Analysis of Radial Vacuum-Assisted Consolidation Using 3D Finite Element Method

In this study, a 3D numerical modelling of a single vertical drains consolidation incorporating vacuum preloading considering both vertical and horizontal drainage is presented. The effects of nonlinear soil compressibility, varying permeability and vacuum pressure distribution along the drain are examined through the dissipation of average excess pore pressure and associated settlement. A selected case history, using 3D finite element method, is employed to analyse two embankments constructed at the Second Bangkok International Airport, Thailand stabilised with prefabricated vertical drains and vacuum preloading. The behaviour of this embankment including settlements and excess pore pressures is then compared with the numerical predictions. This study reveals a close agreement between the predictions results obtained from the numerical model and the field measurements. INTRODUCTION In recent years, a system of vacuum-assisted preloading via prefabricated vertical drains (PVDs) has become the most popular methods of soft ground improvement (Indraratna and Chu, 2005). The utilisation of geosynthetic prefabricated vertical band drains to reduce the drainage path has become an economical and viable option due to its rapid installation (Bo et al.,2003). The implementation of vacuum application via PVDs is cost-effective, and the height of the surcharge embankment can be reduced to avoid embankments stability issue (Shang et al., 1998). The vacuum pressure increases the radial hydraulic gradient of soil surrounding PVDs, thereby increasing the consolidation rate. In order to analyse the behaviour of radial consolidation of soft soils, the unit cell theories and the equivalent plane strain (2D) numerical modeling have been employed to design and predict the soil behaviour (Barron, 1948; Hansbo, 1981; Hird et al., 1992; Indraratna and Redana, 2000). In the analysis, the vertical band drain and its influence zone has to be assumed in the analytical solution (Fig.1a). Either unit cell geometry or permeability in 2D analysis has to be made equivalent to obtain the same consolidation response (Fig. 1b). However, the discrepancy between the measured and predicted consolidation based on the approximate 2D modelling techniques can be found from previous studies (Hird et al. 1992). Borges (2004) described the advantages of 3D FEM for vertical drains. In this paper, a three dimenasional analysis was applied to two embankments at the Second Bangkok International Airport, Thailand using a finite element code (ABAQUS, Hibbitt, et al., 2004). In the numerical analysis, the actual band drains crosssection, influence zone, the parabolic permeability variation in the smear zone and the time-dependent vacuum pressure are considered. Subsequently, the 3D numerical results are compared to the field observations. Flow contact area = 2 l