Experimental and numerical study of coastal clays of Bangladesh

Abstract

Numerical and experimental investigations were undertaken in this research work to investigate how a critical state soil model (Modified Cam Clay model) and a frictional soil model (Mohr-Coulomb model) predict the stress-strain response of coastal clays of Bangladesh. The Mohr-Coulomb model was first used to predict the drained and undrained triaxial shear behaviour of laboratory samples of coastal clays of Bangladesh. The ultimate behaviour is reasonably well predicted using this model. However this model could not predict the undrained stress paths of coastal clays. The Modified Cam Clay (MCC) model using finite element method was used to predict the stress-strain, pore pressure and effective stress path of coastal clays under undrained triaxial condition. The MCC model was found to be a good qualitative predictor of the undrained response of coastal clays. It predicts the elasto-plastic strain hardening as observed experimentally. The Modified Cam Clay model predicts elastic response of highly overconsolidated clays before the onset of yield. The inability to predict elasto-plastic response before the onset of yield is a limitation of the model. However, the Modified Cam Clay model reallistically predicts the stress-strain and pore pressure response of coastal clays under undrained triaxial condition and various overconsolidation ratios. The Modified Cam Clay model was also used to predict the stress-strain and volume change response of coastal clays under drained triaxial condition. The stress-strain and volume change response of coastal clays as predicted by the MCC model matches well with the drained response of the experimental stress-strain and volume change data for coastal clays as available in published literature. An experimental program comprising of a series of model-scale footing and pile tests was carried out in order to investigate the footing and pile response to axial loads in coastal clays. The results obtained from these tests were compared to the footing and pile response predicted using the Mohr-Coulomb and Modified Cam Clay models. The Modified Cam Clay model predictions for the model footing test at consolidation pressure of 50 kPa and 150 kPa was observed to be nonlinear with a gradually decreasing slope which closely approximated the experimental data. Similar nonlinear predictions were observed for the model-scale pile test at consolidation pressure of 150 kPa.