Behaviour of footing on fill slope under cyclic loading conditions

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Copyright: Alam, Md. Jahid Iftekhar
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Abstract
In many practical cases, footings need to be constructed on or near the crest of a fill slope, for example, a bridge abutment supported by an embankment slope. The loading on this type of footing consists of dead load, sustained live load and cyclic live load. In practice, the design of a footing is often based on approximations by considering an ‘equivalent’ monotonic load and the use of a large factor of safety. As this type of footing rest on well-compacted soil and is under dominantly vertical concentric loading, the design issue is mainly about the accumulation of displacement under a large number of loading cycles. Studies on experimental or numerical investigations of this class of problem are very limited. There are severe challenges in utilising the findings of these limited model studies to predict performances under field conditions. In this thesis, an attempt has been made to investigate the behaviour of a footing on unreinforced and geosysnthetic reinforced dense sandy fill slope under cyclic loading using a combination of large-scale laboratory model testing and numerical modelling. Large-scale laboratory experiments of a model footing on a flexible geogid reinforced dense sandy fill was conducted under different cyclic loading conditions. In addition to applying continuous cyclic loading of single waveform to the model footing, experiments were performed to examine the effect of load interruptions. It was found that, there was a negligible effect of load interruptions on the permanent footing displacement and residual soil stress when the load was held at the trough of the cyclic loading amplitude. However, significant increases of both the permanent footing displacement and residual soil stress were observed in each phase of the test when the load was entirely released during hold periods. A number of monotonic and cyclic triaxial tests were conducted to develop a consistent but simple material model for dense sandy soil under cyclic loading conditions. It was found that most of the published soil models, that can capture the unload-reload loop and it’s linkage to the accumulation of permanent strain, are for loose sand. Therefore, an analytical equation for permanent strain, that takes into account the effect of confining pressure, cyclic deviatoric stress, and number of load cycles, was proposed for dense sand. Particular emphasis was placed on linking the stress-strain behaviour of an unload-reload cycle to the accumulation of permanent strain. The proposed soil model was incorporated into FLAC2D to predict the behaviour of the model testing. All the model input parameters were objectively determined from cyclic triaxial test results. The analysis results showed that, for both the unreinforced and reinforced conditions, the soil model behaved well in the numerical analyses, i.e., it presented nil numerical problems. This means the model can be used in a prediction exercise. The predictions for the footing displacements and soil stress (i.e., the relationships between vertical and horizontal permanent footing displacements and residual soil stress with load cycles) compared well with observations from the model tests in terms of overall trends.
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Author(s)
Alam, Md. Jahid Iftekhar
Supervisor(s)
Gnanendran, Dr Carthigesu Thiagarajah
Al-Deen, Dr Safat
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Publication Year
2017
Resource Type
Thesis
Degree Type
PhD Doctorate
UNSW Faculty
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