Numerical Simulation of the Installation of Vibro Displacement Columns in Normally Consolidated Clay Using a Field Case Study

This paper presents the findings of a comprehensive numerical modelling study investigating the performance of vibro displacement crushed stone columns installed in a typical soft clay considering detailed simulation of the installation process. The installation alters the stresses in the surrounding soil and hence the soil-structure interaction (SSI). The volume of the column increases as the surrounding soil is compacted during the displaced installation process, resulting in increased lateral stresses and excess pore water pressures. Subsequently, the excess pore water pressures produced are expected to disperse towards the porous surface over time, during the consolidation stage. The specific details of the installation method are modelled in this paper using the finite-difference software FLAC, with a modelling scheme utilising coupled mechanical and hydraulic conditions. The behaviour of the normally consolidated soft clay and medium clay layers was simulated using the modified Cam clay soil model. The elastoplastic Mohr-Coulomb soil model was used to model the behaviour of the cohesionless silty sand layer and the crushed stone used in the displacement column. The numerical simulation permits a quantitative post-installation evaluation of the lateral earth pressure coefficient and highlights the SSI improvements resulting from the column installation. The numerical model results were validated using field data collected from a full-scale column load test conducted in Port Fouad, located east of Port Said in Egypt. The most effective parameters, such as the clay compression index, crushed stone cushion thickness, and distance between stone columns, were incorporated into a parametric study to evaluate their effect on long-term settlement. Even though a deep mixed column (DM) and a non-displacement reinforced concrete pile behave differently than a vibro displacement column (VDC), most projects used an economic comparison to choose the least expensive system. Therefore, under the same soil and loading conditions, the output of the VDC system was compared to that of a DM column and a non-displacement reinforced concrete pile. The study proposed a simplified method for estimating immediate settlement as well as long-term settlement based on the concept of a composite foundation. Furthermore, the study found that the ratio between the thickness of the top layer of crushed stone and the thickness of the soft clay should not be greater than 0.125 to optimise the cost.