Triaxial Extension Tests Show an Incresase in Shear Strength of Soil with Grass Roots

The role of roots on shear strength has been a matter of searches and also uncertainties. An investigation to identify and quantify the contribution of the vetiver grass Chysopogon zizanioides (L.) Roberty in shear strength of soils was performed by means of triaxial test. Samples were prepared from 4 inch PVC pipe molds where tropical soil was compacted and specimens of vetiver seedling, obtained by tillering, were grown. After several months of growth vetiver grass reached maturation and each mold provided at least four samples with roots inside. Similar samples of the same soil without vetiver grass and others roots were performed on triaxial tests to obtain shear strength of soil alone. The 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 and friction angle were respectively 13 kPa and 34°, and the Mohr-Coulomb envelope of the soil with vetiver roots shows a bilinear shape and shows that an increase in shear strength was obtained due roots acting like a reinforce in soil mass. The shear strength parameters of soil with vetiver roots, cohesion and friction angle were, respectively, 17 kPa and 59° for confining pressure below 75 kPa and 22 kPa and 33° for confining pressure above 75 kPa. Triaxial compression tests were conducted in same soil with and without roots and none increase of resistance was observed. It is an expected behavior due a vertical spread of roots and any reinforcement in the same direction of the compressive force, do not contribute to increase the strength. In conclusion, for extensional stress above 75 kPa confining pressure, the friction angle was the same that the soil without roots although the cohesion was larger. Below 75 kPa the soil shows a very large friction angle due the roots. Therefore vetiver roots contribute for an increase of shear strength in soils under extensional loadings.