| The main environmental geotechnical problem of landfill is the pollution and destruction on environments induced by the municipal solid wastes. In order to control leachate leakage and transport, landfill liners and covers are commonly built in modern landfills. The compacted clay liners (CCL) are widely used in impervious structure of landfill. The clay is used as a retention medium of pollutants present in the leachate. However, the sorption capacity of pollutants onto clay is weak. Hence, there is a crucial need to improve sorption capacity of CCL. A method to amend the liners with granuar activated carbon (GAC), bentonite activated by acid and lime capable of strongly sorbing pollutants is performed in the study. In order to improve the bearing capacity of liners, a method of adding lime into liner-soil materials is proposed. The adsorption property of improved CCL is observed by a series of batch tests. The pollutants transport through improved CCL is investigated by column tests. The diffusion coefficients of pollutants are determined according to data from column tests. The geotechnical tests are carried out to test the feasibility of improved clay used as liner-soil materials. A dynamic model is developed for simulating Cr(Ⅵ) transport in improved CCL. The moisture balance of compacted clay and evapotranspiration (ET) cover are analyzed by numerical simulation. On the basis of tests and theoretical analysis, the main conclusion is as follows:1. The pseudo-second order kinetic model is the best choice among all the kinetic models to describe the adsorption of Cr(Ⅵ) and Zn(Ⅱ) onto the clay. The activation energy values for Cr(Ⅵ) and Zn(Ⅱ) were found to be 22.7 kJ/mol and 26.88 kJ/mol. The value of activation energy suggests that the adsorption process might be a by chemical adsorption. The thermodynamic parameters values indicate that the adsorption process is an endothermic in nature and the rise in temperature favors adsorption.2. The results from batch tests show that the Cr(Ⅵ) sorption to improved clay was nonlinear and characterized by a Langmuir isotherm model. As the soil-solids concentration increases the Langmuir isotherm parameters (qm,b) first decreases logarithmically, but then stabilizes when the soil-solids concentration exceeds a critical value (e.g.; 200g/L); The isotherm parameters also increase linearly with increasing temperature.3. The results of geotechnical tests show that the compacted clay and clay containing GAC or bentonite activated will exhibit volumetric shrinkage≤4%, hydraulic conductivity ≤1×10-7cm/s and unconfined compressive strength≥200kPa. These data indicate that the above soils can be used as liner-soil materials.4. A two-layer liner is designed; the upper soil layer is composed of 90% clay and 10% lime, and the lower soil layer is composed of 97% or 94% clay and 3% or 6% adsorbents (GAC or bentonite activated). The two-layer liner can not only meet requirement of hydraulic conductivity, but enhance its bearing capacity and adsorption capacity to pollutants.5. The diffusion coefficients of Cr(Ⅵ) for liner-soil materials were back-calculated by a one-dimensional numerical transport program. The diffusion coefficients of one-layer liners were as follows: for 90% clay plus 10% lime, 6.4×10-8m2/s; for clay, 3.0×10-10m2/s; for 97% clay plus 3% bentonite activated, 2.2×10-10m2/s; for 94% clay plus 6% bentonite activated, 6.8×10-10m2/s; for 97% clay plus 3% GAC, 2.5×10-10m2/s; for 94% clay plus 6% GAC, 8.3×10-10m2/s. The diffusion coefficients of two-layer liners were as follows: for clay containing 10% lime of upper soil layer, 4.4×10-8-5.2×10-8m2/s; for clay containing 3% or 6% bentonite activated of lower soil layer, 6.1×10-10m2/s or 5.4×10-10m2/s; for clay containing 3% or 6% GAC of lower soil layer, 6.5×10-10m2/s or 7.0×10-10m2/s.6. On the basis of considering flow, diffusion, geochemical reaction and the effect of temperature on adsorption, a pollutants transport model is presented. Solutions to the model were accomplished by finite difference, which predicted Cr(Ⅵ) transport through two-layer liners systems. Results indicate that liner containing activated bentonite exhibited the highest retard capacity, followed by liner containing GAC and CCL. GAC and bentonite activated are two potential materials for their use as sorptive amendments for sorbing heavy metals in CCL. As leachate head, hydraulic conductivity, and dispersion coefficients increase, the Cr(Ⅵ) transport significantly speed up. Hence, they are main factors that influence the pollutants transport. The effect of soil specific heat and thermal conductivity to Cr(Ⅵ) transport is not obvious.7. Volumetric water content of compacted clay and ET cover is simulated under data of precipitation and evaporation of Dalian in 1976. Parametric analyses are conducted to simulate variations in precipitation rate, precipitation event duration, initial volumetric water content and top soil layer thickness. Results show that the moisture content near the surface is highly sensitive to climate loading; and the effect is significantly attenuated and time lagged along with increasing depth. The compacted clay cover has low penetrability; so it can not get effective moisture charge. ET cover acts as a Veservoir that stores moisture during precipitation events and subsequently returns it to atmosphere as evapotranspiration. ET cover prevents desiccation cracks and water leakage. Hence, it has great potentialities used in construction of landfill covers.
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