An Experimental Study On Soil Water Retension Characteristics And Rainfall Infiltration Characteristics Of The Compacted Soils

Many traditional geotechnical engineering problems involve unsaturated soil mechanics,including the seepage of the upper soil layer of large-thick loess,the seepage of landfill sites,as well as the ground drainage system,and the paving of dams or roads.The water retention and permeability characteristics of unsaturated soils are two important aspects that reflect its hydraulic characteristics.This paper takes the compacted loess with different initial conditions as the research material,takes the water-holding characteristics and unsaturated permeability characteristics of unsaturated soil and its microscopic mechanism as key points,and carries out experimental research with the help of various testing methods.The hydraulic characteristics,the micro-mechanism of unsaturated soil under different initial conditions are discussed.The main research contents and conclusions are:(1)During dehydration process using centrifuge,significant axial shrinkage happened.Significant deviations were observed between the volumetric water contents,pressure heads and SWCC when soil shrinkage was considered or ignored.And the lower the initial density,the greater the axial shrinkage for samples before and after the centrifuge test.The soil shrinkage curves in this paper lacked the zero shrinkage,while clear structural and normal shrinkage are observed.In structure shrinkage stage,the water volume reduction is larger than the void reduction.In normal shrinkage stage,the water volume and bulk soil volume reductions are nearly the same.The slope of the shrinkage curve is 1.(2)It can be seen from the density change or height shrinkage curves and SWCC,as well as the data form SEM,MIP and CT analysis,the amount and volume of the macropores are much higher at the lower suction range.The greater compressibility and volume of the amount of larger macropores compromise with the initial notable soil consolidation and significant water loss during desiccation.The drastic decrease in such macropores and the slight increase in the pores inhibited a further and obvious soil consolidation in the higher suction region,and inhibited water to flow out of the soil matrix.In the process of density increase,the pore area of the soil sample decreases first from the large pores,and then slowly transitions to the reduction of the mesopore area and the small pore area.(3)Samples prepared on the dry side of the optimum exhibit typical bimodal shapes.At low water content,capillary effect tends to link silt particles and clay together and generally has a tendency toward flocculation of the colloids during sample preparation,forming grain-clay aggregates.In this case,water mainly stored in the aggregates,while the large voids between aggregates and silt grains is mainly occupied by air.Thus,they present typical bimodal distribution.When samples produced on the dry side of the optimum,density increase affects macro pore systerm.However,they have similar micropore systems.Possible reason for this phenomenon is that the soil aggregates exhibit a comparatively high stiffness at low water content.Such flocculation of the colloids is relatively stable and insensitive to the forces occurred during static compression or dynamic compaction.When water content is at the optimum,water pushes the grains apart during sample preparation and produces a dispersed structure of soil fabric.The capillary effect decreases and the compressibility of the softer aggregates is large and vulnerable to external force,so does the soil fabric between the aggregates.Thus,not only inter-aggregate pore can be influenced but also some larger inter-aggregate pore.The differentiation between the micro-and macro-pore system is often no longer significant,the pores are much more uniform.As a consequence,a multimodal pore size distribution with much lower peak value of inter-aggregate pore normally arises.(4)The SWCC obtained by the improved centrifuge method is in good agreement with the SWCC data from filter paper method for soil samples prepared at optimal moisture content.The two classes of predominant pores of soil samples compacted on the dry-side redult to its double S shaped SWCC.Correspondingly,the compacted soils are expected to lose water in two clear stages during drying.The slope of SWCC is related to the large pores.The water loses more easily in first stage than in later stage because the existence of large volume portion of macro pores or inter-aggregate pore.In the second stage when the suction force exceeds the residual suction,the water loss rate decreases,and the SWCC slopes of soil samples of various densities are not significantly different,especially when the suction force is greater than 10 Mpa.The smaller group of pores,which corresponds to the pores inside aggregations,is independent density.This accounted for the convergence of the drying curves with suctions greater than 10 MPa.Thus,the suction is mostly governed by the micropore structure when it excess 10 MPa in this paper.(5)The infiltration rate of the vertical soil column infiltration test can be roughly divided into three stages: the first stage occurs at the initial stage of infiltration.The infiltration capacity of the soil under the initial conditions is greater than the rainfall intensity,and all rainwater penetrates into the soil.The infiltration rate is theoretically equal to the rainfall intensity,which is called the rainfall intensity control stage.Taking the time point of water outflow at the bottom of the soil column as the end point of the second stage,the saturation range of the upper soil gradually deepens,and the infiltration capacity of gradually saturated soil gradually decreases.The infiltration rate gradually decreases with time and enters the stage of soil infiltration capacity control.The infiltration rate of the soil column at this stage mainly depends on the nature of the soil itself;the third stage after the bottom of the water emerges when the soil column reaches saturation State,the infiltration rate at this time almost no longer changes,and the infiltration rate is equal to the saturated permeability coefficient.(6)The unsaturated hydralic concuctivity can span multiple orders of magnitude during the soil column getting saturation.The permeability coefficient decreases with the increase of the matrix suction which can be divided into three stages: the first stage is that the suction is less than the AEV,and the permeability coefficient remains unchanged,equal to the stable infiltration rate.The second stage is between the air enter value and the residual suction value,and it appears on the unsaturated permeability-volumetric water content relation curve that the permeability coefficient increases slowly with the increase of water content.On unsaturated permeability-suction relation curve,it shows that the hydralic concuctivity decreases linearly with the increase of suction.When suction exceed the residual value,the unsaturated permeability-suction relation curve is basically flat.This stage failed in this experiment because the initial moisture content of the soil column is not 0%.