The effect of strain rate and specimen length on the stress-strain relationship of vegetation roots used in slope stabilization.
The main objective of this investigation was to provide information about the contribution of plant roots to soil shear strength. Reconnaissance of the case study areas was undertaken and geology of the areas established, rainfall patterns collected, type of vegetation obtained, theoretical research on stability of slopes studied, soil samples were collected and laboratory soil tests were conducted. Laboratory tests were conducted at soils laboratories in the University of Nairobi and University of Portsmouth, UK. It was established that unvegetated soils are weaker than vegetated slopes in shear. Tests conducted at Sasumua backslope and Murang’a landslide sites indicated that shear values reached 16 kPa for unvegetated soils while vegetated soil had maximum values of 80 kPa for saltbush, 90 kPa grass and 120 kPa for tree fern. Rooted soils were thus stronger in shear as compared to fallow soils. Shear stress increases at the end of testing for the rooted samples. Observations of the roots after test indicates roots elongated. This elongation can be related to the root biomass density to explain why varying strengths were obtained for different samples of a same species. The perpendicular model of Wu et al. (1979) was used to calculate soil reinforcement by action of roots. The tensile strength - diameter curves (T-D) relationships indicate that root tensile strength decreases with increasing root diameter, and follows a power law equation /(x) = axk, where a and k are parameters obtained from T-D curves. Strong roots have high ^-values and low ^-values and vice versa. Shrubs generally have high a-values, but a great variation is noted within individual plant species. It was observed that smaller diameter roots have higher tensile strengths. Tensile strengths for shrubs decreased from 34 N/mm2 to 4 N/mm2, grasses from 45 N/mm2 to 5 N/mm2and ferns from 30 N/mm2 to 10 N/mm2 for root diameters ranging from 1.5 mm to 6 mm. Maximum root area ratio (RAR) values, defined as the ratio of the sum of the root areas to the area of soil profile they intersect, were located within 0.1 m for all the species, with maximum rooting depth of 0.7 m for fern tree. Shrubs species showed high RAR values between 0.1 - 0.3 m depth. In general, vegetations growing in the Sasumua backslope have shallow roots (maximum root depth 0.7 m) therefore unable to reinforce the soils to stop the landslides occurring at 1 m depths. The results strongly imply that shrubs species have prominent root mechanical properties and it is anticipated that these particular plants have the necessary features to be outstanding slope plants. Overall correlation and regression analysis show that the pull-out resistances of plants have a positive, either weak or strong, linear relationships with all the morphological properties. Bigger plants can resist pull-out force better than the smaller plants. The increase in plant size will normally generate high pull-out resistance. Taller plants will resist uprooting better than the shorter ones. The increase in pull-out resistance of plants that have root systems with extensive number of lateral root is due to the fact that the stronger soil-anchorage is developed by the lateral roots The safety factor decreases with increasing slope angle until the slope fails at a critical angle of about 45° and follows a power law equation of the form /(x) = axk. Moisture content of 50% accelerates failure of slopes. Design charts and graphs have been developed. If 6 (the angle of shear distortion) and </> (the soil friction angle) are known, using the relevant chart, K-Values will be obtained, which will be multiplied by /R-Values (the total mobilized tensile stress of roots fibers per unit area of soil) also obtained from charts. The contribution of roots to shear strength is thus deduced. Tests conducted using a variable tensile test machine indicate that vegetation roots increase their tensile strength at high strain rates. This is the case during a landslide spell, indicating that roots could induce high resistance to the forces impacted and thus offering to the stability of slopes. Several recommendations have been drawn. Deeper rooted vegetation (> 1 m) with high RAR values are recommended for slopes to reduce the potential of shallow landslides from occurring. Vegetation with high surcharge (weight / unit area) should be avoided. Vegetation should be spaced l m apart in order to influence full mobilization of shear strength as the roots integrate with the soil mass. Population living in slopes greater than 30° should be resettled in gentle areas as safety factor for slopes beyond 30° is less than 1.5, and failure can be triggered during a rainy season. Proper drainage system should be designed and constructed over slopes to dissipate surface runoff immediately it occurs in order to avoid premature failure of slopes as moisture content of more than 50% reduces the shear strength significantly. The easy to use design charts developed under this research should be applied for root reinforced soils design process.