Development Of Enhanced Compacted Clay Liner For Preventing Volatile Gas And Study Of Key Performance Parameters

CCLs（Compacted Clay Liner）are prone to cracking because of water evaporation,which eventually forms the dominant pathways.As a result,the impermeability and gas barrier property of horizontal barrier system decreased sharply.Financially supported by the National Key R&D Program of China（No.2018YFC1803100）and the National Natural Science Foundation of China（No.41877248）,the dissertation presents a systematic study of the water retention,permeability,SVOC gas adsorption,strength,gas barrier and anti-shrinkage properties of enhanced clay functional layer material through laboratory tests and in-situ demonstration project.The main results of the dissertation are as follows:（1）The comprehensive properties of attapulgite and diatomite modified clay materials are systematically investigated,the influences of the dosage and ratio of dual improver on the water retention and gas barrier properties of the enhanced clay functional layer materials are clarified,and the permeability and SVOC adsorption properties of the improved clay materials are evaluated.With the increase of the dosage of dual improver,the water retention and gas barrier properties of the enhanced clay functional layer materials are improved.After considering the economic cost,the dosage should be set as 5%.When the ratio of attapulgite to diatomite is4:1,the water retention capacity can increase 82%,and the gas barrier property fulfil the requirement(D_θ≤1.0×10^-6m²/s).The hydraulic conductivity of dual modified clay is decreased by more than 25%,and the adsorption performance parameters K_d and K_oc（Liquid phenol）are 3 to 4 times that of unmodified clay.（2）Through evaporation test,volume shrinkage ratio test,gas diffusion test and unconfined compression test,the effect of mixing water content w₀ on water retention,gas barrier,anti-shrinkage and strength characteristics of the enhanced clay functional layer materials are investigated.The result shows that the range of w₀ should be 26～30%.At this time,the Water-retention rate of dual modified clay after baking at 60℃for 24h is about 44%,which means excellent water retention property.After baking at 60℃for 24h,the volume shrinkage ratio is less than 25%,and no cracks appear on the soil surface.The gas diffusion coefficient is between 8.53×10^-7m²/s and 9.00×10^-7m²/s,which meets the gas barrier requirement.And the unconfined compressive strength meets the strength requirements（q_u≥150k Pa）of the CCL at the bottom of the landfill.（3）The SEM,BET and TG-DSC are conducted to investigate the microstructure,pore characteristics,temperature boundary and surface functional groups of the enhanced clay functional layer materials.The results show that the rod-like attapulgite particles can fill the gaps between the flake-like clay particles,diatomite particles can reduce the size and number of pores in compacted clay liner.The modified clay has a significant increase in micro-porous volume,but the volume of meso-porous and macro-porous is not changed.Therefore,the adsorption capacity of water molecules and other pollutants is enhanced.The temperature of weakly bonded water before and after the improvement are 101.55℃and 110.59℃,respectively.（4）The demonstration project of horizontal barrier system in Xuzhou Tongshan District Coking Plant was completed and the construction process of the demonstration project was put forward.The field tests show that the mixing uniformity of the clay can be improved by spraying.When the compaction times are more than 4,the degree of compaction is more than90%,and the hydraulic conductivity k is less than 1×10^-7cm/s.The anti-seepage performance of CCL improved by 37%and the modified clay strength increased by 7-12%.The results of laboratory tests show that only the gas diffusion coefficient of dual improved CCL can satisfy D_θ≤1.0×10^-6m²/s.Long-term monitoring results show that the thermal insulation performance of modified clay is increased to a certain extent,the long-term volume water content is increased by more than 10%compared with that of the unmodified clay,and the water retention capacity is excellent.