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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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This paper presents the three-dimensional discrete element method (DEM) that was used to study the shear behaviour of fresh and coal fouled ballast in direct shear testing. The volumetric changes and stress-strain behaviour of fresh and fouled ballast were simulated and compared with the experimental results. 'Clump logic' in Particle Flow Code (PFC3D) incorporated in a MATLAB Code was used to simulate irregular shaped particles in which groups of ten to twenty spherical balls were clumped together in appropriate sizes to simulate ballast particles. Fouled ballast with various Void Contaminant Index (VCI), ranging from 20%VCI to 70%VCI, were modelled by injecting a specified number of miniature spherical particles into the voids of fresh ballast. The DEM simulation captures the behaviour of fresh and fouled ballast as observed in the laboratory showing that the peak shear stress of the ballast assembly decreases and the dilation of fouled ballast increases with an increasing of VCI. Furthermore, the DEM also provides insight to the distribution of contact force chains and particle displacement vectors, which cannot be determined experimentally. These micromechanical observations clearly justify the formation of a shear band and the evolution of volumetric changes during shearing. The reduced maximum contact force associated with increased particle contact area due to fouling explains the decreased breakage of fouled ballast. An acceptable agreement was found between the DEM model predictions and laboratory data.

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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.

Geogrids are commonly used in railway construction for reinforcement and stabilisation. When railway ballast becomes fouled due to ballast breakage, infiltration of coal fines, dust and subgrade soil pumping, the reinforcement effect of geogrids decreases significantly. This paper presents results obtained from Discrete Element Method (DEM) to study the interface behaviour of coal-fouled ballast reinforced by geogrid subjected to direct shear testing. In this study, irregularly-shaped aggregates (ballast) were modelled by clumping together 10-20 spheres in appropriate sizes and positions. The geogrid was modelled by bonding a large number of small spheres together to form the desired grid geometry and apertures. Fouled ballast with 40% Void Contaminant Index (VCI) was modelled by injecting a predetermined number of miniature spheres into the voids of fresh ballast. A series of direct shear tests for fresh and fouled ballast reinforced by the geogrid subjected to normal shear stresses varying from 15 kPa to 75 kPa were then simulated in the DEM. The numerical results showed a good agreement the laboratory data, indicating that the DEM model is able to capture the behaviour of both fresh and coal-fouled ballast reinforced by the geogrid. The advantages of the proposed DEM model in terms of capturing the correct stress-displacement and volumetric behaviour of ballast, as well as the contact forces and strains developed in the geogrids are discussed. ABSTRACTGeogrids are commonly used in railway construction for reinforcement and stabilisation.When railway ballast becomes fouled due to ballast breakage, infiltration of coal fines, dust and subgrade soil pumping, the reinforcement effect of geogrids decreases significantly. This paper presents results obtained from Discrete Element Method (DEM) to study the interface bahaviour of coal-fouled ballast reinforced by geogrid subjected to direct shear testing. In this study, irregularly-shaped aggregates (ballast) were modelled by clumping together ten to twenty spheres in appropriate sizes and positions. The geogrid was modelled by bonding a large number of small spheres together to form the desired grid geometry and apertures.Fouled ballast with 40% Void Contaminant Index (VCI) was modelled by injecting a predetermined number of miniature spheres into the voids of fresh ballast. A series of direct shear tests for fresh and fouled ballast reinforced by the geogrid subjected to normal shear stresses varying from 15kPa to 75kPa were then simulated in the DEM. The numerical results showed a good agreement the laboratory data, indicating that the DEM model is able to capture the behaviour of both fresh and coal-fouled ballast reinforced by the geogrid. The advantages of the proposed DEM model in terms of capturing the correct stress-displacement and volumetric behaviour of ballast, as well as the contact forces and strains developed in the geogrids are discussed.3