Hydro-mechanical Properties Of Biochar-clay Mixture

Sanitary landfill is capable of treating almost all municipal solid waste,and it is the most commonly used method of garbage disposal.Compacted clay has the advantages of low permeability and low cost,and is often used as the final cover in landfills.However,due to the uneven settlement of soil caused by natural environment and garbage degradation,which will lead to cracking of the clay covering layer and greatly damages the integrity and sealing of the soil,weakens the engineering characteristics of the soil.Therefore,it is urgent to apply new materials to optimize the engineering characteristics of the clay final cover in landfills,such as water retention,permeability and strength.Biochar has a porous structure with high specific surface area and high adsorption.Moreover,it provides a suitable habitat for microorganisms that can reduce harmful gases.The addition of biochar can improve physicochemical properties of soil,such as relative density,porosity and p H,and so on.The high specific surface area and porosity of biochar can significantly affect the micro-pore structure of soil,and then enhance the water retention,permeability and strength characteristics of soil.It is necessary to explore the possibility of biochar-clay mixture as a substitute material for the landfill final cover material,and it is necessary to study the physicochemical properties and hydro-mechanical properties of biochar-clay mixture as the final cover of landfills.In this paper,the hydro-mechanical properties of biochar-clay mixture were studied on the basis of sorting out the relevant literature at home and abroad,taking biochar and clay as the research object,combined with a series of laboratory tests,microscopic tests and theoretical analysis.This study results can provide a reference for the design and construction of final biochar-clay mixture cover in landfills which made of biochar-clay mixture.The main contents and conclusions are as follows:(1)The physicochemical properties of biochar and biochar-amended maxture were studied.The density,liquid-plastic limit,specific gravity,particle size curve,compaction curve,ash content,p H,chemical element content and functional groups of biochar,pure clay and biochar-clay mixture were determined.The results show that the addition of biochar to clay can reduce the density and specific gravity of soil,and increase the liquid-plastic limit and p H value of soil.With the increase of biochar content,the optimal moisture content of biochar-clay mixture increases gradually,while the maximum dry density decreases gradually.Biochar contains carbon,oxygen,nitrogen,potassium,calcium and other elements,of which carbon accounts for the highest proportion.The main types of functional groups are aromatic hydrocarbons.(2)Using pressure plate,filter paper method and vapor equilibrium technique with saturated salt solution,the soil water characteristics of biochar-clay mixture with different content and particle size was tested and the soil-water characteristic curves of biochar-clay mixture in the range of total suction were obtained.The results show that the content and particle size of biochar can affect the water retention characteristics of biochar-clay mixture.When the particle size of biochar is larger,the distribution of soil-water characteristic curves of mixture with different biochar content is more concentrated,and the effect of biochar content on water retention characteristics of mixture is not obvious;When the particle size of biochar is small,the water retention characteristics of samples are obviously improved with the increase of biochar content.The addition of biochar can reduce the hysteresis of biochar-clay mixture in the moisture absorption and dehumidification stages,improve the stability of biochar modified clay,and slow down the trend of wet expansion and dry shrinkage of traditional clay cover.Combined with the results of scanning electron microscope(SEM)and mercury intrusion(MIP)test,the micro pore structure was analyzed,and the distribution of biochar between soil particles and the variation of pore content and pore size with biochar content and particle size were obtained.The influence mechanism of biochar content and particle size on the water retention characteristics of biocharclay mixture is further explained from the micro level.(3)The saturated permeability coefficients of biochar-clay mixture with different biochar content,particle size and dry density were measured by rigid wall permeameter.The results show that the saturated permeability coefficient of biochar-clay mixture increased with the increase of biochar content,and decreases with the increase of dry density,and the decrease trend is more obvious with increasing of biochar content;With the increase of biochar particle size,the saturated permeability coefficient of biochar-clay mixture firstly increases slowly and then increases sharply.After the permeability test,the nuclear magnetic resonance(NMR)microscopic test was carried out on the biochar-clay mixture samples.By establishing the conversion relationship between the data of MIP and the T2 spectrums of nuclear magnetic resonance(NMR),the variation of pore size of soil samples was quantitatively evaluated by NMR test results,and the variation mechanism of saturated permeability coefficient of soil samples with the increase of biochar content was explained.(4)The air permeability coefficient ka of biochar-clay mixture with different biochar content,biochar particle size ranges and dry density are measured by using the flexible wall air permeability testing device.The results show that for the biocharclay mixture with dry density is more than 90% of the Standard Proctor Maximum Dry Density(SPMDD)of clay,there is an biochar content,and at this biochar content,the air permeability coefficient of biochar-clay mixture reaches the lowest value.When it is less than the biochar content,the air permeability coefficient decreases with the increase of biochar content,and the trend is more obvious at lower dry density.When the biochar content is larger than the value,the air permeability coefficient gradually increases with the increase of the amount of biochar.Comparing the air permeability coefficient of biochar-clay mixture with non-intersecting particle size groups of biochar,the air permeability coefficient decreases with the decrease of biochar particle size.Combined with the micro test results,the micro pore structure and pore size distribution of biochar-clay mixture were analyzed,and the influence mechanism of biochar content and particle size on the air permeability of biochar-clay mixture was explained.(5)The Humboldt pneumatic direct shear test device was used to determine the shear strength of biochar-clay mixture with different biochar content,different initial water content and different initial dry density.The study results show that with the biochar content increases,the shear strength of soil samples gradually increases,and the stress-strain curve of biochar-clay mixture gradually transitions to a strainsoftening type.With the initial water content increases,the shear strength of the biochar-clay mixture continues to decrease when the biochar content and the initial dry density are the same.Under the same vertical load,the shear strength of biochar-clay mixed soil is proportional to the dry density.The mechanism of interaction between biochar particles and clay particles was analyzed by SEM test results,and the root cause of the increase of shear strength of mixed soil with the increase of biochar content was explained from the microscopic point of view.(6)Based on the permeability test results of biochar clay mixed soil with different biochar content and dry density,the air permeability and water permeability function G(kw,ka)= f(?d,?)was established under the double change conditions of biochar content and dry density,and the function was verified by the test group,it shows that the function has good applicability.Based on the soil-water characteristic curve of biochar-clay mixture,the prediction formula of unsaturated soil permeability coefficient of biochar content is established.Considering the relationship between the effective saturation of biochar-clay mixture and the shear strength,the strength prediction equation of biochar-clay mixture is proposed.The results show that the strength prediction equation can be used to predict the strength characteristics of biochar-clay mixture.