تاثير شوري و ريزساختار بر فشردگي پذيري و مقاومت برشي خاك در رطوبت هاي مختلف

چكيده انگليسي :

87 Effect of Salinity and Micro structure on Soil Load Support Capacity and Shear Strength at Different Moisture Contents Elnaz Ghajar e ghajar@ag iut ac ir August 3 2014 Department of Agricultural Machinery Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc language farsi Abbas Hemmat ahemmat@cc iut ac irAbstractSoil compaction may reduce the porosity pore size and pore continuity of the soil and thus reduces air permeability infiltration rate of water into the soil Soil compaction also increases bulk density mechanical strength anddegradation of soil structure and thus leading to reduced crop yield and increased soil erosion and powerrequirements for tillage One of the most important parameters in evaluating soil load capacity is pre compressionstress pc which is often used as a criterion for soil susceptibility to compaction The pc is also used as soilcompressive strength which is the single point that divides the compaction curve into two parts When soil issubjected to stresses smaller than the pc it deforms and recovers elastically However at stresses greater than the pc the soil deformation is irreversible By limiting the mechanical stresses exerted on the soil below the pc therisk of adverse changes in soil structure is prevented One of major problems of agriculture in Iran is salt affectedlands Salinity problems are due to high water evaporation from the soil low rainfall topography and poor quality ofirrigation water These factors have caused many salty lands Despite the large areas covered by saline soils incentral Iran there is little information on the soil sensitivity to compaction at different salinity levels and watercontents Therefore the objectives of this research were 1 to study the effect of salinity and microstructure oncompressibility and shear strength of soil under different soil water contents and 2 to determine the relationshipbetween these two soil strength values In this research silty clay soils with EC valuess of 4 9 and 13 dS m wereobtained from Rudasht region in Isfahan The soil compressive properties were evaluated using plate sinkage test PST and confined compression test CCT Stress sinkage and stress strain curves were drawn for the PST andCCT respectively Soil shear strength parameters cohesion c and internal friction angle were measured by thedirect shear box test The effects of water contents 0 7PL 0 9PL and 1 1PL PL platic limit salinity levels 4 9and 13 dS m and two soil microstructures remoulded and undisturbed on pre compression stress and shearstrength were studied using a factorial experiment in a randomized complete design with three replications Resultsshowed that in PST the maximum and minimum values of pc were obtained with Casagrande method andmaximum curvature methods respectively In CCT the maximum values of pc were obtained with Casagrandemethod and the minimum values of pc were obtained with intersection of virgin compression line VCL and x axis By increasing salinity level and decreasing soil water content and changing soil structure from remoulded toundisturbed the pc increased in both PST and CCT However increasing salinity level and changing soil structurefrom remoulded to undisturbed led to a decrease in compression index Cc With increasing soil salinity soilcohecion and internal friction angle reduced but the changes in soil structure from remoulded to undisturbed increased these parameters Internal friction angle increased with increasing soil water content Increasing soilsalinity led to a decrease in estimated soil shear strength at its pre compression stress point Keywords Pre compression stress shear strength compressibility index saline soil undisturbed soil

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تاثير شوري و ريزساختار بر فشردگي پذيري و مقاومت برشي خاك در رطوبت هاي مختلف

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