EFFECT OF POISSON’S RATIO ON PORE PRESSURES AND DISPLACEMENTS DURING VACUUM CONSOLIDATION

With rapid urbanization, population is moving into the coastal regions around the world. These regions mostly consist of highly compressible and weak soft soil up to great depths. It is essential to improve these soils to achieve desired load carrying capacity. In this environment, surcharge preloading will not only be time taking it will also be needed in large height. Vacuum preloading can be of significant utility compared to the surcharge load. Reduction in pore pressure during vacuum consolidation is a complex phenomenon & is not uniform throughout the soil mass. Near the drainage boundaries i.e., PVDs it will be rapid, and between the PVDs, it will be gradual. However, it is observed that pore pressures show an initial increase between the two drainage boundaries before they finally decrease. This phenomenon of increase in pore pressure due to the application of load is called as Mandel-Cryer effect. The increase is small when compared to the overall vacuum pressure applied. Rapid reduction in pore pressure at the drainage boundary is considered to be the main cause behind this phenomenon. To have a better understanding of this behaviour in vacuum consolidation, a two-dimensional numerical study has been conducted using Plaxis 2D incorporating fully coupled flow-deformation analysis. The pore pressure changes at different depths in the soft clay layer have been shown. The effect of variation in poisson’s ratio on the change in pore pressure is discussed. Displacements obtained with different Poisson’s ratio of soft clay are also compared and presented.