obvious cracks under differential settlement.When the differential settlement reaches 7.5 cm,cracks will occur at the top of the fine-grained soil layer at the edge of the settlement area.The maximum width of the fracture is about 15 mm and the maximum depth is about 22 cm.The maximum width of cracks in fine-grained soil layer is about 17 mm.The width and depth of the fissures in the clay layer are smaller due to its near-saturation state and the action of gravity on fine-grained soil and coarse-grained soil.The differential settlement increases to 22 cm,and the shear surface of the clay layer runs through the whole soil layer.The influence of differential settlement on water transport in the three-layer capillary barrier cover system is obvious.Cracks become the dominant channel for rainwater infiltration,which leads to the increase of rainwater infiltration rate.When the differential settlement is 7.5 cm,the breakthrough time of capillary barrier cover system is shortened by 1 h.The lateral diversion of coarse-grained soil layer increases by 38%,indicating that the coarse-grained soil layer still has good lateral diversion performance when the settlement is small.And the average permeability coefficient of clay layer increases to 1.4×10~-77 m/s.The percolation is 25.8 mm.When the differential settlement is 22 cm,the performance of clay layer deteriorates further.The average permeability coefficient increases to1.7×10~-66 m/s and the percolation increases to 89.9 mm.Rainfall time has a great influence on water transport.The lateral diversion of sand layer after 12 hours of rainfall is 4.1 times larger than that of 4 hours of rainfall,and the percolation of 12hours of rainfall is 113%larger than that of 4 hours of rainfall.When the differential settlement is 22 cm,considering the error of the experiment,the effect of inclination angle on water transport is relatively small.