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

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

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

Soft ground improvement by vacuum-assisted preloading

This paper describes the behaviour of soft soil foundation stabilized with vacuum-assisted preloading at the New Bangkok International Airport, Thailand. An analytical solution considering the variation of soil permeability and compressibility and a finite element analysis based on an equivalent plane strain model developed by the authors are employed to investigate the performance of the test embankment. The converted equivalent plane strain parameters are incorporated in the finite element code ABAQUS. The associated settlement, excess pore pressure and lateral movement are predicted and compared with the available field measurement. The data indicate that the efficiency of the prefabricated vertical drains depends on the magnitude and distribution of vacuum pressure as well as on the extent of air leak protection provided in practice. The height of sand surcharge and consolidation time are significantly reduced in comparison with the conventional method of surcharge alone. The effectiveness of this method, its economies and its merit potential are also discussed

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)

General report TC202: transport geotechnics

Today\u27s 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 hte glob 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 geoysynthetics, numerical modelling and field performance evaluation

Plane-strain modeling of smear effects associated with vertical drains

The present study provides a methodology to include the smear effect of vertical drains in a two-dimensional (2D) plane-strain finite-element model, employing the modified Cam-clay theory. The analysis is conducted by converting the radius of the smear zone and its permeability (axisymmetric) into equivalent plane strain parameters. The introduction of smear effects improves the accuracy of the numerical model that is tested for a Malaysian soft clay, in this study

An analytical model of PVD-assisted soft ground consolidation

Prefabricated vertical drains (PVDs) are widely used to accelerate the dissipation of excess pore pressure in soft estuarine deposits in coastal areas under fill surcharge or vacuum preloading. Vacuum preloading can also control any lateral outward movement of the embankment toe, although excessive inward movement must be avoided. An equivalent 2D numerical modelling is proposed as a predictive tool for multi-drain conditions, but unlike a 3D simulation it has a greatly reduced computation burden. A unit cell model incorporating key factors such as the smear effect, vacuum distribution, nonlinear compressibility and permeability, soil disturbance and large-strain geometry, has also been developed. This paper presents selected work at the University of Wollongong on analytical solution for PVD-assisted ground improvement. The model is applied to a case history at Tianjin Port in China and then compared with the numerical result and field measurements

Performance Appraisal of Ballasted Rail Track Stabilised by Geosynthetic Reinforcement and Shock Mats

Rail tracks serve the principal mode of transportation for bulk freight and passengers in Australia. Ballast is an essential constituent governing the overall stability and performance of rail tracks. However, large repetitive loads from heavy haul and passenger trains often lead to excessive deformation and degradation of the ballast layer, which necessitate frequent and expensive track maintenance works. In Australia, the high cost of track maintenance is often associated with ballast degradation, fouling (e.g. coal and subgrade soil) and associated poor drainage, differential settlement of track, pumping of subgrade soils, and track misalignment due to excessive lateral movements. With increased train speeds, the track capacity is often found to be inadequate unless more resilient tracks are designed to withstand the substantially increased vibration and repeated loads. A field trial was conducted on a section of track in Bulli, New South Wales, and findings indicated that the moderately-graded recycled ballast when used with a geocomposite resulted in smaller deformations in both vertical and lateral directions in comparison to uniformly-graded fresh ballast. Installing resilient (shock) mats in the track substructure led to significant attenuation of high impact forces and thereby mitigated ballast degradation. In addition, a series of full-scale field experiment was undertaken on track sections near Singleton, New South Wales to investigate the effects of geosynthetics on the performance of the track built on subgrade soils with varying stiffness. The finding suggested that geogrids can decrease vertical strains of the ballast layer and a few selected types of geogrids can be used more effectively with soft subgrade soils. This state of the art & practice (SOAP) paper describes the results of two unique full-scale field trials, series of large-scale laboratory tests and numerical models to assess the improved performance of ballasted rail tracks using synthetic grids and shock mats

Geotechnics of granular materials for railways and port reclamation

Rail is one of the largest transportation modes offering freight and passenger traffic in rapidly developing nations, including India. Concerted efforts to improve productivity, modernization and technology upgrading have led to an impressive growth in railways and port infrastructure. Large-scale physical modelling and full-scale field monitoring often provide significant knowledge to better understand performance and to extend the current state-of-the-art in design of rail and port. A series of large-scale laboratory tests were conducted to analyse behaviour of granular materials (ballast, sub-ballast) under cyclic and impact loads. Effectiveness of using geosynthetic grids, geocells and shock mats was also studied. Comprehensive field trials were carried out on instrumented rail track sections and port reclamation sites in the towns of Bulli and Singleton and in Port Kembla (Wollongong), respectively. This keynote paper provides an insight to geotechnical behaviour of granular materials capturing the effects of degradation and deformation in railways and potential of using granular waste for the reclamation project in port