Liverpool has two Caverns. Well, for the purpose of this contrived introduction it does. One bills itself as the cradle of British pop, having claimed legendary status in the Sixties courtesy of John, Paul, Ringo and George; the other, half-a-mile across the city, has temporarily hosted the Eighth Wonder of the World in 2016 thanks to a chap called Dave. How’s that for a claim-to-fame?
It’s not pushing the envelope too far to describe Central Tunnel Down High Level Neck as a ‘cavern’. Honest. Stand at track level and exposed sandstone sidewalls reach 60-odd feet upwards to a segmental brick arch springing off ledges. It’s quite something. Even more unusual is that the tunnel looked rather different 45 years ago before rationalisation changed the shape of Liverpool’s railways.
For the best part of a century, Central station served as the terminus of the Cheshire Lines Committee (CLC) route from Manchester, trains emerging from the tunnel into a vast iron and glass shed. Sub-surface platforms – aligning with those above ground – opened in 1892 when the Mersey Railway arrived from Birkenhead, having passed under the river. Beeching closed the high level station in 1966, but Merseyrail soon emerged as several suburban lines were brought together to form one cohesive network.
Engineering Merseyrail involved new tunnels being pushed under the city centre. Central Tunnel evolved to comprise three distinct parts: » Up Single Bore: a Seventies structure that carries the Up Main from the underground Central station to Duke Street.
» Down High Level Neck: the northernmost 325 yards of the original two-track tunnel through which the invert was lowered to connect with the low level platforms at Central station, accommodating the Down Main and a reverse siding. Included at the south end is a section of concrete infill through which is an archway for a single track, whilst in the middle is a ventilation shaft, rectangular in section. Both the Down High Level Neck and Up Single Bore separately enter Central station via short cut- and-cover concrete boxes.
» Central Tunnel: the remaining 662 yards of unmodified CLC tunnel, heading southwards to Parliament Street.
The intention behind splitting the tracks was to ease the construction and subsequent operation of the proposed Edge Hill Spur, conceived as a link between the city centre and communities further east. Work started on the headings for two bores that would have connected Central station to the historic Wapping Tunnel, engineered by George Stephenson in the late 1820s with the assistance of Charles Vignoles and Joseph Locke. But the spur was abandoned in the Eighties due to economic constraints.
Live and learn
Back in 2010, a collection of routine brickwork defects was recorded through a 60-metre length of arch towards the southern end of the High Level Neck. Brought in to repair them was AMCO Rail, the solution being to erect a scaffold from which sprayed concrete would be applied.
The job is not remembered fondly for a host of reasons, having conspired against the team from start to finish. Plant and material were transported to the scaffold by RRV from Brunswick station – more than a mile back up the line – but this was only available for about three hours each midweek night and six hours on Saturday nights as part of a regular possession regime.
The design for the birdcage scaffold was complex – with multiple lifts – creating a series of cramped spaces which made the work operation difficult. Unable to pump concrete over such a long distance, dry-spray concrete packaged in 25kg bags had to be used, resulting in dust and manual handling issues, not to mention extending the programme appreciably and driving up the cost. The whole thing was a compromise, and a hugely inefficient one at that.
Looking for a positive, the job did get done. It was however not an experience anyone would have chosen to repeat. Then, 18 months ago, the phone rang.
A better way
Dave Thomas, AMCO Rail’s contracts manager, has a can-do philosophy by default. You sense, though, that even his first instinct – faced with the prospect of a return to Central Tunnel – was to check his remaining leave allocation. Not helping matters was Network Rail’s remit: to remediate a section of arch almost three times longer than the previous occasion.
On the plus side, the prospect was raised of access being available via the portal. This was news to Dave; as far as he was concerned, there no longer was a portal. But tucked into a corner behind Network Rail’s maintenance depot – which sits on the former station site – is the old high level tunnel entrance, albeit with an immediate sheer drop down to the track. This opportunity proved critical to the subsequent success of the project – impacting positively on timescales and cost – although the city centre location certainly brought unhelpful space and logistical constraints, with access too tight for articulated vehicles.
The last thought on anyone’s mind was to use another birdcage scaffold, so the AMCO Rail team spent some time in the tunnel considering options. The chosen approach was to establish a crash deck and working platform for 165 metres, high above the railway. This would entirely separate the project from trains passing beneath, bringing 24-hour working, a safer environment and significant efficiencies. “The Eighth Wonder of the World,” Dave calls it with a characteristic glint.
Not to be underestimated were the loading requirements of this structure, taking account of the spray-concrete robot, the thrust of its boom on start-up, a shift’s worth of rebound and the impact risk from a falling ‘cowpat’ two metres in diameter. The firm responsible for erecting it was Crossway, supplier of a similar system at Manchester Victoria during the station roof works. RDG Engineering fulfilled the design work.
Installation took place predominantly over four 29-hour weekend possessions. Road-rail MEWPs from Total Rail Solutions were used to reach up to where holes needed drilling to fix in place the scaffold tubes which support ladder beams, spanning the tunnel. Floor cassettes were then put in place longitudinally between these beams. The variable height and width of the rock excavation did cause some complications, but the team still managed to assemble 54 linear metres of deck over the final weekend. Weekday-nights offered sufficient opportunities for any remaining works to be completed, including the process of boarding-out.
Do the right thing
Able now to touch the soffit, a full condition survey was undertaken from wheeled towers. It soon became clear that the mortar – whilst fine to the eye – effectively consisted of a thin crust with nothing behind it. Much of the brickwork was hollow and nearly all of it needed repointing; large areas would have to be recased.
When a schedule was drawn-up for conventional repairs, so time-consuming was it that another spray concrete intervention proved the obvious way forward, encompassing the full length of arch accessible from the platform. This would put the High Level Neck to bed for 120 years, making the original brick arch redundant and delivering deep whole-life cost savings.
As part of the preparatory works, soot was removed from the brickwork to improve bonding with the new concrete arch. To provide a flat base for it to spring off, a 300mm ledge had to be saw-cut into the sandstone and then broken out. There was also an array of rock bolts to install and a ventilation system. And there was still time for AMCO Rail’s team to take on two further projects, taking down the top six courses from the shaft’s protection wall, as well as fulfilling a grit-blast and paint job on a girder passing over the portal.
The standard methodology for spray concrete arch repairs specifies L-pins at 400mm maximum centres together with A142 steel mesh. Over the area involved, this would have occupied 12 men for three months, fitting more than 12,000 L-pins. Donaldson Associates, the works designer, calculated that – for a structural layer – the thickness of the spray concrete would have to exceed 300mm. Instead it was decided to install adjustable Ram-Arch reinforcement panels, manufactured by Foulstone Forge, 40mm clear of the brickwork to ensure no shadowing. Doing so brought the concrete thickness down to 250mm, sprayed in layers of not more than 100mm. This approach improved the end-product and reduced the associated cost.
The specification for the concrete mix was a slightly refined version of that adopted during AMCO Rail’s reconstruction of Holme Tunnel two years ago. The total order amounted to 740 cubic metres.
Spraying was done on days, the first batch arriving from the supplier, Cemex, at about 08:30 each morning. From the portal, it was pumped to the robotic arm, which was sat on timbers to transfer its load safely into the ladder beams. Operating it was a four-man team from Gunform. One wagon delivered enough concrete to spray for about an hour; thereafter it was all hands to the shovel, filling bags with the rebound and despatching them on a trolley. Half-an-hour later, the cycle would start again.
Checking the Ram-Arch for the following day’s work was one key role for the night shift, ensuring it was sufficiently tight to prevent any sagging. Once the spraying was well advanced, a scaffold team also attended to progress the dismantling of the platform during the three-hour nightly possessions, allowing swift demobilisation of the site when they crossed the finish line.
Outside the box
Spraying concrete above a live railway is another AMCO Rail first. Well, probably. That’s what the company thrives on and has built a reputation for – developing bright ideas to overcome unusual challenges. It’s something the industry must learn to do more of.
The financial benefits speak for themselves. Back in 2010, spraying 60 metres of the High Level Neck cost around £5 million (£83,000 per metre), a consequence of the unproductive methodology imposed by prevailing circumstances. This time, 165 metres has been tackled for £3 million (£18,000 per metre). That’s great news for both Network Rail and the public as its ultimate funder.
There have been sleepless nights, particularly the one before the robotic arm was first introduced to the platform. But it’s all worked seamlessly. Dave reflected: “It’s great when a plan comes together.”
As the Fab Four might have put it: “We Can Work It Out”. Yup, still contrived.
Written by Graeme Bickerdike.
Photos: Four by Three