Pushing a preformed concrete box through an excavation to make a readylined tunnel is not unknown, in Britain or abroad, but hitherto, at least in this country, the boxes have been rectangular and the pushes have been in a straight line. But inserting a curved portal into a curved excavation – pushing it, as it were, literally round the bend – is something that has not been attempted on these shores.
“We have heard rumours of it being done before in China, but we could not find details and we had to work out our own technology,” says Matt Hadden, senior project manager at contractor Morgan Sindall, which is performing the feat on a £200m project on the East Coast Main Line for Network Rail.
Mott MacDonald together with Tony Gee have come up with the design, while Jacobs was appointed by Network Rail as GRIP3 designer. Essex-based Tunnelcraft has supplied expert labour, BAM Ritchies has performed soil nailing, and Bauer Technologies has installed piling. All the reinforced concrete work has been done by Bell Formwork.
A New Rail Junction
On the face of it, a modification to a rail junction might seem neither costly nor complicated, but the constraints around this one have led to a perhaps unique, certainly unusual, solution.
At Werrington Junction, three miles outside Peterborough in East Anglia, a branch line to Spalding and Lincoln meets the main line. Slow-moving freight trains to or from those destinations have to cross over three tracks of the East Coast Main Line. Fast passenger trains therefore frequently have to slow down or stop and wait for them to pass, for up to 10 minutes at a time, delaying passengers, reducing capacity, and slowing schedules and services. It has been a bottleneck that Network Rail has long sought to remedy.
The solution is a dive-under. More officially, it is known as a grade separation. Public consultation back in 2018 preferred it to the alternative of a flyover. The branch lines will pass under the main line, in a curve, before rising to ground level again to emerge parallel to it and then join it further down the line. Hence the need for a curved tunnel.
Why, though, the push through? There are plenty of other ways of making a tunnel. “We looked at other methods,” says James Tweddell, project principal at Mott MacDonald. “Because the main line could not be interrupted for long, all the other options would have involved piecemeal excavations, stopping and restarting, with temporary supports for the main line throughout the operations. Equipment would have been scattered along the line for months. With so many interlocking operations there would have been scope for things to go wrong.
“On a greenfield site this would have been a very simple cutand- cover operation. Working around and underneath a fully operating main line gave instead a complex web of constraints. There were lots of them: environmental – there is a watercourse running alongside; safety, and rail movements were just some; and adjusting for one affected the others.” Hence this solution.
One obvious benefit was that it offered a minimum of closure time for the main line – a short period over Christmas 2019 for realigning some existing track, when a stretch of the main line was shifted 20m to the west to provide space for the south ramp and the TBM reception pit. In addition, 9-10-days are scheduled for early next year for the push through – the only major closure of the main line during the entire project.
Environmental Issues
The planning was detailed, and difficult: “I’ve been working on this for three-and-a-half years,” says Tweddell. “We have done 3D modelling in a great deal of detail.”
Preparatory enabling work was extensive. The site lies within a flood plain so water was always going to be an issue. A stream, Brook Drain, which runs alongside, has been moved westwards to make way for the new structure. It has also been enlarged to help with local flood issues – the new river is now 840m long and 4m deep. It features a meandering channel, with shallow sections called riffles and side pools where aquatic wildlife can take refuge. Environmental surveys found a rare insect, the four-spotted moth (Tyta luctuosa), on the riverbank. The creature is a high-priority UK Biodiversity Action Plan species and a habitat for it was designed into the new channel. A pond as a habitat for the protected great crested newt was also dug.
Ramps north and south lead the branch lines into and out of the tunnel. The 940m-long north ramp has piled walls with overhead bracing. Rotary-bored contiguous piles that are 183 No x 18m long and 900mm in diameter have been used, and there are more than 900 No 8-10m soil nails, finished with sprayed concrete. An integral bridge, of steel and concrete – 55m wide with a 15m span – is part of the project. In total, 120,000m3 of earth will be moved.
With groundwater being “a challenge on the site”, according to Alan Willoner, technical principal of Mott MacDonald, pumping stations were put “each side of the line to get the water out. The north sump pumping station underpinned caisson … has a 6m internal diameter and is 13m deep. The south side one has a 5m internal diameter and is 14m deep.
The construction sequence is interesting. Two parallel tunnels, each of 3.3m external diameter, have been bored along the curve of the route by a Lovat RM132 EPB TBM. They are lined with concrete segments, six to a ring. The rings are asymmetrical – 1m wide on average, but slightly wider on one side than on the other to provide the curvature of the tunnel. Muck removal to skips was by screw conveyor on the TBM onto a conveyor belt passing over the top of the back-up carriages. The skips were raised to the surface by crane for removal.
Geology
“The geology includes three high-pressure confined aquifers that we expected to hit and a need for extensive dewatering,” says Willoner. “These are Kellaway Sands, Cornbrash Limestone, Blisworth Limestone and we also considered the possible influence of water pressure from the deeper Lincolnshire Limestone.”
The TBM began excavating in March, working its way under the main line carrying 125mph Intercity trains. The first breakthrough came in May 2020. Having been carried back to the start, the TBM began boring the second tunnel, where it achieved breakthrough on 27 July. It has now been removed in sections. The exit excavation was around 10m long.
The TBM’s entry shaft, however, was very much longer, at 200m. This is because it was used, once the TBM had cleared it, as the excavation for pouring and casting the concrete portal box.
The tunnels each have a 3.3m external diameter (2.85m internal). They are situated lower than the final position of the dive-under to simply provide a route for the temporary rails that will eventually support and guide the preformed, curved concrete portal box on its 160m path.
The box was constructed and completed in an excavation at one end of the tunnels. Rectangular in cross-section, it measures 9.5m wide by 5.1m high, and its 160m length will comprise 10 No 16m-wide sections. Its curve will follow the 750m radius of the finished tunnel.
Pushing into Place
The 11,000t box will have to withstand not only the stresses of its static working life but also the greater stresses of being pushed on a curved path from the construction site to the final position. For that reason it was built thicker, stronger and heavier than might otherwise be expected. With the concrete 1m thick, Tweddell says, “the sheer amount of steel in it was one of the challenges”. Hadden of Morgan Sindell adds: “We are now back-propping it with steel props inside.
That is to keep it rigid during the push.” When the guide rails have been laid through the TBM tunnels, the box will be jacked onto skids that rest on the guide rails. Then as many as four massive 1,250t-capacity jacks (supplied by ALE, now part of Mammoet) will push the portal forward. The box has been built with pockets precast into the bottom of its walls to give support points for the vertical lifting jacks – quite separate from the four, large horizontal pushers – that will be positioned underneath it and will raise it onto the skids.
“We can push it the whole 160m with just two jacks, and we will hope just to use two; the others are for backup in case anything goes wrong,” says Hadden. The driving jacks will be braced against steel pins set into what Tweddell describes as “a huge great chunk of concrete”. More precisely, the reaction block is supported on 30 bored 1.2m-diameter piles.
Steering is primarily achieved by the tunnel guide rails, but there is a lot of potential for steering by differential pressures in the two thrusting rams, says Willoner, who adds: “At each corner of the box are yet another set of jacks, four at each corner, to assist the process.” They will resist out-of-balance horizontal soil pressures during the push.
The box is considerably larger than the two TBM tunnels. As it moves forward, the soil ahead of it will be removed by conventional excavation from above. And when it is in its final position, the roof of the portal will be covered over again and the rail tracks re-laid on top. Therefore, despite the use of a TBM, this is in effect a cut-and-cover tunnel. But unlike conventional cut-and-covers, the concrete box is prefabricated outside the cutting rather than being poured in situ.
Once the top surface of the tunnels has been exposed the excavators will cut into them. The segments lining the top one-third of each tunnel are sacrificial and will be removed. “On the left-hand tunnel it will be the segments between 11 o’clock and three o’clock; on the right-hand tunnel it will be the ones between nine o’clock and one,” says Hadden. “The two-thirds of the tunnels left in place will be permanently covered by the base slab.”
They will, in effect, form the bottom corners of the finished dive-under.
“The skids are due to arrive at the end of November,” says Hadden. Testing for the great push forward is scheduled for early December. Tweddell adds: “We will give a trial push, of around 10m, to test friction and design parameters some time before Christmas.”
Excavation will keep pace with the pushing, keeping just ahead of the steadily advancing face of the portal, as it may not be wise to have to stop or start a mass of 11,000t more than is necessary. Clearance each side of the box is to be kept to a minimum – around 700mm, says Hadden. A steel cutting shield on the portal’s advancing walls should remove any minor lumps of soil.
The tracks, gantries and overhead lines will, of course, have to be removed and the railway closed while the push and excavation are happening. The push itself is planned to take around three days of a 9-10-day blockade of the line.
In all, the project involves 1.87 miles (3km) of new branch line, culminating in the dive-under. Network Rail estimates that, alongside other upgrades, it will improve capacity on the East Coast Main Line by 33%, equivalent to two extra train paths an hour or 1,050 Intercity journey seats an hour on express trains through Peterborough. It will reduce the fastest journey times between London and Edinburgh by 21 minutes.
Completion is projected for early mid- 2021. East Coast passengers may not even notice the new infrastructure or appreciate the engineering imagination that has created it. They will, though, experience a faster journey without any unnecessary slowing down or stops.
