The ballast layer is designed to be free draining, but when the voids of the granular medium are wholly or partially filled because of the intrusion of fine particles, the ballast is considered to be “fouled.” To ensure acceptable track performance, it is necessary to maintain good drainage within the ballast layer. This paper critically examines the current methods commonly used for evaluating the degree of ballast fouling and, because of their limitations, a new parameter, “void contaminant index” is introduced. A series of large-scale constant head hydraulic conductivity tests were conducted with different levels of fouling to establish the relationship between the void contamination index and the associated hydraulic conductivity. Subsequently, a numerical analysis was executed to simulate more realistic two-dimensional flow under actual track geometry capturing the drainage capacity of ballast in relation to the void contamination index. In the context of observed test data, the drainage condition of the track could be classified into different categories together with a classification chart capturing the degree of fouling. The contents of this paper have already been considered in track maintenance schemes in the states of Queensland and New South Wales.

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The role of ballast fouling characteristics on the drainage capacity of rail substructure

Angles of friction and dilatancy of sand

Physical properties and shear strength responses of recycled construction and demolition materials in unbound pavement base/subbase applications

Seismic passive resistance by pseudo-dynamic method

Microbially induced calcite precipitation (MICP) is used increasingly to improve the engineering properties of granular soils that are unsuitable for construction. This shows MICP technique significant advantages such as low energy consumption and environmentally friendly feature. The objective of the present study is to assess the strength behaviour of bio-cemented sand with varying cementation levels, and to provide an insight into the mechanism of MICP treatment. A series of isotropic consolidated undrained compression tests, calcite mass measurement and scanning electron microscopy tests were conducted. The experimental results show that the strength of bio-cemented sand depends heavily on the cementation level (or calcite content). The variations of strength parameters, i.e. effective friction angle φ′ and effective cohesion c′, with the increase in calcite content can be well evaluated by a linear function and an exponential function, respectively. Based on the precipitation mechanism of calcite crystals, bio-clogging and bio-cementation of calcite crystals are correlated to the amount of total calcite crystals and effective calcite crystals, respectively, and contributed to the improvement in the effective friction angle and effective cohesion of bio-cemented sand, separately.

Influence of cementation level on the strength behaviour of bio-cemented sand

Understanding the complex mechanisms of stress transfer and strain accumulation in layers of track substructure under repeated wheel loading is essential to predict the desirable track maintenance cycle as well as the design of the new track. Various finite element and analytical techniques have been developed in the past to understand the behavior of composite track layers subjected to repeated wheel loads. The mechanical behavior of ballast is influenced by several factors, including the track confining pressure, type of aggregates, and the number of loading cycles. A field trial was conducted on an instrumented track at Bulli, New South Wales, Australia, with the specific aims of studying the benefits of a geocomposite installed at the ballast-capping interface, and to evaluate the performance of moderately graded recycled ballast in comparison to traditionally very uniform fresh ballast. It was found that recycled ballast can be effectively reused if reinforced with a geocomposite. It was also found t...

Field Assessment of the Performance of a Ballasted Rail Track with and without Geosynthetics

Knowledge of seismic active earth pressure behind rigid retaining wall is very important in the design of retaining wall in earthquake prone region. Commonly used Mononobe-Okabe method considers pseudo-static approach, which gives the linear distribution of seismic earth pressure in an approximate way. In this paper, the pseudo-dynamic method is used to compute the distribution of seismic active earth pressure on a rigid retaining wall supporting cohesionless backfill in more realistic manner by considering time and phase difference within the backfill. Planar rupture surface is considered in the analysis. Effects of a wide range of parameters like wall friction angle, soil friction angle, shear wave velocity, primary wave velocity and horizontal and vertical seismic accelerations on seismic active earth pressure have been studied. Results are provided in tabular and graphical non-dimensional form with a comparison to pseudo-static method to highlight the realistic non-linearity of seismic active earth pressures distribution.

Pseudo-dynamic approach of seismic active earth pressure behind retaining wall

Impact loads generated because of wheel and rail defects cause accelerated ballast breakage leading to adverse performance of ballasted rail tracks. In the present study, experimental and numerical investigations have been carried out to develop an understanding of the performance of shock mats in the attenuation of dynamic impact loads and subsequent mitigation of ballast degradation. The investigations include different locations of shock mats considering stiff and weak subgrade conditions. With the provision of shock mats, the magnitude of impact forces decreases, and the time duration of impact ges prolonged. In the case of stiff subgrade, the efficiency of the shock mat in reducing the impact forces is greater when it is located at the bottom of ballast rather than at the top, whereas the reverse is true for weak subgrade. However, the provision of shock mats both at the top and bottom of the ballast bed, irrespective of the subgrade condition, is the best solution for minimising the impact force. The shock mats can bring down the impact-induced strains in the ballast bed by as much as 50%, apart from substantially reducing the ballast breakage.

Sanjay Nimbalkar

Papers

The role of ballast fouling characteristics on the drainage capacity of rail substructure

Field Assessment of the Performance of a Ballasted Rail Track with and without Geosynthetics

Seismic passive resistance by pseudo-dynamic method

Pseudo-dynamic approach of seismic active earth pressure behind retaining wall

Improved Performance of Railway Ballast under Impact Loads Using Shock Mats