Study of the Shear Strength of a Tropical Soil with Grass Roots

Abstract

The role of roots in shear strength has been a matter of research and also uncertainties. An investigation was conducted to identify and quantify the contribution of the roots of vetiver grass (Chrysopogon zizanioides) to the shear strength of soils, by means of triaxial testing. Samples were prepared from 4-inch PVC pipe molds, where tropical soil was compacted and specimens of vetiver seedlings, obtained by tillering, were grown. After 24 months of growth, from each mold of the mature vetiver grass it was possible to get at least four samples with roots inside. Similar samples of the same soil without vetiver grass were submitted to triaxial tests to determine shear strength of soil alone. Confining pressure ranging from 25 to 200 kPa was used in triaxial extension drained tests to determine a Mohr-Coulomb envelope. The shear strength parameters of soil without roots, cohesion intercept and friction angle were respectively 13 kPa and 34°, and the Mohr-Coulomb envelope of the soil with vetiver roots showed a bilinear shape with cohesion and friction angle, respectively, of 17 kPa and 59° for confining pressure below 75 kPa and 22 kPa and 33° for confining pressure above 75 kPa. So an increase in shear strength was obtained, because the roots acted to reinforce the soil mass. Triaxial compression tests were conducted in the same soil with and without roots and no significant increase in resistance was observed. The result was observed due to a vertical spread of roots, since any reinforcement in the same direction of the compressive force does not contribute to increase the strength. In conclusion, for extensional stress above 75 kPa confining pressure, the friction angle was the same as that of the soil without roots, although the intercept of cohesion was larger. Below 75 kPa, the soil showed a very large apparent friction angle due to the roots. Therefore, vetiver roots increase the shear strength in soils under extensional loadings.