Assessing physical soil quality using mechanical indices from applying internal and external stresses to remoulded and structurally intact soil samples

This thesis investigates the application of existing pedotransfer functions, based on the compression characteristics of structurally intact and remoulded samples, for southwestern Ontario to soils outside this physiographic region and attempts to devise analogous indicators using shrinkage tests. Structurally intact and bulk samples were collected from the diagnostic A, B and C horizons of natural, agricultural and pipeline workspace areas from plastic soil series in Texas, Alberta, Ottawa region and southwestern Ontario. Remoulded samples were compressed by a slurry consolidometer and intact samples by static uniaxial compression. Shrinkage characteristic curves were constructed using prescribed matric potentials in both nu(w) and e(sigma ') co-ordinates and fit with a 3 straight line segment model.; Both compression and shrinkage testing indicated convergence of the remoulded and structured curves in e(sigma') co-ordinates. Overconsolidation, as indicated by the displacement of the structured curve below the remoulded curve, often occurred for B, C and clay rich (>36% clay content) horizons. Regardless of mineralogy or land use, a threshold of 0.40 for (e*1kPa-e0) and 0.25 for (ealphaR-e alphaS) indicated overconsolidation and severe plant growth limitations as indicated by macroporosity, plant available water-holding capacity and saturated hydraulic conductivity. In covariate analysis using clay or organic carbon content, significant treatment differences in relative mechanical indices (remoulded minus structured values) often persisted, suggesting that structure in addition to constitutive properties influenced the mechanical responses. The mechanical response to internal stresses was divided into 2 groups of soils based on clay and organic matter contents. Soils with ≤27% clay and >4% organic carbon exhibited no "normal" shrinkage stage when remoulded but often did exhibit a "normal" stage when structured, which was opposite for the other group of soils (>27% clay and ≤4% organic carbon).; The relative displacement of the structured compression or shrinkage curves below the remoulded curves was not entirely explained by the amount of area occupied by optically anisotropic material in the matrix. Clay orientation was an indicator of previous stress history in the soil depending on the inclusion of coarse fragments in the matrix, mineralogy and drainage conditions, but was not a causative factor in the present mechanical behaviour of the soil.