Size really is everything nowadays when it comes to bulk freight movement. For evidence, look no further than the Maersk Triple E class, the latest generation of energy-efficient container ships. Carrying 18,270 20-foot boxes, these 400-metre monsters motor along optimally at 19 knots, consuming a comparatively frugal 21,200 gallons of diesel daily. Yet even these vessels have limitations despite their hefty $185 million price tag: with a draft of 14.5 metres, they are too deep to navigate the Panama Canal. Writes Graeme Bickerdike
On Britain’s railway, delivering similar economies of scale for container traffic has been consistently hampered by the less-than-generous approach taken, for perfectly legitimate practical reasons, by the Victorian engineers who gifted us our network long before the SS Ancon became the first ship to officially take the short cut from Atlantic to Pacific. Our inherited structure gauge is a function of the requirements prevailing at the time of construction, so recent years have seen a number of schemes to steal 20mm here and 50mm there, adapting the infrastructure with care and precision or, when that has failed, by means of a wrecking ball. This has allowed today’s containers to boldly go where no container had gone before. Please excuse the clumsy stumble from steamship to spaceship there.
The latest corridor to benefit heads cross country from Yorkshire to the West Midlands. The Doncaster to Water Orton W12 Gauge Clearance Enhancement project – or D2WO to save time – has been funded by the Strategic Freight Network and delivered by Network Rail’s IP East Midlands team through the multi asset framework agreement (MAFA) awarded to Carillion Rail in summer 2011. Since then, a challenging programme of 50-plus interventions was delivered through the highly collaborative approach adopted by the two parties and their key partners. Costing £25 million, the 102-mile route was cleared for W12 on time, from the end of March 2014.
It’s worth making the point that, in terms of meeting the contractual end-date, gauge clearance is an all or nothing venture: if an external force had thrust a spanner into any one of those interventions, not a single W12 wagon could have been accommodated. Success then is testament to the team’s tenacity in the face of occasional adversity. No, I can’t be more specific. And it was all done with little disruption to the network. Despite significant changes and increased scope, more than 95% of the work was delivered within existing disruptive and Rules of the Route possessions. As things should be, of course.
Too close for comfort
Proving one of D2WO’s more problematic structures was Conisbrough Tunnel, driven through a spur of land that falls southwards to the adjacent River Don. Part of the busy 75mph railway connecting Doncaster and Sheffield, the 235-yard bore comprises masonry sidewalls with a brick arch and invert, the eastern end having been constructed by cut-and-cover.
Opened in 1848, the uneven load exerted on the lining has brought distortion with it over the years, resulting in a number of remedial works. One of these saw two sections of secondary lining inserted – possibly during the 1870s – involving circumferential riveted wrought iron ribs at 8-foot centres and dished, infill brickwork. At 7.5 metres, the shorter section was thought to coincide with a geological fault near the tunnel’s midpoint; the other, 46 metres long, was further towards Sheffield. Their effect was to reduce clearances, prompting the imposition of a 50mph permanent speed restriction (PSR).
In 2001, to provide some track alignment flexibility and improve clearances, the secondary lining was taken out above ballast level and replaced with an array of rock anchors, between 8 and 12 metres in length, and steel mesh. The removal revealed that an 18 metre section of original lining had been pushed upwards at the crown by 750mm, the chosen solution entailing its reconstruction with heavy-duty lattice girders and fibre-reinforced sprayed concrete. Substantial voids above the brickwork were also grouted.
Fast-forward ten years and a proposal emerges to introduce W12-gauge wagons over the route. Initial assessment work found that threading such traffic through the tunnel was feasible without substantive civils work, subject to a track lower. The involvement of Carillion Rail, the principal contractor, came as the GRIP Stage 4 (single option development) process was concluding and the firm was asked to complete an ‘early contractor involvement’ review to assist Network Rail in identifying any delivery risks and determine a target cost. Contract award came in March 2012.
Jacobs was brought in to undertake the design, development of which included a reappraisal of designs which had previously been completed. This investigation was enhanced by a walk-through and survey of the tunnel followed by an evaluation of the findings with the track and civils construction teams. This early incursion revealed that where the secondary invert was still in place, short-ended sleepers had been used which were sitting in notches cut into the brickwork. The implications of this were considerable. As things stood, in order to provide normal upper sector clearance of at least 100mm and a minimum ballast depth below the sleepers of 200mm, there was insufficient space to meet standard design tolerances. This completely changed the complexion and complexity of the project, placing evermore emphasis on that alliance between client and contractor as they sought to manage the associated risks and costs.
Room for manoeuvre
With a history of ground movement impacting on the structure, the key now was to determine whether a partial removal of the invert could be progressed without compromising the tunnel’s integrity. To that end, Jacobs procured the services of Donaldson Associates – with its recent experience modifying Conisbrough Tunnel – to support the design. Involved was a review of records gathered during the company’s previous encounter with the tunnel as well as a series of detailed site investigations. These entailed lifting the track and ballast to fully expose the secondary invert before carrying out a cloud burst survey and coring to establish its thickness and condition. Based on this work, it was concluded that the ribs and brickwork could be safely removed providing at least 300mm of lining remained and the interface between sidewalls and invert was strengthened to relieve the stresses there.
Also contributing from an early stage was DGauge, a specialist consultancy offering access to a third-generation W12 gauge providing tighter tolerances for gauge clearance parameters, typically buying designers tens of critical millimetres. Achieving this demands refined data interpretation methodology and deeper analysis of the output, as well as a risk-based approach that helps to overcome some of the conservatism associated with ‘absolute gauging’. Authority to use the tools as an alternative to industry-standard ClearRoute has to be sought from Network Rail for specific problem structures – of which Conisbrough Tunnel was certainly one – thus minimising both the track lowering and civils work needed to achieve W12 clearances.All this analysis came together to confirm that removal of just the secondary invert would provide sufficient space. With this knowledge, Jacobs was able to create a new series of track alignment iterations to identify the optimum design. Amongst the final requirements were shallow-depth, short-ended EG47 sleepers and cutting back bolt threads on several steel rock anchors which infringed on structure gauge. Fortunately the tunnel is generally dry, so passive drainage was proposed using the natural fall of the primary invert (1:559 towards Sheffield), there being no room to accommodate compliant pipework above it. Improvements have, however, been made at the lower end of the tunnel to collect any water and carry it away to the existing p-way drain.It’s worth stating that consideration was given to a high-fixity solution (slab track) but this was quickly ruled out due to the higher costs and disruption it would have brought.
The tunnel forms part of a key route, not just for local and cross-country passenger services but also overnight freight traffic.
At the races
It was recognised at an early stage that, due to access constraints, the longer section of secondary invert would have to be removed in two parts. To facilitate this, during preparatory works from October to Christmas 2013, its 20 ribs were uncovered along the six-foot and the surrounding brickwork broken away, allowing them to be cut in half. At the invert-sidewall interface, the secondary invert was saw-cut to leave a stub of 450mm, through which 20mm diameter steel dowels were inserted into the primary invert at 400mm centres to provide the required strengthening.
A monitoring system, installed by Carillion’s sister company TPS in early December, comprised 65 prisms arranged radially through 13 cross- sections. With baseline coordinates established, these were checked automatically by a wall-mounted EDM (Electronic Distance Measurement) device four times a day during the substantive works and twice daily at other times, the results then being emailed to an agreed distribution list. Allied to this was an alert strategy and response plan for different thresholds, appreciating of course that the tunnel naturally breathes just as we do. Movement tended to peak during or after the main possessions, with readings of 8-10mm – more than what was anticipated – recorded on several occasions. Subsequent reviews and inspections confirmed sighting issues to be the main culprit, a function of penetrating water and the plethora of furniture on the sidewalls and arch. Some prisms also succumbed to misplaced boots from time to time.
Fulfilment of the core works took place over seven 27-hour possessions through January and February 2014. Given the space restrictions imposed by the tunnel, choreographing which train or machine went where and in what order obviously proved critical, with a ‘racecard’ produced on each occasion detailing all the movements and associated logistics. “It’s a work-of- art in terms of the track construction chain and liaison with the civils team,” asserted Andy Robinson, the project’s design manager.
Highs and lows
First to be removed was the shorter 7.5 metre section of invert, RRVs lifting the track whilst two mini-diggers – one with a breaker, the other with a bucket – were used to tackle the ribs and brickwork. As previously stated, the 46 metre section was done in two halves – starting on the Down side – but here trains were needed to deal with the substantial arisings. Following the removal, the track was returned to its original line and level, but with EG47 sleepers. Work to implement the new alignment took place the following weekend. Attention then turned to the Up side with the process being repeated.
Subsequent to these invasive works, a cloud burst survey was performed to confirm that the structure gauge had been left clear, the onus being on the project team to demonstrate to Network Rail that the work had delivered its objectives.
This applied not just to the tunnel and those 50 other interventions, but to the whole 102-mile route.
Having originally been budgeted at around £400,000, the emerging cost of £900,000 might raise the eyebrows of anyone just crunching numbers. Back in 2012 this looked like a basic track lower; in reality, getting to the realignment stage has demanded investigations, design, civils work and monitoring that no-one had anticipated. “If we hadn’t been able to clear the tunnel in time, it would have put everything else we’d done to waste,” Alan Sheffield, Carillion Rail’s senior project manager, reflected. “And for a long time it’s fair to say that there was some uncertainty. So the headline here is that delivering it really has been a huge team achievement. There’s a great degree of satisfaction getting it signed off as clear.” And removal of the 50mph PSR – the works’ other key objective – is currently being progressed.
Giving birth to the railways brought social and economic revolution to Britain in the 19th century but has disadvantaged us in the 21st. Others learned valuable lessons from our pioneering and now find themselves better placed in terms of infrastructure. We still have to compete though. With a few obvious exceptions, we don’t build new lines anymore – that ship has sailed – so we’re faced with the prospect of evolving what we’ve got. Doing so challenges our engineers, but history proves that’s how you drive progress.