Network Rail has published the results of its review of earthworks management, which was prepared by a task force led by Professor Lord Robert Mair CBE FREng FICE FRS.
The review, which focuses on Network Rail’s capability and methodology for the management of railway cuttings and embankments, particularly in the light of climate change, was one of two commissioned by Network Rail in the aftermath of the Stonehaven tragedy in Aberdeenshire in August 2020, when a train derailed, having struck a landslip following intense rainfall, and three people died.
The other review was conducted by a Weather Advisory Task Force headed up by Dame Julia Slingo FRS, former chief scientist at the Met Office and a world-renowned expert in climatology.
Its task was to help Network Rail to understand the risk of rainfall to its infrastructure, drawing on the latest scientific developments in monitoring, real-time observations and weather forecasting.
The report of that review has also been released.
Earthworks Management Task Force
In his summary of the Earthworks Management Task Force’s report, Lord Mair notes that Network Rail has inherited and manages 190,000 earthworks assets, comprising 70,000 soil cuttings, 20,000 rock cuttings and 100,000 embankments. The vast majority are over 100 years old and many are over 150 years old.
The cut and embankment slopes continue to fail for a number of reasons. Historically, cuttings are overly steep and many have failed previously; embankments were uncompacted during their construction and their foundations were unprepared. The upper few metres of the slopes have often been weakened by weathering.
Complex processes, known as progressive failure, operate in the high plasticity clays forming many of the troublesome cuttings and embankments. Drainage is often inadequate to ensure stability and historically was not designed to do so. Vegetation, which can have beneficial and detrimental effects on slope stability, has not been particularly well managed in the past.
In addition, and of particular significance, weather patterns are changing, resulting in longer periods of prolonged rainfall, more intense rainfall and hotter, drier summers.
The combination of these factors is the challenge Network Rail face in managing its earthwork and drainage assets. The review considered how this challenge is being met and whether improvements can be made by modifying the asset management policy, strategy, standards and procedures, as well as by implementing the latest methods of monitoring and surveillance.
Slope condition data is primarily derived by Network Rail through periodic earthwork examinations. These visual inspections rely heavily on data collected by examiners in the field that may be subjective, potentially encompass a degree of variability and may not be able to identify certain precursors to slope failure. Full condition monitoring and assessment of the extensive Network Rail earthwork asset base is challenging; slope stability cannot be determined on the basis of surface observations from examination alone. The rapid failure of cut slopes is especially difficult to predict, particularly when failures are triggered by locally intensive rainfall.
Recognising this, Network Rail has put considerable resource into the development of a comprehensive asset management system for its earthworks and the task force commended the very substantial effort in achieving this. Nevertheless, it felt that some shortfalls in the earthwork examination and risk evaluation system need to be addressed, in particular reliance on algorithms and statistics as predictors of earthworks failure without verification by some site-specific engineering calculations. There are also limitations of mainly using pre-determined hazard categories to trigger earthwork evaluations and examination frequencies.
An overhaul of the geotechnical and drainage asset management systems is required to bring the existing data and decision support tools together in a common interface. This will provide the Network Rail engineering teams with the capability to properly evaluate and document the vulnerability of earthwork and drainage assets and accurately prioritise intervention activities required to safely manage the infrastructure.
An overriding message from this Review is that soil pore water pressure is a key parameter in determining the past and future stability of Network Rail’s earthwork assets. Pore water pressure regimes have been modified by changing weather patterns and will continue to change. Rates and magnitudes of pore water pressure changes depend on the availability and ability of water to flow into the soil profile, while strong surface flows can also cause soil erosion and flooding in their own right. Hence surface and sub-surface water management is probably the single most important factor in determining if, when and where an earthwork failure will occur.
A holistic approach to water management, considering catchment to outfall and integrating all drainage systems, is needed to manage earthworks stability and track quality. The size, shape and location of all natural catchments draining towards Network Rail’s railway should be established, in order that the water flow rates can be determined for the required design storm return periods, allowing for climate change. The effective control of water and proper understanding and maintenance of drainage assets is of fundamental underpinning importance to the safe operation of the railway network.
The process for the identification of localised water concentration features at the top of cutting crests and the likelihood of failure from washout or earthflow should be fundamentally reviewed to address the fact that many of the 2019/2020 failures occurred in slopes which had not been identified as vulnerable and most of the failures were still observed first by the train drivers. The aim should be to improve the prediction rate for rapid cutting slope failures, a feature of which there is often little or no indication of visible distress prior to failure. A forensic re-assessment of the significant number of previous washout and earthflows, together with the accompanying weather patterns, would be invaluable for calibration of the current examination and evaluation process and provide lessons learnt for future risk assessment.
The report recommends that consideration is given to undertaking drainage inspections with sufficient and professionally qualified competent staff under the control of the RAM-Drainage, as is done for earthworks examinations, rather than the current arrangement where the Network Rail Maintenance off-track team is often overloaded with inspections of drainage systems. More resource is needed for this vital activity. Consideration should also be given to having dedicated drainage maintenance teams across all Routes, rather than drainage being only one of the activities for which Off-Track Section Managers are responsible. Off-Track drainage maintenance should have its own budgets. There is a case for having Off-Track Maintenance Engineers with similar role and grading to the existing Track Maintenance Engineer posts.
Another area for significant investment is the updating of monitoring and surveillance methods. There have been recent advances, some of which have been explored by Network Rail. A notable example is the recently trialled wireless tiltmeter system; this is an extremely promising application of innovative sensor development to the management of earthworks assets. These sensor systems are capable of providing failure detection and reaction via alert alarm systems, as well as data on the performance and condition of a slope or embankment, and possible precursors to failure. Such monitoring systems should be more widely adopted on earthworks slopes and embankments that are judged to be potentially critical, along with pore water pressure measurement where appropriate; pore water pressures are critical but are not often measured.
More regular and frequent use should be made of surveillance technologies with helicopters and drones. There should be more widespread use of helicopter flights for inspections of earthworks, particularly after especially intense rainfall; they can serve as a key mitigation by providing warning or identification of an earthwork failure.
Despite some existing limitations, particularly in respect of current regulations, drone technology is a rapidly expanding area and there is considerable potential for drones to significantly enhance Network Rail’s earthworks management; for example, by locating obstructions on the track before the train driver, identifying changes, and mapping features that lead to localisation. InSAR is a promising satellite technology which is also developing rapidly, with substantial developments in AI and machine learning, and should be given further attention.
Routine analysis of track geometry data is a potentially valuable technique for the early detection of embankment instability; further development work is required to establish automatic data processing.
International experience indicates that the most promising surveillance technologies for slope and landslide management are LiDAR and photogrammetry (both aerial and land-based). Wireless sensors have been shown to be effective for monitoring of slope movements, provision of warning systems and detection of flexible barrier deformations. There is significant potential for these advanced surveillance and monitoring technologies; they need to be managed centrally, replacing several aspects of the well-established visual examination procedure but run in parallel with retained parts of that inspection.
With climate change, more intense rainfall and higher frequencies of extreme rainfall are likely. This will result in an increase in washout and earthflow slope failures, particularly in cuttings. The intense rainfall is associated with cyclonic storms that can occur at any time of year; however, they are particularly prevalent in the summer period.
Shallow slope failures and washouts on cut slopes appear to predominate. Of the 252 reportable failures in 2019/2020, 190 were in cuttings and 62 in embankments. Localisation makes the prediction of precisely where they may occur almost impossible.
Nevertheless, advances in monitoring technologies and surveillance techniques, in combination with data on past failures, can lead to identification of which geologies and geometries are especially vulnerable. In such cases, a pragmatic approach would be to install instrumented barriers that will provide temporary restraint and notification of an instability event.
A key challenge for Network Rail’s earthwork asset management is to focus on mitigating the effects of earthworks failures, as currently it is not reasonably practicable to detect nor prevent all earthwork failures. Mitigation measures are crucial to reduce the consequence of a failure should it occur.
Network Rail has carried out a comprehensive review of its vegetation management standards following the Varley Report on vegetation. Further work should be undertaken to develop vegetation management and bioengineering techniques to stabilise earthwork slopes as a cost-effective preventative and remedial intervention technique.
Vegetation management needs to balance the negative impacts of vegetation (blocked ditches and pipes, leaf fall, tree fall, desiccation adjacent to and beneath the track) against its positive impacts (reducing surface erosion, providing root reinforcement, avoiding channelling of flows, maintaining surface pore water suctions). The potential negative impacts of de-vegetation should be carefully considered. Vegetation should be treated as an asset and the railway system regarded as a wildlife corridor.
The task force recommends that Network Rail build on its comprehensive asset management system and progressively adopt a broader and more integrated approach to the management of earthworks, drainage and vegetation, taking account of changing weather patterns. There is a need to breakdown the historic silos between these interdependent assets across the organisation to support the delivery of a safe, cost-effective and sustainable railway infrastructure into the future.
Martin Frobisher, Network Rail’s safety and engineering director, said: “The UK’s railway is one of the safest in Europe, but something went tragically wrong at Stonehaven last summer and our thoughts remain with the families and friends of Brett McCullough, Donald Dinnie and Christopher Stuchbury.
“Heavy rain caused ground slips on many occasions across the entire network last year and, although tragic accidents are thankfully incredibly rare, and none other than Stonehaven caused injuries, it is clear that extreme weather presents a significant challenge to the way we safely and reliably manage railway infrastructure.
“We do a vast amount to tackle the effects of climate change already but there is more to do. We established two independent, expert taskforces led by world-class specialists to investigate the problems we face and, crucially, to guide us as we make substantial improvements.
“We will carefully consider every single recommendation and develop a science-backed improvement plan, to target available money and technology in the best possible way. This is a real breakthrough.”
Transport Secretary Grant Shapps said: “The incident at Stonehaven in August was devastating, and my thoughts remain with the families of Brett McCullough, Donald Dinnie, and Christopher Stuchbury, who tragically lost their lives.
“We must do everything we can to keep our railways safe, and I immediately commissioned this report so that lessons would be learnt without delay. This document sets out how our investment can enhance the resilience of our rail network against climate change and incidents of extreme weather in the future.
“We will be considering these findings carefully as we progress these works at pace.”