Triaxial compression test of soil–root composites to evaluate influence of roots on soil shear strength

In order to evaluate influences of roots on soil shear strength, a triaxial compression test was carried out to study the shear strength of plain soil samples and composites comprised of roots of Robinia pseucdoacacia and soil from the Loess Plateau in Northwest China. Roots were distributed in soil in three forms: vertical, horizontal, and vertical–horizontal (cross). All samples were tested under two different soil water contents. Test results showed that roots have more impacts on the soil cohesion than the friction angle. The presence of roots in soil substantially increased the soil shear strength. Among three root distribution forms, the reinforcing effect of vertical–horizontal (cross) root distribution was the most effective. Increase in soil water content directly induced a decline in soil cohesion of all test samples and resulted in a decrease in shear strength for both plain soil samples and soil–root composites. It was concluded that the triaxial compression test can be effectively used to study influences of roots on soil shear strength.