Study On The Performance Of The Environmental Geotechnical Enrineering Of Capillary Barrier Of Biochar-amended Soil

A capillary barrier(CB)is a cover system that uses the contrast in hydraulic properties between fine and coarse-grained layers under unsaturated conditions to minimize downward water movement.The performance of a CB depends on its capacity to store water.Therefore,increasing the water storage capacity is an important research area.Biochar has long been proposed for amending agricultural soils to increase soil-water retention capacity and therefore promotes crop growth.Recent studies revealed the potential use of biochar-amended soil in landfill final covers to reduce water infiltration into a landfill.However,the effects of biochar application ratio and degree of compaction(DOC)on soil-water retention characteristics of biochar amended with soil are not well understood.The use of dredged sediments is widely concerned by researchers.However,the use of dredged sediments in CB cover systems has never been investigated.The objectives of this study were to develop a modified capillary barrier system to reduce water infiltration into a landfill and to investigate the potential use of biochar amended with dredged sediments(BADS)as capillary barrier forming materials.The study consisted of laboratory experiments and numerical modeling.The laboratory test was conducted to get the soil-water characteristic curve(SWCC)using the GDS UNSAT.In order to quantify the influence of the degree of the compaction and biochar amended ratios on the water storage characteristics of the dredged sediments,a series of soil samples were introduced.The SWCCs of biochar amended with dredged sediments with different biochar amended ratios(0,10,and 20%,w/w),and different DOCs(70,80,and 90%)were measured.Furthermore,the hydraulic conductivity coefficient curves of different soil samples was fitted using the Fredlund&Xing method serving the numerical modeling research.Two-dimensional seepage analyses were performed using the finite element software SEEP/W.The numerical modeling was subsequently carried out on a 8m long inclined capillary barrier system to investigate the the capillary barrier effect,especially the water storage capacity of BADS under different rainfall intensities of 20mm/h and 40mm/h.The cover,being inclined by 1V:8H,is composed of 40cm finer-grained layer and 40cm coarse-grained layer.The cover was set to the top of a residual soil layer which is 20m wide and 10m high.Different materials were chosen to form the finer-grained layer.Three representative sections and six typical points were determined to consider the pore-water pressure and volumetric water content.Compared the pore-water pressures and volumetric water content,the capillary barrier performance could be evaluated reasonably.There were several conclusions can be drawn after the laboratory test.The volumetric soil water content was increased with the increasing biochar amended ratios and DOC.Compared with the DOC effect,the ratio of biochar made a much bigger difference.Under a high suction(such as 150kPa),the volumetric water content of BADM at 70%and 90%DOCs for the 10%BADS were increased by 15.6%and 22.1%,when compared with dredged sediments,while the increases of 20%BADS were 38.7%and 27.2%,respectively.It proved that the BADS had a better water storage capacity.Using the Fredlund&Xing method to fit the SWCC,the value of parameter a of 10%and 20%BADSs were 29.376kPa and 86.306kPa,respectively.And the values had increased by 320%and 930%compared with dredged sediments.About the numerical modeling,several conclusions can be drawn,either.At the beginning of the rainfall,the capillary barrier could impede the infiltration effectively.But the capillary effects faded slowly as rainfall goes on.The capillary barrier performance lasted 4 hours when the rainfall intensity was 40mm/h s while the lasting time was 6 to 8 hours when the rainfall intensity was 20mm/h.Furthermore,the soil-water retention capacity was increased with the biochar application ratios increaing.Under a same situation,the BADS had a higher volumetric water content compared with dredged sediments.For example,at point F2,the volumetric water content of BADM at 90%DOCs for the 10%BADS and 20%BADM were increased by 31.7%and 36.9%,when compared with dredged sediments under a rainfall intensity of 20mm/h for 10 hours,while the increases of 10%BADM and 20%BADS were 5.8%and 6.6%when the rainfall intensity was 40mm/h.Compared with the light rain,the volumetric water content and pore-water pressure of the soil was higher under heavy rain.Such as at point F2,after 10hr rainfall,the volumetric water content and pore-water pressure of the soil were 45.2%and 160.2 kPa under heavy rain while the values were 32.7%and 40.8 kPa under light rain.Furthermore,the capillary barrier with biochar improved the volumetric water content in both finer grain layer and coarse grain layer.