Hydraulic,mechanical And Solute Migration Behaviours Of Compacted Clay Upon Leachate Intrusion And Freeze-thaw Cycles

Compacted clay is widely used in the anti-seepage system at the bottom of and around the side of landfill sites due to its low permeability.In seasonally frozen soil areas,the compacted clay liner at the bottom is subjected to dual damage from freeze-thaw cycles and infiltration of leachate at the beginning of construction;As the height of the landfill increases,the leachate head also continuously increases,and the compacted clay liner around the side will be subject to the dual destructive effects for a long time.In this paper,sodium ion（Na⁺）,copper ion(Cu²⁺)and chemical oxygen demand（COD）are used to represent the three typical components of the leachate,namely,inorganic salt ion,heavy metal ion and organic matter.The leachate containing separate components and compound components（COM）is prepared,with distilled water as a reference.The single and compound impact of these three components on the compacted clay specimens before and after the freeze-thaw cycle is relatively comprehensively explored.The main content is as follows:（1）Based on swelling-shrinking tests and microstructural observation,the mechanism of the influence of freeze-thaw cycles and infiltration of leachate on the swelling and shrinkage characteristics of compacted clay was revealed.Leachate invasion inhibits the swelling behaviour of compacted clay,but,in the case of COD,the specimen undergoes a primary swelling again at the end of soaking.The volume shrinkage deformation after Na⁺invasion first increases and then decreases with the increase of concentration.The freeze-thaw induced cracks close after saturation,but open again after shrinkage.After Na⁺invasion,new pore groups generate between 10-100μm;the impact of Cu²⁺invasion is weak;after COD intrusion,both V_m and V_M increased;the compund leachate（COM）intrusion reflects the impact of Na⁺.（2）The influence of freeze-thaw cycles and infiltration of leachate on the hydraulic characteristics of compacted clay was illustrated through filter paper method tests and flexible wall permeability tests.Freeze-thaw cycles weakens the water retention capacity of compacted clay in the capillary stage.When N_FT=0 and matric suction s_m<1000 k Pa,COD invasion enhances water retention capacity,while Na⁺and Cu²⁺invasion reduces it;When s_m>1000 k Pa,Na⁺invasion enhances water retention capacity;COM intrusion reflects the impact of Na⁺.When N_FT=10 and s_m<1000 k Pa,the invasion of Na⁺,Cu²⁺,and COD weakened the soil’s water retention capacity;When s_m>1000 k Pa,Na⁺and COM intrusion enhanced the water retention capacity of the specimen.After Na⁺invasion,the hydraulic conductivity k first increases and then decreases with increasing concentration;the impact of COD and Na⁺invasion is similar,but high concentration of COD gives rise to a furthur reduction;k increases with increasing concentration of Cu²⁺.Anti-seepage characteristics of compacted clay deteriorates after freeze-thaw cycles.When N_FT=10,the soil structure becomes loose,osmotic consolidation and precipitation of heavy metal ions further reduces k.The permeability of compacted clay is determined by ratio of the V_M to V_t（total pore volume）V_M/V_t.（3）Triaxial consolidation undrained compression tests and one-dimensional consolidation tests were conducted to clarify the influence of freeze-thaw cycles and infiltration of leachate on the mechanical properties of compacted clay.After the invasion of Na⁺and Cu²⁺,the friction strength of the specimen decreases due to cracks left from the compression of the diffusion double layer,while the cohesion increases due to the cementation effect of salt and the osmotic consolidation caused by high concentration of solute;after the invasion of COD,the friction strength first decreases due to the increase of pore ratio caused by the expansion deformation under low concentration conditions.Under high concentration conditions,it increases due to osmotic consolidation,while the cohesion increases due to microbial cementation.After leachate invasion,the compression deformation of the specimen decreases.When N_FT=10,the compression deformation increases;After COD intrusion,it further increases.The pre-consolidation pressure p_c of the specimen increases after leachate infiltration.（4）Based on permeability tests and hydrodynamic dispersion tests,the solute migration patterns of compacted clay under freeze-thaw cycles and leachate invasion were investigated.Compared to a single solute condition,different solutes migrates competitively under mixed solute conditions,and the hydrodynamic dispersion coefficient D of the three solutes decreases.When N_FT=10,the breakthrough time of pollutants is significantly shortened（that of COD is the shortest）,and the pollution depth is extended,making the compacted clay difficult to meet the general requirment.