Physical modeling of the effect of roots of grass in the slope stability

The developments in infrastructure require adequation and construction of new roads and therefore stable slopes adapted to the weather and its variations associated with the current climate change. In Colombia, many of the slopes are reinforced with vegetation of different species, which are selected depending on the climatic conditions related to the altitude (between 0 to 3000 msl in Colombia). The vegetation contributes to the slope stability in two manners: (i) mechanically, as the roots act as small anchors in tension increasing the shear strength to the soil, and (ii) plant transpiration, which contributes to the increase of suction in the soil and therefore increasing the shear strength. Despite that this practice is very common in the country, the design continues to have a very important empirical component.

The objective of this research is to study through physical models the mechanical contribution of the root of three different grass species (Vetiver, Brachiaria, and San Agustin) on the deformation field, the shape of the failure surface, and the increase of the factor of safety in a clayey slope. In order to do this, physical models in the geotechnical centrifuge of the Colombian School on Engineering Julio Garavito were performed. For the root of grass simulation, glass fiber was selected considering scaling laws for physical modeling in the geotechnical centrifuge for an acceleration field of 100 x g. To model each grass, the glass fiber was mixed with clay with a percentage in mass that depends on the tensile strength specific to each root. The contribution of the grass-reinforced soil in the undrained shear strength was obtained through triaxial tests in samples of clay and grass-reinforced clay. The theoretical increase of the factor of safety of each grass-reinforced slope was computed using the finite element software Slide by Rocscience. Finally, physical models of slopes in the geotechnical centrifuge with and without reinforcement equivalent to each grass were performed. Deformation fields in each model were analyzed through particle image velocimetry technique using the software GeoPIV_RG.

As a result, the numerical and the physical models show that the movement produced in the slope reinforced with vetiver grass is lower than the movement obtained with the Brachiaria grass and San Agustin grass, respectively. This is because the root morphology generates a specific size, number, and depth of roots that affects the global stresses of the slope and therefore consequent deformations. The results obtained in the physical models allow the designers to predict the behavior of the reinforced slope and to estimate the order of magnitude of the reinforcement that should be reached in the field for each species of grass. It is important to continue investigating the effects of vegetation on slope stability as a solution that can reduce the environmental impact compared to other solutions that also involve higher construction costs.