Investigating the Effect of Direction of Grass Roots on Shear Strength of Soil and Stability of Embankment Slope

The use of grass to protect the slopes from erosion of road embankments is a common practice. However, quantifying the changes in shear strength of such bio-structured (with grass roots) soil is a complex procedure. One of the important factors affecting the shearing resistance of such soils is the direction of grass roots with respect to the slope angle. An experimental investigation was taken up to quantify the changes in shear strength of soil due to the direction of grass roots. Direct shear tests were conducted on field samples from a slope with grass roots and compared with remolded samples without roots. The field samples were obtained by pushing the samplers in a sloping ground in vertical, horizontal, and in 45 degrees inclination. Direct shear tests were conducted on these samples by keeping the direction of the roots in perpendicular, parallel, and inclined to the failure plane during the direct shear test. Thirty-five samples were tested with in-situ moisture content, and fifteen samples were tested in saturated conditions. For the samples tested with in-situ moisture conditions, the test results indicate that the soil with the vertical and inclined root direction experienced a similar overall increase in cohesion and a near negligible change in friction angle. However, the samples with roots parallel to the failure plane experienced the least increase in cohesion in comparison with the other two root directions. A similar trend was observed for the samples tested in saturated conditions. Additionally, slope stability analysis was performed using software SLIDE (version 7) to determine the effects of the root direction on the factor of safety.