Standard state of soil dispersions for rheological measurements

The structural degradation of land is increasing all over the world; hence any effort to characterize it effectively is of great practical importance. By rheology we were able to compare the stability and resilience of structured soils containing the same clay minerals. The shear resistance of soil dispersions can be measured over a limited range of solid to water ratios. A precise methodology for rheological characterization of soil dispersions which contain as much water as they can hold in equilibrium, introduced here as standard state, was developed within a European specific targeted research project (acronym: INDEX-GOCE-CT-2003-505450) to determine parameters indicating structural degradation of European soils. We also aimed to find a measuring method for non specialists to follow structural changes in soils. Based on the entire INDEX sample pool, it was shown that soils with particles smaller than 1 mm can be measured in a reproducible way. First the water content of dispersions in a standard state (WCSDinStS) containing the maximum occluded water had to be determined, which itself is a characteristic of soil quality. Then the method for preparation of dispersions was standardized at 25 + 0.1 °C and 1 bar, and the rheological measurements were performed with a stress controlled rheometer using plate-to-plate and vane sensors. The flow character of all concentrated soil dispersions was viscoplastic and showed thixotropy, with two exceptions. Rheological parameters, i.e., initial stress, thixotropic hysteresis area, extrapolated (Bingham) yield value, plastic viscosity and absolute yield stress, of soil dispersions were determined. They changed in parallel, and were related to the particle adhesion, sensitivity to mechanical effects, and the strength of the physical network built up during the gradual aggregation of particles in soil dispersions. It was shown that the fragile networks of particles are thixotropic, so that external forces above the measurable limits destroy them, but they recover on standing for a longer time period. In the present paper, the development of a precise methodology for structural characterization of several clayey systems is described in detail showing only some examples from the thousands of rheological measurements, and in the end a useful comparison with field test measurements is shown.