In the recent past, several accidents due to derailing of trains have been reported in various parts of India. Openly accessible data shows that over 40 per cent of all rail accidents in India have been cases of the train going off the rails. Derailment is caused by three main factors. There can be mechanical reasons, like broken rails, or lack of maintenance of tracks. Derailment caused by mechanical reasons has reduced over the years. Derailment can also be caused due to functional reasons, a result of high speed, poor signalling or human error.
The third and most important reason for derailment is geotechnical. Derailment is caused by misalignment of rails and sleepers because of problems with the foundation on which it rests. The railway tracks are placed on parallel concrete sleepers, which in turn are carefully adjusted on a layer of stones, or gravel, of specific sizes, also called ballast. This ballast is crucial to the stability of the railway tracks.
The ballast that is seen under the railway tracks in India are mostly prepared by crushing locally available rocks to generate specific-sized stones. In India, stones measuring 20 mm to 65 mm are permitted to be used as ballast. These sizes are slightly bigger than in many other countries, where even 9 mm ballast is used.
The ballast has to fulfil two key objectives. They must have high shear strength to provide maximum stability and minimum track deformation. And, they must have high permeability to allow adequate drainage of water from the tracks in the rainy season. The size of ballast is also referred to as ballast gradation. Now, ballast of larger sizes does not allow for very tight packing; the void is large. This is good for the drainage function, but not ideal for providing maximum stability to the tracks. Because of the loose packing and presence of large voids, when load is applied on the ballast, for example, by a running train, it tries to rearrange itself. In the process, many of the stones break, parts of them getting converted into small granules.
Besides these granules, there are also several foreign materials that get mixed into the ballast due to various processes. The trains, for example, carry coal or iron ore. Some of these drop from the wagons and get mixed with the ballast. Dust and mud, and a variety of other materials such as plastics and soil, too, settle in. This is called ballast fouling. Initially, this fouling leads to filling up the voids and helps in packing the ballast tightly. But with time, it starts to create problems. It facilitates the movement of the ballast and makes it easier for the sleepers to shift.
This in turn can change the delicate alignment of the rails in the track, which can lead to a derailment.
To deal with a problem like this, Indian Railways has to periodically carry out what is called deep-screening of the tracks. In this process, the tracks are lifted and the fouling is cleaned before replacing the tracks. This exercise, as of now, is randomly carried out.
Prof Anbazhagan P and his team at the department of civil engineering at the Indian Institute of Science, Bangalore, have now tried to develop a method through which it would be possible to identify the tracks that need immediate deep-screening by studying the nature and extent of ballast fouling. The team has defined an “optimum” fouling point and “critical” fouling point to check state of fouling in the track. If the ballast has reached “optimum” fouling point, deep-screening is recommended immediately. If “critical” fouling point is reached, a track must not be used until it is deep-screened.
Anbazhagan says this new idea emerged out of large-scale experimental studies during his Endeavour postdoctoral fellowship in University of Wollongong, New South Wales. His team carried out research by building a model track with different kinds of fouling conditions. They developed a model to predict the extent and the nature of fouling in tracks, using ground penetrating radar. Their studies were then verified by on-site real track sampling at selected locations.
They showed that it was possible to assess when the fouling on a track would reach its “optimum” point and when can a deep-screening be scheduled. This kind of advance analysis can help the railways in prioritising its deep-screening works and focus its attention on the tracks that are more likely to experience misalignment because of fouling.
Anbazhagan says there is a need for further research in this area, in terms of finding improved materials to be used as ballast. His team is already working on exploring possibilities of using natural or waste materials in the ballast. Also, he points out, his current research is to focus to define the “optimum” ballast gradation that would be the best suited for performing both the functions of allowing drainage and providing stability to the tracks. Many countries have already defined this “optimum” gradation for their own railway lines. He says these studies can be useful for the existing railway system and future high speed rails.
A method to identify which railway tracks need the most urgent attention, by defining an “optimum fouling point” and “critical fouling point” for the ballast beneath.
Dr Anbazhagan Panjamani & Team, Department of Civil Engineering,
Indian Institute of Science, Bengaluru
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