Geotechnical aspects of polder embankment in selected coastal area of Bangladesh

Abstract

Polder embankments are vulnerable to breaching during storm surges. To investigate the causes of failure of embankments, polder no. 14/1 was selected for this study. This polder is in Koyra Upazilla under Khulna district. This polder is near Sundarbans. Two SPT bore holes were done near Patakhali closure, one at the toe of existing embankment and another adjacent to the closure. Due to very soft and non-plastic soil, undisturbed soil samples could not be collected from any borehole. Disturbed samples were collected from two boreholes at 1.5 m interval along with field SPT-N value. Bulk soil samples were collected from adjacent to closure at 1.5 m depth by manually excavating the top soil. Two in-situ field density test using sand cone method were performed; one on the top of the existing embankment and another at the toe of the existing embankment. Necessary index tests were done to classify the soils. Double hydrometer ratio tests were performed to determine the dispersiveness of the soil. Standard Proctor Compaction test were performed on bulk sample. The sub-surface soil consists mainly of lean clay in the upper layers up to the depth of eleven meters whereas the layers below this consist of non-plastic silt with sand. Since the soil is non-plastic i.e. the soil possesses no cohesion, it is vulnerable to erosion and so if any failure takes place in the embankment due to any cause it propagates quickly and cannot be stopped and becomes a catastrophe within days. Double hydrometer ratio test results on lean clay show that the soil samples collected from the surface and the boreholes are medium dispersive. When water flows through it or in contact with it the soil particles are easily washed away with water. So, it might be the cause of failure of the embankment. In situ density of soil is very low. Usually no compaction is done during construction of polder embankment. Because it is very expensive to carry compaction machine to the site as it is very remote place in Bangladesh. So the embankment remains at high void ratio which is susceptible to piping and erosion.