Shear strength characteristics of sandy soil reinforced with vetiver root

Abstract

Bioengineering techniques are extensively used for protecting slopes in many countries of the world. Vetiver is widely available in Bangladesh and its root morphology is effective for slope protection. In this context, effectiveness of bioengineering using vetiver grass was studied.

Strength deformation characteristics of vetiver rooted soil was determined by consolidated drained triaxial tests. Drained consolidated tests were conducted on 30 reconstituted samples with initial moisture content of 15%. Among these, three samples were prepared using clean sand only. Other remaining samples were vetiver root reinforced soil prepared with varying root lengths (RL) and root content (RC). Root lengths and root contents were varied to observe the effect of root parameters on shear strength characteristics. Root lengths of 2.54 cm, 3.8 cm and 6.4 cm were used with root contents of 1%, 2.5% and 3.5% respectively.

Consolidated drained triaxial test show that root can significantly contribute to the improvement of soil shear strength, thereby enhancing factor of safety of slopes. The presence of root increases the apparent cohesion of soil. However root content was not observed to have any significant effect on angle of internal friction. For each root content, there is an optimum root length for which apparent cohesion is maximum. When the sample is constructed with 1% RC and 3.8 cm RL, apparent cohesion is maximum. With 2.5% RC, maximum apparent cohesion is achieved for 2.54 cm RL. Furthermore increasing RC to 3.5%, the overall apparent cohesion decreased. The maximum value of apparent cohesion of 3.5% RC is reached for 2.54 cm RL. This appears to imply that, for each RC, there is an optimum RL for which apparent cohesion is maximum. Whilst, with increasing RC, the overall value of apparent cohesion appears to decrease.

With the computed values of apparent cohesion determined from results, FS of hill slope is computed for slope failure. For 1%, 2.5% and 3.5% RC, FS were 7.76, 3.34 and 1.83 respectively whilst for control sample, the computed FS was 1.32.

Finally, it can be concluded that, overall shear strength of rooted soil has increased as a result of increase in apparent cohesion. But there is an optimum RC and RL for which apparent cohesion is maximum and thereafter with increasing RC, apparent cohesion decreased.