Effectiveness of vetiver grass in slope protection in a coastal area of Bangladesh

Abstract

The main objective of this research is to determine the performance of vetiver grass in slope protection against erosion, tidal surge and flood especially in the coastal regions of Bangladesh. In order to identify the geotechnical characteristics of coastal soils, investigations have been carried out at selected locations. Besides, thirty three soil borings data from BRTC, BUET have been collected for the coastal regions (Khulna, Bagerhat and Patuakhali) of Bangladesh. The top layer of the sub-soil of the coast is generally silty clay or clayey silt. The depth of this layer varies from 4 to 12 m below the Existing Ground Level (EGL). The field SPT N-value of this layer varies from 1 to 12. Below this layer, soil is generally loose sand or silt. The thickness of this layer varies from 2 to 13 m. The field SPT N-value of this layer varies from 6 to 20. Below the loose sand or silt layer, soil is generally dense sandy silt or stiff clayey silt. The thickness of this layer varies from 2 to 9 m. The field SPT N-value varies from 8 to 30. Specific gravity of the soil sample varies from 2.66 to 2.70. The mean grain size (D50) and fines content (Fc) of the top silty clay or clayey silt layer varies from 0.01 to 0.05 mm and 82 to 100%, respectively. Liquid limit (LL) and plasticity index (PI) of this top layer varies from 35 to 54% and 9 to 29%, respectively. The top layer soil contains 1 to 32% clay, 51 to 98% silt and 1 to 25% sand. Dry unit weight and moisture content of this clayey silt or silty clay layer varies from 10.8 to 14.1 kN/m3 and 30 to 54%, respectively. Cohesion and angle of internal friction of the top soil varies from 19 to 26 kPa and 11 to 13 degree, respectively. Embankment failures occur continuously throughout Bangladesh. From a strictly economic point of view, cost of ramification of these problems is high, and the State budget for such works is never sufficient which confines rigid structural protection measures to the most acute sections, never to the full length of coastline and embankment. From field survey, it is observed that the general reasons of embankment failure are erosion due to rain splash, wave action and overtopping of storm surge. Poor maintenance practice, overturning or uprooting of trees are also among the other reasons of embankment failure. It was found that the traditional practice for protection of embankment is to use cement concrete blocks, stone or wood revetments, geobags, geotextile and plantation etc. These are expensive and sometimes not effective due to improper design and construction fault for the designed life. On the other hand, protection of embankment slopes using vetiver grass (Vetiveria zizanioides) is being used in many countries. A device was developed to determine the in-situ shear strength of the vetiver rooted soil matrix and the bared soil. Block samples (29×15×19 cm3) were sheared at different depths under different normal loads at the field to know the in-situ shear strength. It is found that the shear strength of vetiver rooted soil matrix is 1.84 times higher than that of bared soil. Again, the failure strain of vetiver rooted soil matrix is 7.48 times higher than that of bared soil. The cohesion and angle of internal friction of vetiver rooted soil matrix is significantly higher than those of the bared soil. The enhanced effective soil cohesion due to vetiver root matrix is 2.06 times higher than that of the bared soil. This means that vetiver grass is able to increase the safety of embankment slopes against natural forces such as rainfall erosion, flood, and cyclonic storm surge. It is found that factor of safety of the embankment protected by vetiver grass is 1.76 to 2.06 times higher than that of embankment without any protection. The cost of slope protection by vetiver grass is significantly lower than the cost of other available slope protection measures. Therefore, it can be said that vetiver grass might be a cost effective and sustainable green solution for the protection of coastal embankments against natural disasters in Bangladesh coast. Keyword: Coastal embankment, low-cost, vetiver grass, rooted soil, in-situ shear strength