Two decades of advancement in process simulation testing of ballast strength, deformation, and degradation

© Copyright 2018 by ASTM International. This paper describes salient features of a set of large-scale ballast testing equipment developed at the University of Wollongong, Australia, and how the test results and research outcomes have contributed to transforming tracks in the Australian heavy haul and commuter networks, particularly with regards to the strength, deformation, and degradation of ballast. Ideally, ballast assemblies should be tested in prototype scale under actual loading conditions. This is because a reduction in particle sizes for testing in smaller equipment can reduce the internal angle of friction (shearing resistance) of the granular assembly in a macro sense, and the angularity of the particles in a micro sense, and hence the volumetric changes during the shearing process. In response to the worldwide lack of proper test facilities for ballast, the University of Wollongong has, since the early 1990s, designed and built a number of large-scale process simulation triaxial testing rigs. They are all custom made to minimize any boundary effects and also to evaluate the deformation and degradation of ballast, particularly the size, shape, and origin of aggregates used as ballast in Australian tracks. This triaxial process simulation equipment was originally used to characterize the behavior of coarse aggregate used for state railway standards for monotonie loading, but since then it has been fitted with dynamic actuators to simulate actual track conditions involving the true cyclic loading nature while also capturing the wheel-rail dynamics that correspond to high-speed commuter rail and fast heavy-haul operations. These tests invariably demonstrated completely different stress-strain and volumetric characteristics of ballast compared to conventional static or monotonie testing of the same test specimens

True triaxial testing of geogrid for high speed railways

This work describes a series of novel experimental tests to determine the potential of geogrids to confine granular layers within ballasted railway lines operating at speeds close to critical velocity. This is important because at low train speeds, vertical stresses are dominant, but when approaching critical velocity conditions, dynamic horizontal stress levels are magnified. Therefore the majority of previous geogrid investigations have been performed assuming constant horizontal stress levels, thus making them more relevant for lower speed lines. To investigate settlement under high relative train speeds, ballasted railway track samples were subject to combined vertical-horizontal cyclic loading. Three areas were explored: (1) the performance benefit from placing geogrid at the ballast-subballast interface, (2) the performance benefit from placing geogrid at the subballast-subgrade interface, (3) the effect of subgrade stiffness on geogrid performance at the subballast-subgrade interface. Testing was performed using a unique large-scale true triaxial apparatus which had the ability to vary stress levels in three Cartesian directions. Compared to the control conditions, the geogrid offered a settlement improvement of approximately 35% when placed at the ballast-subballast interface, and 10–15% when placed at the subballast-subgrade interface. Regarding subgrade CBR, it was found that the geogrid offered the greatest performance benefits when the subgrade was soft. Therefore it was concluded that for the ballasted rail structures under test, when subject to elevated levels of horizontal stress, geogrids reduced settlements compared to non-geogrid solutions

Mineral precipitation and the associated reduction of hydraulic conductivity in a PRB

Permeable reactive barriers (PRB) have been used worldwide for the in-situ treatment of con-taminated ground water. One such contamination found in Australia is acidic groundwater enriched with high concentrations of dissolved aluminium and iron produced in acid sulfate soil (ASS) terrains. To treat this acidi-ty in groundwater, a PRB was installed in the Shoalhaven Floodplain and then monitored over a seven year pe-riod. This remediation process was satisfactory but there was a small decrease in efficiency due to a secondary mineral precipitation (aluminium and iron oxy/hydroxides) which reduced the hydraulic conductivity of the PRB. Numerical modelling carried out through MODFLOW and RT3D software showed that this reduction in hydraulic conductivity due to secondary mineral precipitation was 3% at the entrance of the PRB after seven years of installation. This result was satisfactory considering that the predicted longevity of the barrier was 19.5 years, assuming a mean groundwater velocity of 0.05 m/day

Implications of ballast degradation under cyclic loading

In spite of recent advances in track geotechnology, the understanding of the mechanisms of ballast degradation is vital for improved design to withstand high speed cyclic loading. The research conducted at Centre for Geomechanics and Railway Engineering (CGRE) at University of Wollongong (UOW) has shown that ballast degradation is influenced by various factors including the amplitude, frequency, number of load cycles, particle size distribution, confining pressure, angularity and fracture strength of individual grains. A series of cyclic drained triaxial tests were conducted using a largescale cylindrical apparatus designed and built at UOW for frequencies ranging from 10-40 Hz. A low range of confining pressures to resemble \u27in-situ\u27 track conditions was applied. The results showed that permanent deformation and degradation of ballast increased with the frequency. Variation of the resilient modulus with respect to the degree of degradation is also discussed. Two-dimensional discrete element method (DEM) and finite element method (FEM) simulations were also carried out to capture the behaviour of ballast and the numerical results were compared with the laboratory and field data. These results quantifying the geotechnical behaviour of ballast on the micro and macro scale are described in the paper. Practical implications of these findings are discussed through field monitoring of full-scale instrumented track sections at Bulli in New South Wales

Consolidation analysis of a stratified soil with vertical and horizontal drainage using the spectral method

A novel use of the spectral method to determine excess pore water pressure during vertical consolidation of multi-layered soil with time constant material properties is presented, considering a unit cell with combined vertical and radial drainage. Equal strain conditions are assumed in the analysis. The novel adoption of material properties that vary in a linear fashion with depth allows arbitrary distributions of properties to be modelled. By incorporating surcharge and vacuum loading that vary with both depth and time, a wide range of consolidation problems can be analysed. The spectral method is a meshless approach producing a series solution to the consolidation problem based on matrix operations. Accuracy can be improved by increasing the number of terms used in the series solution. The model is verified by the analysis of selected case studies characterised by: analytical free strain consolidation with thin sand layers (surcharge only); laboratory test and embankment trial with surcharge and vacuum loading; and ground subsidence caused by groundwater pumping

Mathematical Modeling and Field Evaluation of Embankment Stabilized with Vertical Drains Incorporating Vacuum Preloading

This study presents the analytical modeling of vertical drains incorporating vacuum preloading in both axisymmetric and plane strain conditions. The effectiveness of vacuum pressure (i.e. both constant vacuum pressure and varied vacuum pressure) applied along the drain is considered. A multidrain plane strain model is employed to analyse an embankment at the site of Second Bangkok International Airport (SBIA) stabilised with prefabricated vertical drains. At this site, a significantly reduced height of sand surcharge was applied by reducing the pore pressures through vacuum preloading. The results of FEM analysis confirm the efficiency of vacuum preloading in comparison with the conventional method of surcharge alone

Rapport général du TC202 Géotechnique pour les infrastructures de transport

Today’s needs of urban transportation including roads, railways, airports and harbours demand significant resources for infrastructure development in view of rapid and efficient public and commercial (freight) services. In most cases, authorities have had difficulties in meeting these service demands due to the rapidly growing public, industrial, mining and agricultural sectors in many parts of the world. In order to maximise efficiency and to reduce the costs of maintenance, sound technical knowledge is required. This general report presents major technical advancements around the globe encompassing 33 articles from 19 countries and it is classified into 6 key categories, namely: compaction and subgrade improvement, laboratory testing, theoretical advancements and contributions to design, applications of geosynthetics, numerical modelling and field performance evaluation.De nos jours, les besoins en transports urbains (routes, chemins de fer, aéroports aériens et maritimes) nécessitent d’importantes ressources pour le développement des infrastructures en vue d’assurer des services commerciaux rapides et efficaces. Dans la plupart des cas, en raison de la croissance rapide des secteurs public, industriel, minier et agricole, les autorités se trouvent confrontées à des difficultés pour atteindre les services escomptés. Un savoir technique est alors nécessaire en vue de maximiser l’efficacité et de réduire le coût d’entretien. Le présent rapport général expose les avancées techniques majeures à travers le monde synthétisant 33 articles émanant de 19 pays ; six thèmes clés sont classés : compactage et amélioration des assises, expérimentation en laboratoire, développements théoriques et contributions au dimensionnement, applications des géosynthétiques, modélisations numériques et évaluation des performances sur le terrain.(undefined

Filtration of broadly graded soils: the reduced PSD method

Granular filters are used in earth structures, such as embankment dams, to protect fine soils from erosion due to seepage forces. Successful filtration requires that the filter voids are fine enough to capture some of the coarse fraction of the base soil. These retained particles are then able to capture progressively finer base soil particles, and eventually a filter interface forms that is able to prevent any further erosion. This process is called self-filtration. Lafleur et al. (1989) examined self-filtration in cohesionless, broadly graded base soils. It was found that the extent of mass loss before selffiltration occurs was greater in broadly graded materials: hence a finer filter was required to reduce this mass loss. Filters for cohesive base soils are commonly designed using the Sherard & Dunnigan (1985) design criteria. While these criteria have been developed from extensive laboratory data, they may not be applicable to all fine base soils, particularly broadly graded materials. In this paper, a series of filtration tests on various base soils are described. Data from the current study, and the published results of laboratory tests from several sources are compared to examine the filtration of broadly graded base soils. Based on this analysis, a new design procedure is proposed for filters to protect fine base soils, which determines the ability of the coarse fraction of the base soil to retain the fine fraction (i.e. a self-filtering base soil)