Numerical modelling of geosynthetic reinforced embankments over soft ground improved with deep cement mixed columns

Abstract

Embankment construction for infrastructure projects has increased considerably during the past few decades in coastal and low lying regions where soft clay is common. Geotechnical engineers dealing with foundation design under such site conditions face a real challenge due to the low shear strength and high compressibility characteristics of soft clay. A wide range of ground improvement methods have been developed to increase the bearing capacity of soft ground and thereby to increase the use of soft ground for the construction activities. The methods based on consolidation are available for the soil to gain stiffness and strength over a long period of time. However, they are often not economical due to the uncertainty in soil conditions and limited time available to complete the projects. Therefore, alternative innovative construction technologies are needed. One such solution is geosynthetic reinforced column supported (GRCS) embankments, which is an attractive sustainable technology for the 'fast-track' construction environments. This method provides many advantages against conventional consolidation based ground improvement techniques. In addition, this is economical compared to reinforced-concrete piled embankments. Consequently, this method has increasingly become popular among geotechnical engineers. Even though, a significant amount of research effort has been made on this area, the behaviour of these embankments is not well understood yet. This thesis describes a comprehensive numerical study conducted using the finite element method to improve the current knowledge on the behaviour of GRCS embankments. Two- and three- dimensional finite element modelling based on coupled theory of nonlinear porous media were carried out incorporating the strain-softening behaviour of cement stabilised columns beyond yield. Numerical model verification was carried out using a case history of a GRCS embankment constructed in Hertsby, Finland. Numerical modelling results of the case history were used to investigate arch formation within the embankment fill layers during embankment loading and subsequent consolidation. In addition, application of currently available design procedures for the design of GRCS embankments was investigated in detail using these results. Since column yielding was not dominant in the first case history, a deep cement mixed (DCM) column supported embankment, which is a part of the Ballina Bypass Alliance (BBA) project in northern NSW, Australia, was selected to investigate the embankment behaviour when column yielding is substantial. These results clearly show that the incorporation of strain-softening behaviour is essential to investigate the actual behaviour of GRCS embankments (especially the settlements, tension and strain in the geosynthetic, pore pressure generation and dissipation, and failure modes at the ultimate limit state), if the DCM columns yield under the embankment load. Thesis presents extensive two- and three- dimensional parametric studies based on the coupled theory of nonlinear porous media, incorporating the full geometry of the embankment and strain-softening behaviour of DCM columns. Influencing factors considered in the parametric study were elastic modulus of DCM columns, spacing and diameter of DCM columns, elastic modulus and permeability of soft foundation soil, stiffness and location of the geosynthetic reinforcement, embankment height and friction angle of the fill material. Embankment behaviour during the parametric study was investigated by comparing maximum total and differential settlements, maximum tension in the geosynthetic, maximum lateral deformation of columns, efficiency coefficient of columns and arching ratio. After that, results of two- and three- dimensional parametric studies were compared. Finally, three-dimensional parametric study results were summarised to emphasise the influence of each factor on the performance of GRCS embankments. An investigation of failure modes of GRCS embankments showed that the bending failure of columns and subsequent slip surface shear failure are critical for internal stability of GRCS embankments at the ultimate limit state (ULS). Consequently, new analytical equations for the stability calculation against bending failure were derived considering all possible driving and resisting moments, and an inclined failure plane as observed during the FEM analysis. Finally, the research is concluded with a summary of major conclusions, important design criteria and recommendations for future research on the design of GRCS embankments.

Date of Award	2013
Original language	English