Railway asset engineers and their project colleagues currently face a huge challenge to keep the railway safe and reliable, with a budget said to be 25% less than what is required to maintain the current asset condition.
All asset types face the same challenge, but geotechnical earthworks face another challenge with climate change and the need to keep the assets safe in the extreme weather events we are experiencing. Not only do the existing earthworks need to kept safe, but they also have to support change with for example new electrification schemes. This can require a combination of elements including new Overhead Line Equipment (OLE) structures; bridge works – reconstruction, jacking, and parapet raising; track – lowering, lifting, and slewing; and signalling, with new signals and signal gantries.
All this has to be delivered on an asset which in many cases does not comply with modern geotechnic standards. There are over 190,000 earthwork assets (approximately 19,000km) which were constructed well before the development of geotechnical engineering techniques. Earthwork construction during the 19th century was undertaken on a huge scale, which has left a legacy of over-steep embankments and cuttings, made with questionable construction techniques.
‘Embankments and cuttings on the early railways’ By A.W. Skempton, says that: “Thomas Telford’s report (1829) on a railway embankment, clearly shows they were formed by end-tipping at the full height; a process, moreover, of which Telford specifically disapproved on the grounds of delayed consolidation and an increased tendency for slipping.
“Shortly after opening the line another very similar slip occurred… as a quick first-aid measure Brunel had a row of timber piles driven at the toe of the slope, penetrating 8ft into undisturbed ground”.
To make matters even more challenging, Rail Engineer reported in December 2023 how historic mining features affecting railway earthworks must be managed safely and efficiently to ensure projects such as new electrification can be delivered cost effectively. Rail Engineer recently met up again with Gerry Manley, to learn how changes to earthworks can be made safely and cost efficiently to accommodate electrification work on the Midland Main Line, and the concept of a Minimum Viable Product (MVP).
Minimum Viable Product
There are several ways that mining risk to legacy earthworks can be managed during construction projects, and it is something which can be very time consuming and expensive. The MVP principle however is that a project can only deliver interventions to meet the client’s requirements, standards, legislation, and budget.
The term MVP came from the production industry and is a version of a product with just enough features for it to be usable. It aims to avoid over engineering products and to minimise cost. It contrasts with the traditional ‘stealth mode’ method of product development, where businesses make detailed plans for products, spanning a considerable time period and incurring huge costs.
Rail needs to be an affordable option for travellers and freight operators, otherwise customers will use other modes of transport which are not as inherently safe or carbon friendly. So, over engineering rail assets can result in increasing the overall societal risk of transport. This is why cost must always be a factor in managing safety and is a fundamental part of So Far As Reasonably Practical (SFARP) legislation and MPV.
The Construction (Design and Management) Regulations 2015 (CDM 2015) also recognise SFARP and say that the principal designer must, as far as reasonably practicable, ensure the elimination of the risks associated with the design. If this is not possible (for instance because of disproportionate costs) then the remaining risks must be reduced or controlled. This is also one of the founding principles behind the management of mining risk for electrification projects and an MVP.
The Midland Main Line (MML) electrification is a significant upgrade to the railway which has been ongoing since 2015. The project involves electrifying sections of the line to allow electric trains to operate, thereby supporting the UK Government’s environmental goals and improving the overall efficiency of rail transport. The electrification is part of a broader initiative to modernise the rail network and reduce reliance on diesel engines.
Gerry explained that an early mining report of just one short 2.4km section of Route Section Three from Syston North Junction to Sheet Stores Junction and Trent Sidings, suggested that 100 bore holes would be required to determine the mining risk. This could have cost approximately £4 million and had significant programme impact. However, using the Guidance for Electrification in Mining Risk Areas & Associated Civils (Issue 205, Nov-Dec 2023) and applying MVP avoided the need for the time consuming and costly bore holes, while managing the risks arising from electrification work on the existing assets, including earthworks, in an area with a mining history.
Overhead Line Equipment
The foundations for new OLE structures can be split into two groups, either laterally loaded piled foundations or vertically loaded pad foundations, including mass gravity foundations.
Laterally loaded piled foundations are the most common, so these should be considered first. These are typically either driven / vibrated Circular Hollow Section (CHS) piles or augered. Augered piles take longer to install and are more expensive than CHS. Therefore, in the absence of significant mining issues, CHS is generally the more attractive option and offers best value. However, where there are noise or vibrational concerns, augered piles may be more appropriate.
If there is evidence to indicate mining void migration and/or Loss of Surface Support (LoSS) in the area, this needs to be considered and, where appropriate, low vibrational auguring may be considered.
Pad foundations are more expensive, time consuming, and disruptive to construct than either CHS or augered piles, but they may be appropriate where shallow bedrock prevents piling or the use of Overhead Line Equipment Master Index (OLEMI) side bearing foundations, for example. On earthworks, pad foundations also impose their load vertically directly onto the top of embankment slopes, where laterally loaded piles have nominal vertical load making them much more appropriate.
Gerry explained that there has been much discussion over the years on whether laterally loaded piles in embankments should be designed to withstand earth pressures in the event of embankment instability. However, this would result in significant increase in size of foundation, and substantial cost escalation, required to essentially allow a failing embankment to ‘flow’ around the pile foundation. There is little benefit to be gained following a landslip from having a vertical OLE stanchion with the track ‘hanging in the air’. Plus, the prediction of which earthworks will be prone to future instability is not an exact science, but rather sound engineering judgement of what could happen.
So, a leaning OLE stanchion can prove a very useful indicator of potential earthwork instability before any effect on track is evident and impact on the safe operational railway. Therefore, not only would designing piles to accommodate such loads result in substantial cost increase and be contrary to MVP, but it would also remove a potential valuable early indication of earthwork movement.
Trials were undertaken on the Transpennine Route Upgrade West projects looking at the risk of mining to electrification schemes. This established that, where the depth to bedrock exceeds 20 metres, CHS piles were appropriate in similar ground conditions without the need to consider mining risk any further. Where depth to bedrock is less than 20 metres, further consideration of other factors is required, to determine whether CHS piling is within acceptable risk limits. These include: (i) whether the stiff glacial till is ≥ 10 metres thick (a glacial till is a product of the glacial processes and are complex, hazardous soils); (ii) if multi-seam extractions are present; and (iii) if worked seam strata dip > 20°. The trials also established that auger pile construction vibration was significantly less than that from typical railway traffic.
New signal structures
There are a range of options available for new signal structures. These range from straight post, cantilever, and gantry. There are also various foundation options, from mass gravity pads, screw piles, CHS piles and augered piles.
For cantilever and gantry structures, foundation options are very similar to those for OLE structures, including typically up to six-metre pile length. Generally, on an electrification scheme there are very few new signal structures compared to the number of OLE structures. Therefore, where possible, employing the same foundation type could be advantageous and cost effective as they can be installed with the same resources and in the same time frame.
In cuttings and level ground, and on embankments with sufficient cess width for pad foundation, straight post signal pad foundations do not typically exceed 1.5 x 1.5 x 1.2 metres deep, and are therefore considered ‘lightweight elements’.
For straight post signals on embankment crests or upper slopes, pad foundations may be difficult to construct, so CHS, augered or screwed piles are likely to be more appropriate.
Conclusion
The concept of a Geotechnic MPV approach for electrification project work is a very welcome initiative to provide a more consistent mature ALARP and effective approach to managing mining risk.
In the years ahead there will need to be significant efforts and resources required to manage earthworks. However, there is likely to be an unprecedented level of competition for funding, between different assets and regions within Network Rail, and competition between railway funding and other public funding. The industry needs to identify new ways of working, like the geotechnic MPV to drive down cost, and it cannot afford to remove every risk on the network. Otherwise, there will be no investment and routes will close.