Abstract:
Embankments can significantly shield people, farms, and livestock from flooding during floods. Embankment stability analysis is necessary to evaluate structural integrity and prevent flood-induced failures because of its high susceptibility to flooding. This study aims to evaluate the geotechnical stability of the existing embankments at specific floodplain locations. For this study, the Ganges-Padma (Noria and Rajbari) and Brahmaputra-Jamuna (Jamalpur) River systems, three of Bangladesh's most flood-prone river regions, have been chosen. SPT and ERT assessments have been conducted, and soil samples are collected from each embankment location. In the laboratory, various geotechnical parameters have been analyzed, including wash sieve analysis, hydrometer tests, liquid limit tests, direct shear tests, one-dimensional consolidation tests, and UU tri-axial tests. One of the main goals is to correlate the resistivity results from the ERT with the data from the boring logs, which has enhanced the accuracy of subsurface characterization and given a comprehensive understanding of soil strata.
In this study, a non-homogeneous embankment section with an impermeable foundation modeled in Geo-Studio has been successfully verified for validation purposes using GEO5 2D. Later, the GEO5 program has been used to simulate the existing embankments numerically. The slope stability analysis using GEO5 software has been carried out to evaluate the factor of safety (FoS) under different water levels, rapid drawdown, and surcharge conditions. Under various surcharges, the embankment shows a critical instability, posing a high risk of failure. The lowest factor of safety (FoS) has been observed in steep slopes, while flatter slopes demonstrated higher stability. For every study location, the impact of severe flood-induced seepage is also examined by assuming the water level on the riverbank at 1.5 m lower than the crest level.
In the Jamalpur study area, the results of the slope stability analysis indicate that an increase in water level leads to a decrease in the factor of safety, whereas a decrease in water level causes the factor of safety to increase. For different analyses methods, the FoS vary from 4.04 to 14.91 for a normal water table of 10 m. Rapid drawdown conditions also have an impact on the factor of safety. When the groundwater table (GWT) is 10 meters and the original water table (OWT) is 17 meters, the FoS vary from 1.59 to 16.9. In a similar way, the findings indicate that the factor of safety decreases with increasing surcharge and increases with decreasing surcharge. The Bishop method shows a peak factor of safety of 11.46 for a circular slip surface with no surcharge, whereas the Janbu method shows the lowest factor of safety of 1.1 for a polygonal slip surface under the maximum surcharge of 150 kN/m2. According to the seepage analysis, pore water pressure increases at a rate that is parallel to the water level; additionally, the pore water pressure decreases when the water height falls. The lowest measured pore water pressure is 265.3 kPa at a reduced water height of 7 meters, while the highest recorded pore water pressure is around 416.9 kPa at an 18-meter water height. The seepage investigation also demonstrates that suction rises with rising water levels. Conversely, a low water level results in high suction. Suction values are -96.07 kPa at an 18-meter water height and -183.4 kPa at a low water level of 7 meters. It has been found that the amount of point inflow or outflow increases as the water level rises. The inflow and outflow are maximum at 30.92 m3/m/day when the water level is 18 meters, and they are lowest at 1.86 m3/m/day when the water level is 7 meters.
The results of the slope stability analysis in the Rajbari study area show that the FoS fluctuates as the water level changes. Based on the stability analysis results, the Janbu method provides the lowest factor of safety (FoS) of 1.44 for a polygonal slip surface at a high water level of 10.5 m, while the Sarma method yields the maximum FoS of 3.27 for a normal water table of 5 m. According to the stability analysis for rapid drawdown conditions, when the groundwater table (GWT) is 3 meters and the original water table (OWT) is 7.7 meters, the highest factor of safety (FoS) for polygonal slip surfaces is 2.94 for Sarma method, while the lowest FoS for polygonal slip surfaces is 1.37 for Janbu method. As the surcharge decreases, the stability study results show that the FoS increases. A polygonal slip surface under a maximum surcharge of 150 kN/m2 provides a minimum FoS of 0.99 for Janbu method, but a polygonal slip surface without a surcharge has a maximum FoS of 9.56 for Sarma method. According to the seepage analysis, as the water level rises, the rate of pore water pressure also rises. The pore water pressure reaches its maximum of approximately 381 kPa at a water height of 8 meters, while the lowest is 304.6 kPa at a low water height of 1 meter. The seepage research also shows that as water levels rise, suction falls. It has been observed that the suction value is -116.54 kPa at a low water level of 5 meters and -55.8 kPa at an 8-meter water height. When the water level is 10 meters, the inflow/outflow is high at 2.17 m3/m/day; when the water level is 1 meter, it is 0.06 m3/m/day.
In the Shariatpur study area (Noria), a river bank stability analysis has been carried out. There are no embankments in this area. Therefore, a riverbank slope and an apparent surcharge of 30 and 60 kN/m2 are considered for the stability analysis. The findings of the stability analysis demonstrate that the Bishop method provides a maximum FoS of 6.49 for a circular slip surface without surcharge, whereas a minimum FoS of 2.15 under a polygonal slip surface subjected to the maximum surcharge of 60 kN/m2 for Janbu method. It has also been shown that for a normal water table of 2 m, the Spencer method for a circular slip surface provides the maximum FoS at 5.69 for the water level variation. In contrast, a polygonal slip surface at a high water table of 3.5 m yields a FoS of 1.77 using the Morgenstern-Price method. For rapid drawdown conditions, when the groundwater table (GWT) is at -2 meters and the original water table (OWT) is at 3.5 meters, the stability analysis indicates that the highest FoS for the polygonal slip surfaces using the Morgenstern-Price method is 3.99, whereas the lowest FoS using the Janbu method is 1.77 for circular slip surfaces. The seepage analysis results have shown that the pore water pressure increases with the water level. At a minimum water level of -3 meters, the pore water pressure measures 177.5 kPa, while at a maximum water level of 3.5 meters, the pore water pressure reaches approximately 261.10 kPa. It also reveals that the suction value is -49.77 kPa at a water height of 3.5 meters and -99.51 kPa at a low water level of -3 meters. It has been demonstrated that the amount of point inflow or outflow increases as the water level rises. When the water level is 3.5 meters, the inflow/outflow is high at 4.68 m3/m/day, and when the level is -3 meters, it is 0.13 m3/m/day.
The results of the seepage analysis have shown that the seepage velocity and water pressure have a major impact on the slope stability. The findings of the numerical study demonstrate that the high water levels substantially affect seepage conditions, which may cause failure during floods. Although field tests, laboratory investigations, and numerical studies can identify potential risks and failure processes, these studies clearly cannot offer total confidence regarding the long-term stability of an embankment. To ensure the stability of an embankment, steps to lessen the effects of floods, such as better drainage systems, better embankment designs, early warning systems, frequent monitoring, proper geotechnical assessment methods, and improved disaster mitigation planning for Bangladesh's flood-prone areas are needed.