Use of vegetation and geo-jute for slope protection

Abstract

Embankment and slope failure 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 embankment. 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 vegetation is being used in many countries successfully. The main objective of this research is to search most suitable vegetation and to investigate its application for slope protection. Use of vetiver grass and geo-jute in stabilizing road embankment and slope against rain-cut erosion has been investigated. Field trials have been made in embankment and pond slope. Three sites have been selected in three different geographic regions in Bangladesh. A pond at Godagari, Rajshahi was selected to test the effectiveness of vetiver plant (Vetiveria zizanioides) in protecting pond slope. The sides of the pond were steep (V: H= 1:1.5 to 1:2). The bank soil is sandy silt (D50=0.016 mm) and that’s why bank soil is highly erodible. In rainy season top soil washed away by runoff and made the pond water muddy. In dry season, the top soil of pond slope flew away by wind. Vetiver grass was planted at three sides of the pond. After vetiver plantation, it was found that the soil erosion reduced and the water of the pond became clear which made it suitable for bathing and other purposes. A section of district road embankment at Keraniganj (northern approach road of Itavara Bridge), Dhaka of Roads and Highways Department was chosen for field trial. Soil erosion is one of the major causes of destruction of the road. The soil of the side slope was sandy silt. Mean grain size D50, cohesion, c and angle of internal friction,  of the embankment soil are 0.048 mm, 11 kPa and 270, respectively. Vetiver grass was planted along the road slope with and without geo-jute. Performance of vetiver and its growth was monitored for one year. The water holding capacity of the selected geo-jute is 375% compared to its dry weight. In dry season it helps to retain the soil moisture. Hence, it helps vegetation to grow even in the dry season. On the other hand, in rainy season it protects soil erosion. So geo-jute is helping for growing vetiver in all the seasons. Primarily geo-jute which is known as soil saver was used to control soil erosion. Geo-jute is a biodegradable material which becomes composed fertilizer in course of time. Hence high strength long rooted vetiver grass will take place the function of geo-jute by this time. Its roots grew more than 25 cm within three months and its leaves grew more than 30 cm in a month. Vetiver grass can grow in sandy silt. It has high tolerance to weather variation. Moreover, it is not a grazing type grass. So, it is sustainable vegetation. Its root has high tensile strength which grabs the soil firmly. Geo-jute was laid over the exposed surface which helped to control soil erosion primarily. As the geo-jute degrades with time, vetiver grasses grew up and took over the function of geo-jute. Geo-jute prevented the displacement of soil particle and mulch the soil. Hence, vetiver grew fast. Geo-jute survived more than a year. Trial project of vetiver by Local Government Engineering Department (LGED) has also been studied. It is found that vetiver grass plantation was effective in protecting the road slope more than 100 years. However, periodic maintenance is required for better performance. The vetiver system is cheaper and more environment friendly than other methods. No special equipment or skilled labour is required for implementing this method. However, periodic maintenance is required for this system. The cost of slope protection by vetiver grass and geo-jute is significantly lower than the cost of other available slope protection measures. It is found that vetiver plantation increases the stability of the slope by 50%. Finally, it can be said that vetiver grass plantation with geo-jute is a cost effective and sustainable green solution for the protection of slope against rain-cut erosion in Bangladesh. Keywords: Bio-technology, geo-jute, embankment, slope, low-cost, vetiver grass.