Stability of embankment slopes subjected to seepace

Abstract

Bangladesh is badly affected by floods every year due to the sudden onrush of rain waters on its extensive network of rivers during the monsoons, most of it coming from neighbouring countries. Many highway and flood control embankments of the country are subjected to high water tables on one-side. The resulting seepage through the embankment may result in significant reduction in the stability of the embankment slope on the country side. These embankments are generally constructed with locally available cohesive soil. Due to improper compaction and lack of quality control in embankment construction practice in Bangladesh, many of these embankments are likely to have low shear strengths. Moreover, steep slopes are not uncommon. It is therefore of concern if the slopes of these embankments will be stable under adverse conditions of floods and earthquakes. An extensive numerical analysis is carried out on the slope stability of earthen embankments subjected to high flood level on one side using the computer program PC-ST ABL. The program calculates the factor of safety against the instability of a slope by the method of slices, based on two-dimensional limiting equilibrium. Simplified lanbu's method with correction factor and Simplified Bishop's method have been used to obtain the minimum factor of safety on circular slip surfaces, commonly observed during slope fuilures in cohesive soils. The embankment is assumed to be homogeneous, isotropic and without any drainage filters which is quite compatible with common practice of embankment construction in Bangladesh. The shape of the phreatic surfuce is assumed to be that given Casagrande's method. Effective shear strength parameters (c', q/) are considered for the stability analysis. The influence of various parameters such as embankment height, slope angle, soil strength parameters, unit weight on the slope stability for conditions of high flo.od level is studied. Embankment height is varied from 3m to 12m. Slope varies from IH:IV to 3H:IV. The range considered for shear strength parameter c' is 10 kPa to 30 kPa while that for cp' is 10° to 40°. Extreme condition of flood related seepage is studied by considering the water level on the river side to be 0.5m below the crest level. Some slope stability analyses are also done for embankment on softer foundation. Pseudo-static limiting equilibrium analysis is performed to study the effect of horizontal ground motion generated by earthquakes on an embankment already affected by seepage. The cases of 0.15g and 0.25g peak ground acceleration are considered. Finally, design charts have been developed, similar to those of Bishop and Morgenstern (1960). These charts are based on dimensionless parameters c'/yH, slope and cp'. They give the minimum factor of safety for the conditions of extreme seepage, no seepage and seepage with earthquake. As further design aids, minimum slope required for different embankment heights and soil properties are presented for several cases. These charts are expected to be useful for rapid preliminary design or stability reassessment of flood control embankments in Bangladesh.