Experimental Study On Hydraulic And Mechanical Properties Of Humus Soil In Municipal Solid Waste Landfill

The mining and beneficial utilization of old landfills has great significance in solving the problems of exhausting the capacity of existing operating landfills,wasting land resources,and polluting the site and the surrounding environment during the rapid urbanization process.The physicochemical properties,mechanical properties,and beneficial utilization pathways of humus soil in municipal solid waste landfill（later referred to as humus soil）as the largest significant proportion of landfilled waste is currently unclear,which leads to the feasibility of landfill mining and resource utilization being hindered.Therefore,under the support of the Hubei Outstanding Youth Fund（2021CFA096）and relying on the Hubei Province Key Laboratory of contaminated sludge and soil science and Engineering and the IRSM-CAS/HK Polytechnic University Joint Laboratory on Solid Waste Science.Taking the humus soil in an old municipal solid waste landfill in China as the research object and investigating the physicochemical properties,hydraulic and mechanical properties of humus soil were studied by laboratory tests and theoretical analysis.The main experimental study includes the analysis of the influence of different landfill ages on the physicochemical and basic mechanical properties of humus soils,the influence of typical factors（landfill age,dry density and particle size）on the microscopic pore and hydraulic properties of humus soils,and the experimental study of unsaturated mechanical properties of humus soils,the results of which provide a reference basis and mechanical parameters for the beneficial utilization of humus soils.The main conclusions are as follows:（1）The physicochemical properties and environmental indexes of humus soil at different landfill ages were tested.The results show that the electrical conductivity,organic matter content,total salt content and most of the heavy metal elements decrease with increasing landfill age,while the total nitrogen,phosphorus and potassium content and specific gravity increase with increasing landfill age.On the whole,humus is a good material for planting,but due to the excessive Hg content,it is not recommended to be used as a cultivation substrate for crops,and measures such as mixing and chemical treatment can be taken to reduce the Hg content before it is used as planting soil for green plants.In addition,humus can be used as engineering backfill material for beneficial utilization.Humus soils of high landfill age can be directly used for engineering backfill,if directly backfilled with humus soil of low landfill age due to the high organic matter content may occur long-term settlement,so can be considered to take heat treatment,mixing or modification and other treatment measures before being used as engineering backfill material for resource utilization.（2）The basic mechanical properties of humus soils at different landfill ages were investigated,and the results showed that the unconfined compressive strength of humus soil increased from66.27 k Pa to 133.73 k Pa,an increase of about 102%,as the age of the landfill increased from 17 a to 23 a.The cohesion and internal friction angle of the shear strength index increased from 27.28k Pa to 66.54 k Pa and decreased from 35.80°to 25.81°,respectively;the saturation permeability coefficient decreased from 1.09×10^-6 cm/s to 2.83×10^-7 cm/s,a decrease of one order of magnitude;its compression coefficient a_v1-2 and compression modulus E_s1-2 varied in the ranges of 0.149-0.143MPa^-1 and 9.51-9.72 MPa,respectively,both being medium compressibility soils.On the whole,humus is a good material for daily or temporary landfill cover,and it is more feasible to be used as engineering backfill material as the age of landfill increases.（3）Based on the centrifuge method and nuclear magnetic resonance technique,an experimental study on the hydraulic properties of humus soil under the three influencing factors of landfill age,initial dry density and maximum particle size were carried out,and the effects of different factors on the fitted parameters in the soil-water characteristic curve of humus soils were analyzed.The results show that as the landfill ages increased from 17 a to 23 a,the saturated moisture content of humus soil decreased from 32.65%to 30.15%,and the air occlusion value increased from 133 k Pa to 320 k Pa;as the initial dry density increased from 1.62 g/cm³ to 1.82g/cm³,the saturated moisture content of humus decreased from 37.54%to 30.15%,and the air occlusion value increased from 64 k Pa to 320 k Pa;as the maximum particle size increased from 1mm to 5 mm,the saturated moisture content of humus decreased from 31.75%to 28.90%,and the air occlusion value decreased from 375 k Pa to 141 k Pa.The fitted parameters of the VG model for humus soil with different physical properties a,n and m varied from 0.00266 to 0.01562 k Pa^-1,1.2169 to 1.2760,and 0.1782 to 0.2163,respectively.Based on the experimental results and nuclear magnetic resonance technology,the prediction model of the soil-water characteristic curve of humus soil was established to achieve a rapid assessment of the hydraulic properties of humus soils under different influencing factors.（4）Based on indoor experimental research,the effect of water content on the mechanical properties of unsaturated humus soil was analyzed.The results show that the unsaturated permeability coefficient of humus soil increases with the increasing saturation,and decreases with the increasing matrix suction.With increasing water content,the unconfined compressive strength and the internal friction angle of humus soils show a linear decreasing relationship,while the cohesion shows an exponential decreasing function.The relationship between the shear strength of unsaturated humus soil and the matrix suction was also established by combining Coulomb law and the Van Genuchten model.