Erosion control and slope stabilization of embankments using vetiver system

Abstract

Embankment failures happen continuously throughout Bangladesh. Traditional practices for protection of embankment are to use cement concrete blocks, stone or wood revetments, geobags, geotextile and plantation etc. But these are expensive and sometimes not so effective for their design life. On the other hand, vetiver grass (Vetiveria zizanioides) is being used for slope protection in many countries efficiently. The main objective of this research is to determine the strength of vetiver grass rooted soil in protecting embankment slope’s failure that happens mainly due to rain-cut erosion. In-situ shear strength of vetiver rooted soil matrix and bared soil was determined for block samples. Block samples (29×15×19 cm3) were sheared under different normal loads at the field to determine the in-situ shear strength. It is found that the shear strength, τmax of vetiver rooted soil matrix is 1.8 times higher than that of the bared soil. Horizontal deformation at failure, δhf of vetiver rooted soil is about 5.5 times higher than that of bared soil. Direct shear tests were conducted on reconstituted soil samples for varying root contents (0% to 12% of dry weight of soil having root length of 2.54 cm under different normal stresses (i.e., 10.96 to 19.98 kPa). Test results show that τmax of vetiver rooted soil are about 1.4 times higher than that of bared soil depending on the percentage of root content. δhf of vetiver rooted soil increases about 2.2 times in comparison to that of bared soils with the increase of root content from 0.5% to 12%. Strength varies with the increase of root content. However, it is found that shear strength (τmax) increases with the increase of root content up to 9% rooted soil. However, the strength decreases at higher root contents. On the contrary, δhf increases with the increase of root content. Direct shear tests were also conducted on laboratory reconstituted soil samples prepared with different root lengths varying from 1.25 cm to 5.00 cm under different normal stresses (i.e., 10.96 to 19.98 kPa) having 6% root content. Test results show that the shear strength, τmax of vetiver rooted soil is about 1.4 times higher than that of bared soil. Similarly, δhf of vetiver rooted soil increases 3.0 times in comparison to that of bared soils with the increase of root content from 1.25 cm to 5.00 cm. To observe the growth rate in submerged condition, vetiver grass was planted in both silty sand and silty clay soil. It is found that vetiver can survive under submerged condition for 3 months. The shoot of vetiver grass planted in Pubail soil (silty clay) and Keraniganj soil (silty sand) grew up to 109 cm and 114 cm, respectively in 3 months. Root length grew 28 cm in Keranigang soil and 13 cm in Pubail soil during this period. In field condition, the shoot of vetiver grass grew up to 175 cm in 13 months while the root grew 25 cm during the same period. It is clear that growth rate of vetiver grass in silty sand is higher than that of in silty clay soil. It is found that vetiver can survive under pH levels varying from 4.6 to 7.7 and Electrical Conductivity (EC) ranging from 0 to 15.9 ds/m. Using the cohesion and angle of internal friction obtained in the study, the stability of embankment slopes were estimated using ‘infinite slope method of slope stability analysis’. From the analyses, it is found that vetiver grass plantation is able to increase the factor of safety of embankment slope by 1.50 times. From the erosion test, it is observed that erosion can be reduced by vetiver grass plantation. Vetiver grass reduces the erosion by 71%. So, it can be said that vetiver grass plantation can protect the embankment from rain-cut erosion and shallow depth slope failure. Keyword: Slope stabilization, shear strength, rain-cut erosion, vetiver grass