Study of the Shear Strength of a Tropical Soil with Grass Roots M.I. Miranda Neto, C.F. Mahler 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. Keywords: soil reinforced, soil stabilization, vetiver grass, triaxial extension test, tropical soil strength parameters, bioengineering. 1. Introduction Many contributions have been made in recent de- cades to improve knowledge about the behavior of soil re- inforced with metal or synthetic or natural fibers such as roots, subjected to direct shear or triaxial compression tests. One of the pioneering studies in this respect was performed by Gray & Ohashi (1983). They concluded that the main role of fibers is to increase the soil shear strength, and that a confining pressure exists below which the fiber has a ten- dency of be pulled out of the soil. Later, Gray & Al-Refeai (1986) indicated that rougher fibers tend to be more effec- tive in increasing the shear strength and Maher & Gray (1990) showed that bilinear shearing envelopes of rein- forced soils have a breaking point, named the critical con- fining pressure, below which the reinforcement tends to be pulled out. After Gray & Ohashi (1983), other researchers (Mi- chalowski & Zhao, 1996; Zornberg, 2002; Michalowski & Cermák, 2003; Heineck & Consoli, 2004; Gao & Zhao, 2013) have carried out theoretical studies to develop pre- dictive models of the improvement of shear strength due to the addition of fibers in the soil. Some researchers have fo- cused on the behavior of the addition of discrete randomly distributed synthetic fibers (Freitag, 1986; Feuerharmel, 2000; Casagrande, 2001; Casagrande & Consoli, 2002; Heineck et al., 2005; Casagrande, 2005; Consoli et al., 2007; Sadek et al., 2010; Palacios, 2012), fibers and cement (Consoli et al., 1998), or natural fibers from vetiver roots (Focks, 2006; Barbosa, 2012). This study is another contribution to knowledge of the behavior of reinforced soil. This article examines the behavior of a tropical soil with natural inclusion of vetiver grass roots subjected to extensional forces. Since the roots’ preferential direction is vertical, it is expected that soil rein- forced with predominantly vertical roots subjected to tri- axial compression tests should behave differently than reinforced soil under triaxial extension tests. Therefore, given that the root system is predominantly vertical, the ex- tension test would better simulate the role of the roots in the reinforcement of the soil than compression tests since they would not be subject to buckling. In geotechnics, the axial extension would be, for example, an unloading by excava- tion and the lateral extension would be from passive earth pressure by jack reaction or earthquake. Soils and Rocks, São Paulo, 40(1): 31-37, January-April, 2017. 31 Manoel Isidro de Miranda Neto, D.Sc., Associate Professor, Departamento de Engenharia Civil, Universidade Federal Fluminense, Niterói, RJ, Brazil. e-mail: manoel.isidro@gmail.com. Claudio Fernando Mahler, D.Sc., Full Professor, Departamento de Engenharia Civil, Universidade Federal do Rio de Janeiro, Rio de Janeiro, RJ, Brazil. e-mail: mahler@coc.ufrj.br. Submitted on May 25, 2016; Final Acceptance on March 16, 2017; Discussion open until August 31, 2017.