Coupled discrete-continuum method for studying load-deformation of a stone column reinforces rail track embankments

Stone columns are being increasingly used as a cost-effective and environmentally friendly method for reinforcing soft soils of rail track embankments. Deformation behavior of stone columns reinforced soft clay has been the subject of an extensive number of experimental and modelling studies during last decades. A continuum-based numerical method provides valuable insights into the settlement, lateral deformation, stress and strain-rate dependent behavior of stone column at macroscopic scale. However, due to the discrete nature of stone columns, which are comprised of granular aggregates, they cannot be properly modelled by the continuum methods. This paper presents a novel coupling model of discrete element method (DEM) and finite difference method (FDM) to investigate the load-deformation behavior of stone columns considering micromechanical analysis. In the coupled discrete-continuum model, the soft soil domain under track embankment is modelled by the continuum method using FLAC and stone column is modelled by the discrete element method using PFC2D. A force-displacement transmission mechanism is introduced to achieve the interaction of both domains in which the DEM transfers forces and moment to the FDM and then the FDM updates displacements back to the DEM. The predicted load-deformation results are in good agreement with the data measured experimentally; indicating that the proposed coupling discrete-continuum model could capture the deformation behavior of stone column reinforced soft soils

A study of the geogrid–subballast interface via experimental evaluation and discrete element modelling

This paper presents a study of the interface of geogrid reinforced subballast through a series of large-scale direct shear tests and discrete element modelling. Direct shear tests were carried out for subballast with and without geogrid inclusions under varying normal stresses of σn= 6.7 to 45kPa. Numerical modelling with three-dimensional discrete element method (DEM) was used to study the shear behaviour of the interface of subballast reinforced by geogrids. In this study, groups of 25–50 spherical balls are clumped together in appropriate sizes to simulate angular subballast grains, while the geogrid is modelled by bonding small spheres together to form the desired grid geometry and apertures. The calculated results of the shear stress ratio versus shear strain show a good agreement with the experimental data, indicating that the DEM model can capture the interface behaviour of subballast reinforced by geogrids. A micromechanical analysis has also been carried out to examine how the contact force distributions and fabric anisotropy evolve during shearing. This study shows that the shear strength of the interface is governed by the geogrid characteristics (i.e. their geometry and opening apertures). Of the three types of geogrid tested, triaxial geogrid (triangular apertures) exhibits higher interface shear strength than the biaxial geogrids; and this is believed due to multi-directional load distribution of the triaxial geogrid

Improved performance of railroad ballast using geogrids

© Springer Nature Singapore Pte Ltd. 2019. Geogrids are commonly used to stabilise and reinforce ballast, and over the various laboratory tests have been carried out to determine how geogrids affect the interface between geogrid and ballast aggregates. This paper presents a critical review and interpretation of the results of large-scale direct shear tests and cyclic tests on key parameters such as the interlocking effects of aperture size and the location of geogrids. Field investigations from sites at Bulli and Singleton as well as findings from Discrete Element Modelling, including the influence zone of geogrid and the linear relationship between geometric anisotropy and stress ratio, are examined and discussed. It also includes a presentation and discussion of analytical modelling for quantifying the geogrid reinforcing effect (pullout tests)

Modelling of geogrid-reinforced ballast under direct shear and impact loading

Railways provide an efficient and economic transport mode in many parts of the developed countries including Australia, China, South Korea and the USA. Ballast layer is designed as a load bearing layer for rail tracks and to be free draining, but when the ballast voids are wholly or partially impeded due to the intrusion of fine particles or ballast breakage, the ballast can be considered to be fouled. Ballast degradation causes a reduction in the drainage capacity of ballast, thereby reducing the track resiliency and triggering high maintenance costs. Geosynthetics are commonly used in railway construction for reinforcement and stabilisation purposes. When railway ballast becomes degraded, the beneficial effect of geosynthetics could significantly decrease. A series of drop-weight impact tests and direct shear tests for ballast with and without the inclusion of geosynthetics are carried out in the laboratory. Discrete element modelling (DEM) is also carried out on ballast with and without the inclusion of geogrids. Load-deformation and ballast breakage responses obtained from the DEM simulations are in reasonable comparison with those measured experimentally. The research outcomes of this study can provide a fundamental laboratory and computational framework to assist practicing engineers in track design considering the role of geosynthetic inclusions

Improved Performance of Ballasted Rail Tracks Using Plastics and Rubber Inclusions

Current railroads require significant upgrading to meet the challenges of heavier loads at higher speeds. Due to excessive track degradation, the Australian rail industry spends large amounts on frequent track repair and maintenance, as well as ground improvement prior to track construction where soft and saturated subgrade soils pose considerable difficulties in design and construction. Moreover, the degradation of ballast particles under impact loading seriously hampers the safety and efficiency of rail tracks, which leads to speed restrictions and more frequent track upgrading. Hence, there is a need for innovative design solutions that can extend the service life of tracks to cater for faster and heavier train traffic. The use of planar geosynthetics and recycled rubber mats placed at the interface of ballast and subballast layer has proven an effective approach to mitigate ballast degradation and improve track longevity. This paper presents the current state-of-the-art knowledge of rail track geomechanics conducted at the University of Wollongong (UOW) including topics relating to laboratory testing and computational modeling approaches. The load-deformation responses of rubber mat/geogrid-stabilised ballast are studied in the laboratory using a large-scale drop weight impact testing facility, and Track Process Simulation Apparatus (TPSA). Numerical modelling using discrete element methods (DEM) are used to model geogrid-reinforced ballasted tracks, capturing both the discrete nature of ballast subject to various types of loading and boundary conditions. These results provide promising approaches to incorporate into the existing track design routines catering for future high speed and heavy haul trains

DEM modelling of geocells reinforced subballast in rail tracks

This paper presents a study of the load-deformation behaviour of geocell-reinforced subballast under cyclic loads using laboratory tests and discrete element method (DEM). A series of large-scale laboratory tests with and without geocell inclusions are carried out using a track process simulation apparatus to study the beneficial effect of the geocells in decreasing the lateral and vertical deformations of railway subballast. Numerical studies conducted in the DEM can capture the reinforcement effect of geocells, considering micromechanical analysis subjected to a given frequency and load cycles, the predicted load-settlement response of the subballast with and without geocell agrees well with those measured experimentally. This finding indicates that the proposed DEM model is able to capture the deformation behaviour of the subballast reinforced by the geocells. These observations clearly emphasise the beneficial effects of the geocell in decreasing the deformation of subballast from a micromechanical perspective

Behavior of fresh and fouled railway ballast subjected to direct shear testing: Discrete element simulation

This paper presents the three-dimensional discrete element method (DEM) that was used to study the shear behavior of fresh and coal fouled ballast in direct shear testing. The volumetric changes and stress-strain behavior of fresh and fouled ballast were simulated and compared with the experimental results. Clump logic in particle flow code in three dimensions (PFC3D) incorporated in a subroutine was used to simulate irregular-shaped particles in which groups of 10-20 spherical balls were clumped together in appropriate sizes to simulate ballast particles. Fouled ballast with a various void contaminant index (VCI) ranging from 20 to 70% VCI was modeled by injecting a specified number of miniature spherical particles into the voids of fresh ballast. The DEM simulation captures the behavior 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 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. © 2014 American Society of Civil Engineers

The use of reproductive healthcare at commune health stations in a changing health system in Vietnam

Background: With health sector reform in Vietnam moving towards greater pluralism, commune health stations (CHSs) have been subject to growing competition from private health services and increasing numbers of patients bypassing CHSs for higher-level health facilities. This study describes the pattern of reproductive health (RH) and family planning (FP) service utilization among women at CHSs and other health facilities, and explores socio demographic determinants of RH service utilization at the CHS level

Exploration of an innovative draw solution for a forward osmosis-membrane distillation desalination process

© 2017, Springer-Verlag Berlin Heidelberg. Forward osmosis (FO) has emerged as a viable technology to alleviate the global water crisis. The greatest challenge facing the application of FO technology is the lack of an ideal draw solution with high water flux and low reverse salt flux. Hence, the objective of this study was to enhance FO by lowering reverse salt flux and maintaining high water flux; the method involved adding small concentrations of Al2(SO4)3 to a MgCl2 draw solution. Results showed that 0.5 M MgCl2 mixed with 0.05 M of Al2(SO4)3 at pH 6.5 achieved a lower reverse salt flux (0.53 gMH) than that of pure MgCl2 (1.55 gMH) using an FO cellulose triacetate nonwoven (CTA-NW) membrane. This was due possibly to the flocculation of aluminum hydroxide in the mixed draw solution that constricted membrane pores, resulting in reduced salt diffusion. Moreover, average water fluxes of 4.09 and 1.74 L/m2-h (LMH) were achieved over 180 min, respectively, when brackish water (5 g/L) and sea water (35 g/L) were used as feed solutions. Furthermore, three types of membrane distillation (MD) membranes were selected for draw solution recovery; of these, a polytetrafluoroethylene membrane with a pore size of 0.45 μm proved to be the most effective in achieving a high salt rejection (99.90%) and high water flux (5.41 LMH) in a diluted draw solution