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Yet the same cannot be said of railway embankments and cuttings, most of which were built more than 150 years ago before soil mechanics was invented. The requirement was to minimise costs and land take on the basis of ‘what had worked so far’.
As a result, cuttings were overly steep, embankments uncompacted and drainage inadequate. Hence railway earthworks failures have always been a problem. Since 2004, there has been an average of 100 earthworks failures per year. However, in recent years this number has doubled owing to longer periods of prolonged, intense rainfall and hotter, drier summers.
Nevertheless, with increased monitoring and additional expenditure on earthworks (£1.3bn in the current five-year control period), there has been a significant reduction in high-risk earthworks failures and derailments. Unfortunately, one such failure caused last year’s derailment at Carmont, which was the first fatal UK train accident since 2007. In a sad irony, this was due to a failed crest drain that was installed 10 years ago during work to stabilise the steep cutting immediately adjacent to the derailment site. This drain was overwhelmed after the local topography had directed large amounts of water into it after 75% of the area’s average monthly rainfall fell in three hours.
Learning lessons
As it is not practicable to rebuild thousands of miles of earthworks to modern standards, the risk from such earthwork failures cannot be completely eliminated. Yet there are lessons to be learned from the Carmont accident. Some of these will come from the formal investigation by the Railway Accident Investigation Branch which will, for example, consider the design and construction of the failed drain. In addition, Network Rail established two task forces led by independent experts.
One, under Lord Robert Mair, reviewed the management of earthworks while the other, led by Dame Julia Slingo, considered weather forecasting. The resulting reports, which are available online, were published in March and aim to ensure that Network Rail has the expertise, technologies and systems to better manage earthworks, and make the best use of weather data.
Lord Mair’s 543-page report provided an academic treatise on the soil mechanics of railway earthworks and their failure mechanisms. It concluded that the dominant reason for continuing failures is exposure of over-steep slopes to rainfall not previously experienced. It noted the strong correlation between earthworks failures and rainfall over the past two decades. It also highlighted the crucial role of drainage systems, which do not always get the maintenance priority they deserve and are installed to default designs that take no account of runoff and water flow.
His report also considered the asset management of earthworks and vegetation. And it highlighted the importance of monitoring, as it is not possible to prevent all earthwork failures. Techniques used to instantaneously detect failures include distributed acoustic sensing by optical fibres and surface-mounted inclinometers. In addition, less responsive monitoring is needed to collect condition data over a period. For example, comparing satellite radar images over time can detect ground deformations of the order of millimetres.
Forecasting and 'nowcasting'
It was also felt to be more practical to “search for the haystacks” – lengths of vulnerable slopes – as “predicting exactly where failures will occur is like looking for a needle in a haystack”. In this respect, localised failures indicate that the rest of a similar slope is prone to failure.
Dame Julia Slingo’s report considered how Network Rail could make use of the best possible weather forecasts. It noted that kilometre-scale forecasting 1-3 days in advance is now possible following the development of accurate landscape models and improved understanding of the physics of thunderstorms and convection. In addition, nowcasting can detect the severe convective storms within the next 1-2 hours by using optical flow techniques to extrapolate weather radar images.
Lord Mair and Dame Slingo’s reports reflect the expertise of their members and offer best-practice solutions. While it is not practicable to detect or prevent all earthworks failures, the reports’ recommendations will significantly reduce the risks.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.