Hogging the limelight is something Crossrail is very good at. And so it should be: successfully threading a new railway through the capital’s subterranean clutter – to a few millimetres’ tolerance and without mishap – is outstanding by any measure. It’s a feat the industry should be shouting about, loudly and proudly. But Crossrail is not the only show in town when it comes to tunnelling. We’re doing a bit up North too, with a machine hand-built in Oldham.
The £400 million North West Electrification Programme might be a financial drop in the ocean alongside the £14.8 billion being spent under London, but it has transformational potential as part of Network Rail’s wider £1+ billion investment across the region’s strained rail network. When government gave the go-ahead for the Liverpool-Manchester phase in 2009, installing stanchions and stringing wires between them probably sounded like a quick-win, but there’s so much more to it.
As this magazine has described previously, negotiating sufficient clearance for overhead line equipment can involve bridge building, track lowering, platform modifications, signal resiting, drainage renewals…you get the idea. The expansion of the north-west’s programme to include Huyton-Wigan, Manchester-Preston (by December 2016) and Preston-Blackpool (by winter 2016/17) has added much to that workload.
The latest intervention – arguably the most eye- catching – impacts on the heavily-trafficked commuter route between Bolton and Manchester. Commanding attention this time are the 295-yard twin bores of Farnworth Tunnel, or Clammerclough as they were originally known. They don’t make names like that anymore. But what’s proposed here is not another nibbling away of the invert to steal a few inches. This one is brutal.
Hold on tight
Engineered by John Hawkshaw of Severn Tunnel fame, Farnworth’s first bore was constructed between 1835 and 1838, the intention being for it to host two tracks.
It did just that for 42 years. English Heritage regards the structure as historically significant, dating from the “pioneering first phase” of railway building and thus warranting a Grade II listing.
Formal opening took place on 17th May 1838 when directors of the Manchester Bolton & Bury Railway invited friends and financiers to join them on an “experimental trip” up the line. One correspondent observed that “By a judicious contrivance of the engineer, the tunnel has been lighted by a shaft sunk from the surface, about equidistant from either end, and covered by a glass dome; and the unpleasant sensation felt by many persons in the dark tunnels on other lines is thus obviated. The time occupied in passing through by a first class train is barely 30 seconds.”
To aid their recovery from this 20mph white-knuckle ride, about 200 gentlemen reconvened at the Ladyman’s Hotel in Salford for dinner and “choice wines”. Numerous toasts followed. By the time Hawkshaw rose to say a few words, the gathering was described as “convivial”. I bet. He reported that “all the cuttings, embankments and bridges on this line have been made of more than ordinary width with a view to the ultimate laying down upon the road of a third line of rails.” Hawkshaw had a vision of future capacity constraints, but unfortunately it wasn’t clear enough to engineer the tunnel with a more generous structure gauge. When Pullman coaches were introduced on the route from Manchester to Carlisle, the only way to accommodate them was by driving a second bore for the Down line. Work on it took seven months, opening on 5th December 1880.
Here we go again
The latest stage in Farnworth’s evolution will be played out between now and October. But like the old broom that’s had a new head and handle, if you drive a bigger bore on a different alignment, is it still the same tunnel? Obviously not. Hawkshaw’s structure will cease to exist – filled with lightweight foamed concrete, along with its shaft and 13 cross passages. The whole thing will then be rebored so that the two tracks can be reunited, along with their associated OLE.
In developing a way forward, the project team considered a dozen options, from the tunnels’ complete removal to a conventional track lower. Two key factors determined their selection – a desire to improve the track alignment and thus increase linespeed, and the deteriorating condition of the tunnels. Surveys of the Down bore identified an assortment of defects: hollow brickwork, open joints and bulging, transverse and longitudinal cracks. Most of these were a product of water ingress. Even without electrification, there would have been a need to act in the foreseeable future; the structure is effectively life-expired.
Preparatory works for the reboring have been ongoing since February, closing the railway at weekends. The main phase gets underway on 2nd May, taking the Up tunnel out of commission. Thereafter, single line working will be introduced through the Down tunnel to maintain a weekday service. Crossovers have been installed for this purpose, together with a dedicated panel in the signalling centre at Manchester Piccadilly.
The impact on paths is considerable, but Network Rail and the two train operators – Northern and First TransPennine Express – have been collaborating closely to ensure eight trains per hour can run during the peaks, with longer formations to maintain carrying capacity. In some places, this has necessitated the provision of temporary platform extensions. To keep things moving, the stations at Farnworth, Kearsley, Moses Gate and Clifton will be closed until 5th October and served by buses.
In November 2014, Network Rail appointed Buckingham Group as principal contractor for the project which incorporates the reconstruction of a retaining wall and road bridge at the west end of the tunnel, as well as moving Farnworth Station’s platforms northwards to meet the new track alignment. As you’d expect, the opportunity is also being taken to progress other jobs elsewhere along the route. J Murphy & Sons are carrying out the actual tunnelling operation to a design by OTB Engineering.
What’s involved is very different to the approach seen on Crossrail. London Clay is soft and quite predictable, ideal for the rotary Tunnel Boring Machines (TBM) we’ve become accustomed to seeing. But Farnworth poses a different challenge, with ground comprising concrete, brickwork, stone and timbers from the old infilled tunnel, surrounded by untouched glacial tills. So what’s being deployed here is a classic open-faced shield, fabricated up the road in Oldham by Tunnel Engineering Services. Measuring 8.83m in diameter, it’s the biggest they’ve ever built.
The machine has been designed around a system of trapezoidal precast lining segments from Buchan Concrete Solutions. Ten of these segments are assembled to form one ring, 600mm in thickness and with a diameter of 8m. Each ring provides a 1.4m advance.
Housed within the machine’s main 3.24m shell are two telescopic cutting booms, controlled by joystick from the operator stations behind them. Mounted one above the other to excavate the upper and lower halves of the working face, the booms are supported on slewing bearings which slide along a transverse beam. This arrangement enables the excavator bucket (or drum cutter if the ground requires it) to reach both the full width of the face and the conveyor onto which spoil is deposited for removal.
Whilst mining, the shield moves itself forward – and steers – using 20 shove rams that push against the last ring of lining segments, exerting a maximum combined force of 2,200 tonnes. The ground above is supported by a series of fore-poling plates at the crown, incorporated within a hood that extends 2.275m beyond the main shell. A modular spoil control platform, positioned by a series of rams, offers protection from falling material at the face. To control dust and generally improve the working environment, there will be water sprays and forced ventilation from the western end through an access way formed within the concrete fill.
Behind the operator stations is the lining’s rotary segment erector, supported by a ring beam mounted on a slew bearing with integral drive gearing. The system has two heads operating in opposite directions, each with a 3-tonne capacity and using a hydraulic pick-up fitted to a parallel motion linkage mounted on the beam. The build takes place within the machine’s tailskin, at the end of which is a double row of brush seals; these retain the grout which is injected through sockets in the segments to fill the void between the extrados and the bored face.
As it progresses, the shield tows a series of gantries. These house the modular hydraulic power packs, cable storage reels, grout pumps, agitating tanks and a handling system to hoist the lining segments from dumpers and feed them into the build area. Running front-to-back through the machine is a central conveyor, taking spoil from the base of the shield hood onto a secondary belt across the top of the gantries. From here it is discharged into the dumpers for disposal.
After a trial assembly at the factory, the machine will be transported to site in six pieces and rebuilt in a launch pit at the Kearsley (east) portal where a large steel frame is being erected for it to push against whilst excavating the first few yards through the portal’s new concrete headwall. The expectation is for progress to be made at a rate of around six metres per day, its movement along the curved alignment being assisted by a laser guidance system. A reception pit will await the machine at the Farnworth end where it will be dismantled. Thereafter an in-situ concrete invert slab will be poured through the tunnel prior to the p-way works getting underway.
Although the control provided by the open-face approach results in far less ground disturbance than with a rotary TBM, you might reasonably ask what effect the mining activity will have on the ailing Down tunnel next door, through which trains will still be passing. At their closest point, the intrados of the two existing bores are less than five feet apart, although they don’t follow an entirely parallel course.
One key part of the preparatory works has involved the spraying of a 200mm fibre-reinforced concrete lining through the Down tunnel during a series of 54- hour weekend possessions. This lining incorporates two layers of welded mesh fabric reinforcement and is supported on dowel bar anchorages at the toe of the sidewalls. The result of this is to make the pre- existing brick lining redundant. Through one section, there was insufficient clearance to adopt this solution so brickwork had to be removed at the crown and steel ribs fitted. A high-strength grout has also been injected to provide further support by stiffening the ground on the Up side. Not surprisingly, the structure gauge is now tight, demanding the imposition of a 10/15mph speed restriction. Nevertheless, a clearance of 50mm is still provided at W6A gauge, with a further 30mm tolerance for inconsistencies in the sprayed lining.
Allied with the strengthening is an optical deformation monitoring system to determine absolute displacements in three dimensions. Supplied by Murphy Surveys, this will be carried out using a total station and reflective prisms established at 10m centres through most of the tunnel, reducing to 5m centres in the section with ribs. Real-time alerts will be provided via a web portal. Inclinometers are also being installed in the surrounding ground, together with a geodetic survey of the portals.
It’s clear that the challenges posed at Farnworth are amongst the stiffest yet faced by the north-west electrification team, both in engineering terms and operational impact. Those in the know insist the tunnelling itself is quite straightforward but, given the risks involved, its reliance on a highly-skilled workforce is beyond doubt.
With Crossrail’s tunnel drives almost at an end, here’s an opportunity to refocus the spotlight from the outstanding German machinery that has worked wonders under London, onto home-grown engineering and innovation. After all, it was John Hawkshaw and his contemporaries who pioneered railway tunnelling as we know it. We’ll return to see how it all performed when the curtain comes down in October.