Doctor of Philosophy
Department of Civil, Mining and Environmental Engineering
Tennakoon, Nayoma C., Geotechnical study of engineering behaviour of fouled ballast, Doctor of Philosophy thesis, Department of Civil, Mining and Environmental Engineering, University of Wollongong, 2012. http://ro.uow.edu.au/theses/3814
Railways are among the fastest and most economic transport modes in Australia, and the improvements in track performance are the direct results of the increase in the volume of rail traffic, as well as the need for reducing the cost of rail maintenance. Ballast is the uniformly graded coarse aggregate placed underneath the sleepers whose main purpose is to facilitate rapid drainage and provide structural support for the heavy loads exerted by the passage of trains.
When the ballast voids are wholly or partially filled with the intrusion of fine materials, particle breakage and pumping of soft subgrade soil, the track can be considered as being “fouled”. In Queensland, on the average, 20% decrease in load carrying capacity is due to ballast fouling. The reduced void space in ballast significantly affects its hydraulic conductivity by reducing the drainage capacity of the track. In order to ensure acceptable track performance and longevity, it is pertinent to maintain rapid drainage conditions within the ballasted bed. However, fouling reduces the drainage capacity of the ballast, excess pore water pressure can be generated under the passage of fast moving trains (cyclic load), which further compromises track resiliency while contributing to increased maintenance costs. In addition, fouling causes differential settlement of the track and also decreases its load bearing capacity due to the reduction in internal friction of the granular assembly.
The design aspects and maintenance cost of ballasted tracks can be significantly reduced if an accurate estimation of different types of fouling material and associated degradation mechanisms can be properly quantified. To maintain the required track geometry with fouled ballast, frequent maintenance activities should be carried out. However the decision for practicing engineers of where and when to perform the maintenance operations due to lack of adequate information of fouled ballast condition is often difficult to make. Therefore, it is important to understand both the mechanical and hydro-mechanical characteristics of fouled ballast for different proportions of fouling, because, this will significantly assist towards optimising the time frame for maintenance while effectively minimising the maintenance costs.
A series of large scale hydraulic conductivity tests with specimen size of 500 mm x 500 mm high, were conducted with different proportions of fouling to study the relationship between the extent of fouling and hydraulic conductivity. Since the hydraulic conductivity obtained from laboratory experiments were one-dimensional given that two-dimensional flow conditions may prevail in reality, a numerical analysis was conducted using SeepW (2007a) to quantify the drainage capacity of ballast under different degrees of fouling. Subsequently, a quantitative classification for drainage in relation to the degree of fouling, which is very useful tool for practical engineers, is presented in this thesis. The analysis showed that both the location and extent of fouling played an important role when assessing the overall drainage capacity of track. Based on the research outcomes, ballast cleaning using the undercutting method is recommended when the Void Contaminant Index (VCI) of the top 100mm of the ballast layer exceeds 50%. When the shoulder ballast is fouled by more than 50% VCI, then it should be either cleaned or replaced to maintain acceptable drainage. This is because, if shoulder ballast is fouled to a high level (VCI > 50%) then ‘poor drainage’ of the track can occur even if the other ballast layers are relatively clean.
In order to establish the relationship between the extent of fouling and the associated strength-deformation properties, a series of large scale (300 mm diameter by 600 mm height) monotonic triaxial tests were carried out for different levels of fouling for confining pressures in the range of 10-60 kPa. Based on the laboratory findings, a novel empirical relationship between the peak deviator stress and VCI has been proposed to assist the practitioner in their preliminary track condition assessment. At a significantly high level of fouling (VCI> 50%), a considerable drop in shear strength can affect the stability and load carrying capacity of the track.
A series of large scale cyclic drained triaxial tests has been conducted to investigate the effects of fouling on settlement and ballast degradation, along with a number of loading cycles, simulating realistic field loading conditions. The experimental investigation showed that an increase in fouling always increases the permanent axial strain at a given number of cycles, while an increase in fouling lowered the compressive strain at final N> 500,000 cycles. This suggests that at excessive level of fouling the dilation of the ballast layer is initiated as the number of load cycles increases. Not surprisingly, the corresponding magnitude of resilient strain was shown to decrease with an increasing degree of fouling. Apart from the detrimental effects discussed earlier, fouling leads to a reduction in particle breakage.
A constitutive model for clay fouled ballast is formulated using bounding surface framework under monotonic loading and drained condition. The model is validated with the large scale triaxial experiments carried out in this research.