THE EFFECT OF GRASS-ROOTS ON SHEAR-STRENGTH VARIATIONS WITH MOISTURE-CONTENT

This study investigated the relationship between vane shear strength and moisture content for root-free and root-permeated sandy clay loam and clay soils, with a view to determining the effect of root density on the relationship. The results indicate that for both root-free and root-permeated soils, shear strength increases exponentially with decreasing moisture content. As expected, increases in root density considerably increase the magnitude of shear strength at all moisture contents investigated. In all cases, roots increased the magnitude of the shear strength of the clay soils much more than the sandy clay loam soils. Relative to the root-free soils, the roots increased shear strength in both soils by at least 500%. In the clay soils a higher relative increase of up to 850% is achieved in samples with a root density of 1.80 g cm-3. Also, relative to root-free soils, root permeated soils were found to exhibit a higher rate of shear strength increase with soil drying. This, it is suggested, is because of the higher shading effect of the shoots of the root-permeated samples leading to a slower rate of soil drying than in the root-free samples.