Characterization of compacted coal wash as structural fill material

In this paper, detailed laboratory investigations were conducted on coal wash produced at coal mine, Wollongong, New South Wales, Australia. Geotechnical tests were conducted to determine the particle size distribution, compaction characteristics, shear resistance and collapse potential. The compaction tests were conducted under dry and submerged condition to examine the compactability and the strength of the coal wash. The test shows that compacted coal wash has good potential as effective fill for embankments, and land reclamation. Although when coal wash is compacted under submerged condition, increased level of compaction has minimal effect

The role of compaction energy on the small strain properties of a compacted silty sand subjected to drying - wetting cycles

The elastic properties of a soil are usually investigated to describe its engineering behaviour. The results of previous studies indicate that the effect of changes in suction on the elastic response at a small strain level of soils is significant during compaction and post-compaction periods. Limited efforts have been focused on quantifying those post-compacted responses due to the changes in suction induced by wetting and drying cycles. During their service life, most earth structures experience changes in hydraulic behaviour owing to climatic changes. These seasonal fluctuations in turn impact on the geomechanical performance of compacted soil. In this paper the aspects related to the elastic properties of compacted soils subjected to cycles of drying and wetting are described. Particular emphasis is placed on the effect of compaction energy on the hysteric behaviour (i.e. amplitude of the hysteresis loop) and its dependence on the initial stress state conditions and suction history. The results not only confirm the importance of the current suction in governing the shear and compression velocities and associated moduli, but they also suggest that subsequent drying-wetting cycles or suction history can further induce hysteretic changes, particularly along the wetting paths

A mixture of coal wash and fly ash as a pavement substructure material

The reuse of waste materials in engineering projects has become the subject of many research efforts worldwide as it provides economical as well as environmental benefits. Coal wash (CW) and fly ash (FA) are example waste materials that can be used as alternative aggregates in transportation infrastructure projects, specifically as base and subbase materials in roads. Class C FA has been extensively used as a stabilizing material due to its hardening potential. However, Class F fly ash, a non-pozzolanic material when used alone, has not been considered in past research projects. In this study, Class F fly ash is mixed with coal wash as a void filler to enhance its compaction efficiency and produce a compact and well interlocked structure. A laboratory testing plan is performed to assess the geotechnical properties of the mixtures with 0%, 7%, 10% and 13% FA content and it includes compaction tests, unconfined compressive strength tests, California Bearing Ratio (CBR) tests, collapse potential tests and permeability tests. The mixture with 7% FA is selected as the optimum mixture and its potential for tensile cracking under service loads is further investigated using four-point bending tests. Also, the resilient modulus and permanent deformations of the mixture are evaluated under different dry-back conditions using multistage repeated load triaxial tests

Consolidation Analysis of Soft Ground Improved by Stone Columns Incorporating Foundation Stiffness

© 2020 American Society of Civil Engineers. The consolidation of soft ground improved by stone columns is normally analyzed under equal strain or free strain conditions. In this study a new consolidation model for stone columns is proposed to capture actual field conditions that lie between these two hypotheses, where a permeable foundation layer on top of the unit cell is introduced. By considering the stiffness of this layer, a closed-form solution can be derived, which indicates a considerable difference between the equal strain and free strain conditions. The influence of foundations with varying values of stiffness is examined, and the results demonstrate that as the foundation layer becomes stiffer, the time needed to achieve a 90% degree of consolidation decreases and so does the differential settlement, but the steady stress concentration ratio increases. This is also confirmed by a parametric study carried out under varying dimensionless ratios with respect to soil modulus, column spacing, and permeability. A computational example is provided to show the implications of these results on actual design. Finally, a case study is presented to illustrate that the proposed model is able to provide more realistic predictions of settlement and stress concentrations on top of the unit cell

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 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

Frontier technologies in design and construction for sustainable transport infrastructure

Railways are expected to be one of the main modes of future transport in rapidly developing countries with high population densities, including Sri Lanka. In spite of recent advances in rail track geotechnology, ballasted tracks progressively degrade under heavy cyclic and impact loading. Field studies often provide significant knowledge to better understand track performance and to extend the current state-of-the-art in design. Therefore, comprehensive field trials were carried out on two instrumented rail tracks in Bulli and in Singleton, New South Wales, Australia. In these studies, several track sections were reinforced with different types of geosynthetics placed beneath the ballast embankment, with the aim of reducing track settlement, increasing track resiliency, and decreasing ballast degradation. The effects of impact loads and its mitigation using shock mats are discussed. A series of isotropically consolidated drained triaxial tests were conducted on both clean and clay-fouled ballast with varying fouling levels to establish the relationship between the extent of fouling and the associated strength-deformation properties. The outcomes of this research are now elucidated in view of industry practices. This keynote paper provides a fresh insight to design and performance of rail tracks capturing particle degradation, fouling and the use of geosynthetics in track design

Application of Recycled Rubber Mats for Improved Performance of Ballasted Tracks

This paper summaries the results of a study on the use of recycled rubber mats for improved performance of ballasted tracks. One solution used to minimise ballast degradation (breakage) is to use an innovative recycled rubber mat, known as rubber-energy-absorbing drainage mat (READ), manufactured from end-of-life tires to provide a cost-effective solution to conventional tracks. When placed underneath the ballast, the energy-absorbing nature of the rubber mats decreases the load that is transferred to the ballast, so the ballast experiences less deformation and breakage. In this study, a series of large-scale triaxial tests are conducted to investigate the performance of the READS in the attenuation of cyclic and dynamic loads and subsequent reducing of ballast degradation. Numerical modelling using the Discrete Element Method (DEM) is conducted to investigate the improved performance of ballast in a micromechanical perspective. Evolutions of contact forces and contour stress distributions during cyclic tests are investigated through coupled DEM-FEM model