Study Of Failure Modes And Mechanism Of Evapotran-spiration Landfill Cover Caused By Differential Subsidence

The evapotranspiration landfill cover is a newly developed covering layer in recent years. It is mainly composed of fine-grained soil suitable for plant growth, such as silt, silty clay. Like a sponge, it absorbs and stores water as rainfall, and releases water through surface evaporation and plant in other time. This type of cover layer has some advantages like, use of local materials,and low cost. therefore it has broad prospects in the northwest area of China. As landfill waste has the characteristics of complex composition, high compressibility, and uneven distribution, during the degradation of the landfill, local subsidence can be easily produced. The local subsidence will damage the cover layer, forming the dominant channel for water, which significantly reduces the barrier function of the coating layer. Therefore, a better understanding of failure modes of the evapotranspiration landfill cover and their mechanisms behind induced by local subsidence, will be useful to impact of local subsidence to the impermeable performance of the cover layer, and developed a new design code. The main work and conclusions are as follows:(1) A trapdoor simulator, developed by Zhejiang University, was employed to carry out centrifugal model tests in order to investigate the destruction process of the evapotranspiration cover composed of unsaturated soil caused by differential subsidence. The test results show that the evapotranspiration cover caused by subsidence exists two failure modes:internal failure and external failure. An arch-shape failure surface is formed above the subsidence in internal failure mode; while vertical failure planes which emanating from the sides of the subsidence occur in external failure mode. It is also found that soil layers with different water content can occur different failure modes.(2) Using the trapdoor simulator and a newly developed rainfall simulator, two centrifuge model tests were carried out to investigate the rainfall-induced failure of an unsaturated soil layer underlying differential subsidence. The variation of the water content and the matric suction of the soil layer along depth with time was measured. The test data reveal that the mechanism of the external failure of the soil layer happened during rainfall is mainly due to the decrease of the soil strength after it absorbing water.(3) A numerical model by FLAC was developed to study the stress and strain distributions in the soil layer with uniform water content under the differential subsidence. It is found that the soil stress field in the soil layer redistributes during the settlement of the trapdoor and a soil stress arch is formed spontaneously. It implies that different failure modes of soil layer occur due to the different shapes of soil arching.(4) An internal failure mechanical model and an external failure mechanical model were developed based on the structure arch hypothesis and the Terzaghi’s limit equilibrium method by considering the unsaturated characteristics of the soil in the evapotranspiration cover system. A formula of an arch-shape failure surface and that of a safety factor were proposed for the internal failure and the external failure modes, respectively. It is found that the internal failure arch height mainly influenced by the internal friction angle of soil (total stress method), which decreases with it. The safety factor of the external failure mode is mainly affected by the apparent cohesion, which also decreases with it. During rainfall, the infiltration of water effectively increase the moisture content of the soil layer, which in turn induces the matric suction of the soil layer, as well as the soil strength, decrease. In the end a sudden collapse of the soil layer occurs.(5) Based on the theory of failure mode of evapotranspiration cover system in this study, an design approach is proposed for evapotranspiration cover system to resist differential subsidence, which promotes from the approach proposed by Giroud. By the analysis of the proposed approach on a real cover system it shows that the HDPE geomembrane used in the cover can effectively prevent the damage caused by differential subsidence.