Influence of vetiver root on strength of expansive soil-experimental study

Grassroots have received more attention than the traditional method as soil reinforcement materials, especially the use of vetiver and other vegetation protection methods to treat expansive soil slope, have been tried and applied. To study the influence of grassroots on the strength properties of expansive soil, the laws of vetiver root growth over time and its vertical distribution of root content(delta) were firstly investigated by the experiment of planting vetiver. Then different delta and depth of planted soil were obtained. Simultaneously different delta and water content(omega) of grafted soil were made. With the direct shear test, the shear strength parameters of root-soil with different delta were analyzed. The shear test on root-soil composites with different delta was carried out to compare the strength characteristics of planted and grafted soil. The results showed that the delta of vetiver decreased with the increase of depth, and the delta of each layer increased with the growth period. The delta of 180d was 70.5% higher than that of 90d. The cohesion(c) of root-soil can be increased by more than 97%, and internal friction angle(phi) can be increased by more than 15.4% after 180 days. The c of 90 d vetiver root system can be increased by more than 18%, and the phi can be increased by more than 1.5%. At each depth, the c and phi of composite soil increases with the increase of delta, and the increment of cohesion (Delta c) and the increment of internal friction angle (Delta phi) increase with the increment of delta. But the increase in the omega will weaken the shear strength parameters of root-soil. Under the condition of the planted root system and grafted root system, the influence degree of delta on strength parameter of root-soil is different, and the law of strength parameters versus delta of grafted soil of 365d is similar to that of planted soil of 90d. And the root reinforcement of grafted soil is weaker than planted soil. Hence the grafted soil can ' t accurately reflect the root-soil interaction of the existing root system.