Abstract
In Bangladesh, failure of embankment is occurred by numerous floods in almost every
year. To protect embankments from mass failure, some measures are taken such as the
use of sandbags and concrete blocks on embankment slope. These are sometimes
helpful to prevent surface erosion, but no measure is taken to increase the geotechnical
stability of the embankment to prevent its mass failure. Considering the socioeconomic
condition, some appropriate measures are necessary to improve the embankment soil.
In terms of cost, strengthening of the embankments by geosynthetic reinforcement is
one of the advantageous measures which can be applied in Bangladesh. However, the
properties of geosynthetic materials, and the interface behaviour between soil and
geosynthetic are important in the design and construction of reinforced soil structures.
But, the research on the interaction between soil and geosynthetic varying their physical
properties has not been done enough so far which is a major challenge in geotechnical
engineering. In this study, at first, the causes of embankment slope failure in Bangladesh
were investigated from geotechnical point of view. Afterwards, considering the possible
use of geosynthetics for reinforcing river embankment in the field, the interaction
behaviour of soil and reinforcement was evaluated through experimental and analytical
studies. Some important observations are made, and a new analytical technique has been
proposed for soil-geosynthetic interaction behaviour based on the test results.
The investigation on embankment failure in Bangladesh which is caused by huge floods
along the Padma and Jamuna rivers was carried out through field observations and
laboratory tests. The study found that the commonly used Jamuna river embankment
material contained high percentage of poorly graded sand which is not recommended as
fill material. On the other hand, the Padma riverbank material having comparatively
higher silt content generates excessive pore water pressure which is considered to be
one of the reasons of causing mass failure along the bank. Furthermore, the bank failure
mechanism was seen initiated with the formation of tension crack. By conducting
seepage and stability analysis of embankment, it is shown that the factor of safety would
be overestimated if the saturation line is predicted using conventional stability analysis
practices. Based on these facts, it was realized that some appropriate measures are very
much necessary for the improvement of embankment material in Bangladesh.
Focusing on the application of geosynthetics in the field, three types of backfill soils (sandy, clayey, and Toyoura sand) and four different geosynthetics (one geotextile and
three geogrids) were selected for evaluating the stress-strain relationship and their
interaction behaviour under various loading conditions in direct shear method. Test
results revealed that the relationship of normal stress and soil-geosynthetic interface
shear strength is mostly parabolic which is unlike the linear relationship usually
considered in case of soil itself. All types of geosynthetics showed distinct dilatancy
characteristics with Toyoura sand whereas very small effect was observed for both the
sandy and clayey soils. Based on the test results, a simplified model for stability
analysis is proposed, taking into account the interaction between soil and geosynthetic,
which showed good agreement with the experimental results.
The stress-strain relationships of Toyoura sand with different geosynthetic interfaces
were investigated by direct shear test. The results revealed that the geometric properties
of the geosynthetics have influences on the interface shear resistance. Besides,
numerical analysis was carried out using elasto-plastic interface model in FE analysis
for geosynthetic reinforced soil considering the shear banding and strain softening
behaviour of soil. Using the parameter values from the test results, the analysis has
obtained similar results with the experiments. This method could be used in the field for
the analysis of geosynthetic reinforced soil having strain softening and dilatancy
behaviour.
In this research, some significant information regarding geosynthetic reinforcement as
well as its interface behaviour with soil, under various conditions, have been revealed
through their stress-strain relationships, which was indistinct before. A new stability
analysis method, based on the parameters of a simple hyperbolic relationship, has been
proposed which was well verified with the experimental results. The elasto-plastic
numerical analysis, incorporating strain hardening-softening characteristics of soil, was
implemented successfully to predict the interface behaviour of soil with geosynthetics.
The outcome of this research has demonstrated that geosynthetics could be used
effectively to improve the stability of embankments in Bangladesh. In this regard, the
proposed model and analysis technique would be applied to address the interface
problem for the design of geosynthetic reinforced embankment.