Geotechnical assessment of coastal polders against cyclones in Bangladesh

Abstract

The stability of coastal embankments is a major concern for Geotechnical Engineers. Bangladesh is one of the countries where a majority number of people live near the coast. Every year severe tropical cyclones, sometimes with storm surges, hit the coastal area where earthen embankments work as the first line of defense. Since the 1960s, the government of Bangladesh has invested millions of dollars in the improvement of the coastal embankment of Polders to save lives and livelihoods. However, each year embankments breach at different locations of Polders. Recently, the government invested thousands of millions of taka in Coastal Embankment Improvement Project. The objective of the thesis is to test some of the newly improved Polder embankments to identify the geotechnical characteristics of the Polder embankments (existing geometry, types of embankment soil, and their geotechnical parameters), and to assess the safety status of the Polder embankments at different locations if the Polders were faced cyclonic storm surges of the same intensity the region experienced during the previous few decades. For the study purpose, the coastal area has been divided into seven regions. One Polder embankment from each region, a total of seven in seven regions, has been selected for this study. Standard Penetration tests were performed, and soil samples were collected from each selected Polders. Geotechnical parameters such as soil type, liquid limit test, and direct shear test, tri-axial test, unconfined compression tests were performed in the laboratory. The embankments have been numerically modeled in PLAXIS 3D, a finite element software, and GEO5, a limit equilibrium software. The model has been validated for a newly constructed superdyke at Chittagong. The safety status and settlement values of the superdyke model are compared with field observation values. The effect of mesh size and soil models are analyzed for the validation model. For this study, the Mohr-Coulomb model is used for Silty Sand and Fine Sand, and the Soft Soil model is used for Soft clay, Silty Clay. The PLAXIS model of the selected Polder embankments is analyzed for normal consolidation, rapid drawdown, slow drawdown, and very slow change in water level. Additionally, the coastal embankments of Polders are analyzed against surge height and thrust forces for two severe Cyclones: 1991 Cyclone and 2007 Cyclone SIDR. The safety factor of the Polder embankments ranges from 0.68 to 2.5 for different cases. In some analysis cases, some Polder embankments collapsed, and the factor of safety could not be calculated. Based on numerical study results, safety maps for Polder regions have been prepared for the above two cyclones. From the field test, laboratory investigations, and numerical study, it is evident that although the government has invested thousands of millions of takas for coastal embankment improvement projects, the coastal embankments of Polders are still unsafe against storm surges and need further improvement.