A system of vertical drains combined with vacuum preloading is an effective method to accelerate soil consolidation by promoting radial flow. This study presents the analytical modeling of vertical drains incorporating vacuum preloading in both axisymmetric and plane strain conditions. The effectiveness of the applied vacuum pressure along the drain length is considered. The exact solutions applied on the basis of the unit cell theory are supported by finite element analysis using ABAQUS software. Subsequently, the details of an appropriate matching procedure by transforming permeability and vacuum pressure between axisymmetric and equivalent plane strain conditions is described through analytical and numerical schemes. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average excess pore pressure, consolidation settlement and time analyses. Finally, the practical implications of this study are discussed.

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A system of vertical drains with surcharge load to accelerate consolidation by shortening the drainage path is one of the most popular methods of soft ground improvement. The conventional radial consolidation theory (including smear and well resistance) have been commonly employed to predict the behaviour of vertical drains in soft clay. Its mathematical formulation is based on the small strain theory, and for a given stress range, a constant volume compressibility (m v ) and a constant coefficient of lateral permeability (k h ) are assumed. However, the value of m v varies along the consolidation curve over a wide range of applied pressure (∆p). In the same manner, k h also changes with the void ratio (e). In this paper, the writers have replaced m v with the compressibility indices (C c and C r ), which define the slopes of the e-logσ' relationship.Moreover, the variation of horizontal permeability coefficient (k h ) with void ratio (e) during consolidation is represented by the e-logk h relationship that has a slope of C k . In contrast to the conventional analysis , the current study highlights the influence of the C c /C k (or C r /C k ) ratio and the preloading increment ratio (∆p/σ′ i ) on the consolidation process. The analytical predictions are compared with the experimental results using a large scale consolidation chamber, and these predictions show good agreement with the measured data. Finally, an embankment case history taken from Muar Plains, Malaysia is analysed based on the current solution, and compared with field measurements.

Abstract:In this study, the extent of the smear zone and the reduction of permeability and water content within the smear zone were investigated using a large-scale consolidometer. The installation of vertical drains by means of a mandrel causes significant disturbance of the subsoil surrounding the mandrel, resulting in a smear zone. The extent of the smear zone for Moruya clay ͑New South Wales, Australia͒ was estimated on the basis of normalized permeability and the reduction of water content by taking undisturbed samples ͑horizontally and vertically͒ at different locations. This study reveals that a significant reduction in water content and horizontal permeability takes place towards the drain, whereas the variation in the vertical permeability is negligible. The smear zone for Moruya clay was found to be 2.5 times the equivalent radius of the mandrel with the horizontal permeability varying from 1.09 to 1.64, an average of 1.34 times smaller than that of the undisturbed zone. Finally, a correlation between the permeability decrease and water content reduction within smear zone is proposed.

Abstract:In this study, an attempt is made to analyse the extent of the smear zone caused by mandrel driven vertical drains, employing the cavity expansion theory for soft clay obeying the modified Cam-clay model. The predictions are verified by large-scale laboratory tests, where the extent of the smear zone was estimated based on the indications such as the pore pressure generated during mandrel driving, change in lateral permeability and the water content reduction. This study reveals that the radius of smear zone is about 4-6 times the equivalent vertical drain radius, and the lateral permeability (inside the smear zone) is 61-92% of that of the outer undisturbed zone. Finally, the predicted size of the smear zone using the undrained cavity expansion solution is incorporated in the finite element code PLAXIS to study the performance of a test embankment selected from the Sunshine Motorway, Queensland, Australia. A good agreement between the predicted values and field measurements was found.