Effectiveness of Geogrid for Protecting Polder in Sundarban Coastal Region

Abstract

Numbers of large scale polders are constructed to protect low-lying coastal area of Sundarban in Bangladesh during 1960s. At present, there are number of polders occupying an area of about 1.2 million hectares, which are vulnerable to breach and cause immense misery to millions of people every year. The purpose of this study is to assess the safety factors (FoS) of those Polder embankments during diurnal tidal cycle in coastal area and provide solution to improve the stability of embankment through installation of geogrid layers. To investigate vulnerability of the embankment failure, soil samples are collected from several borehole locations in Satkhira, a coastal site of Sundarban area. Field test—SPT and several Laboratory tests—direct shear test and Sieve analysis test have been carried out to determine shear strength, angle of internal frictions and grain size distribution. Slope stability analysis are performed using Geo5 software under several conditions—high water table (HWT), low water table (LWT) and rapid drawdown (RD) with side slope are 1:1, 1:1.5 and 1:2 and embankment height are 5m, 7.5m and 10m. The safety factor has been found lowest at rapid drawdown condition because of rapid recession of external water level. The lower FoS have been found 0.68, 0.89 and 0.92 which can be improved to 1.23, 1.34 and 1.24 respectively by installing several geogrid layers. Simplified Bishop Method have been used in Geo5 to perform the stability analysis under various conditions. In our investigation, HDPE biaxial geogrid have been taken to strengthen of polder embankments, which are timesaving, cost effective and efficient, side by side suitable for Sundarban area. The findings and solutions proposed by the study will help geotechnical engineers to reduce the vulnerability of Polder embankment under similar coastal conditions.