The influence of ballast fouling on track settlement

The contamination of railway ballast (referred to as ballast fouling) is commonly caused by ballast particle degradation, external debris fouling due to surface spillage and clay fouling due to subgrade pumping. Intrusion of these fines in the ballast layer can impede rapid drainage which is necessary for good track performance. There is little research relating both fouled ballast and track settlement in a South African context. This paper discusses the effect of coal contaminated ballast on track settlement by use of a large scale “box test” apparatus. A series of tests was conducted on ballast aggregate, typically used on heavy haul railway lines in South Africa, with varying percentages of coal dust material (0, 8.4, 16.9, 25.3 and 33.8 % by weight of aggregate). Testing was conducted at 10 Hz for 100 000 cycles with a moisture content of 5.0 %. Results indicated an increase in settlement of between 11.7 and 40.2 % per 10 % increase in coal dust contamination. It is envisaged that this research will aid the South African railway industry in identifying the critical levels of coal fouling which will hence contribute towards reducing maintenance costs as well as improving railway safety and network reliability.Papers Presented at the 2018 37th Southern African Transport Conference 9-12 July 2018 Pretoria, South Africa. Theme "Towards a desired transport future: safe, sufficient and affordable"

The stiffness of unsaturated railway formations

The rational design of a substructure to support a rail track requires an estimation of the stiffness value of the formation on which it is to be built. Stiffness values derived from back-analyses of deformations of the ground beneath the track have been found by the authors to be much higher than those predicted from laboratory element testing on saturated specimens. This may be because of differences in compaction between field and laboratory, or because suctions created by lack of saturation play a key role in controlling stiffness, and therefore the performance of the track when in use. To test the latter hypothesis a laboratory study has been carried out on material representative of that found in South African railway formations. This was tested at constant dry density and various water contents, with matric suctions determined using different established techniques, and very-small-strain stiffness levels obtained from resonant column testing. A suction stress characteristic curve was developed to identify the contribution of suction to the overall effective stress for this material. The results show that suction can indeed be an important contributing factor to the magnitude of stiffness. For material tested at constant dry density, the stiffness initially increases with reducing compaction water content, and therefore with increasing suction. It subsequently reduces back towards the saturated value as the compaction water content approaches zero, even though the matric suction continues to increase. The relative increase in very-small-strain stiffness due to suction depends, to a large extent, on the net normal stress during the stiffness measurement. The effect of matric suction is proportionately greatest at the low net normal stress levels that apply for shallow infrastructures such as rail formations. Also, the operational stiffness depends not only on the current water content (and therefore suction), but also on the water content at which the material has been compacted.The Engineering and Physical Sciences Research Council’s ‘Rail Research UK’ programmehttp://pif.sagepub.comhb2016Civil Engineerin

Stochastic rail life cycle cost maintenance modelling using Monte Carlo simulation

The need for decision support systems to guide maintenance and renewal decisions for infrastructure is growing due to tighter budget requirements and the concurrent need to satisfy reliability, availability and safety requirements. The rail of the railway track is one of the most important components of the entire track structure and can significantly influence maintenance costs throughout the life cycle of the track. Estimation of life cycle cost is a popular decision support system. A calculated life cycle cost has inherent uncertainty associated with the reliability of the input data used in such a model. A stochastic life cycle cost model was developed for the rail of the railway track incorporating imperfect inspections. The model was implemented using Monte Carlo simulation in order to allow quantification of the associated uncertainty within the life cycle cost calculated. For a given set of conditions, an optimal renewal tonnage exists at which the rail should be renewed in order to minimise the mean life cycle cost. The optimal renewal tonnage and minimum attainable mean life cycle cost are dependent on the length of inspection interval, weld type used for maintenance as well as the cost of maintenance and inspection activities. It was found that the distribution of life cycle cost for a fixed renewal tonnage followed a log-normal probability distribution. The standard deviation of this distribution can be used as a metric to quantify uncertainty. Uncertainty increases with an increase in the length of inspection interval for a fixed rail renewal tonnage. With all other conditions fixed, it was found that the uncertainty in life cycle cost increases with an increase in the rail renewal tonnage. The relative contribution of uncertainty of the planned and unplanned maintenance costs towards the uncertainty in total life cycle cost was found to be dependent on the length of inspection interval.Transnet Freight Rail and the Railway Safety Regulator.http://journals.sagepub.com/home/pifhj2018Civil Engineerin