For nine days, one stretch of Britain’s busiest express railway had no rails on it at all.
Below the gap, a concrete box the length of a city block was creeping sideways into position.
Trains from London kept running the whole time, on tracks shifted out of the way.
At Werrington, just north of Peterborough in the English flatlands, the goal was to thread a freight line beneath the main line so slow freight would stop conflicting with the expresses above.
So how do you push 11,000 metric tons of curved concrete under a live railway and hand the tracks back early?
Why closing the line was never an option
The East Coast Main Line carries some of the busiest express traffic in Britain, and closing it long enough to dig an open crossing would have meant weeks of rerouted trains. Every alternative was worse. Carrying the line on temporary truss bridges while crews excavated underneath would have required massive foundations because of the skew angle, and a bored tunnel would have sat much deeper with more extensive approach works and added risk on a site in a flood plain.
A jacked portal was chosen because most of the construction could happen off line, with just one short track closure and minimal risk of unplanned disruption. A skeleton main line service was maintained throughout by diverting passenger trains onto one of the realigned freight tracks.
The route beyond leads onto the Great Northern and Great Eastern joint line, running through Spalding to Lincoln and beyond. Threading a structure this long under live tracks and around a curve, without a boring machine, had never been done before in Britain.
The box that weighs more than the Eiffel Tower
The proportions are hard to picture. The box measures 508 feet long, 31 feet wide and 17 feet high, with walls a little over 3 feet thick, and the curve runs on a radius of about 2,460 feet. Network Rail noted that the curved concrete box is heavier than the Eiffel Tower, which comes in at roughly 10,100 metric tons all told.
Because the tracks do not run straight at Werrington, the tunnel could not either, and a gentle sounding curve becomes a serious problem when the load is this rigid. Jacking becomes impractical for large or long tunnels because friction forces grow too great for the jacks, and curves add an extra order of complexity. The curved box was therefore built alongside the main line using a specialist sliding formwork system, in staged concrete pours, before the ground under the railway was touched.
That sequence matters: every foot of casting happened in open air before a single foot of live track was risked.
How the jacks moved a record and a railway
Guide tunnels were bored under the line first, and hydraulic jacks then propelled and steered the box through them. The concrete rode prepared surfaces rather than dragging raw against the earth, lubricated slide paths installed in those access tunnels rather than brute force against the clay.
A total of 34 jacking pockets in the base slab drove each stage of the push. Four 1,250-ton jacks were slung from the portal roof and attached to a thrust slab at the rear, with reaction frames weighing 17.5 metric tons craned between pockets as the work advanced in 5 meter pushes at a rate of about 5 feet an hour.
Work began at 1am on a January morning in 2021, with signalling removed first, then track and overhead lines. The push was finished in just over five days, and the track was handed back 52 minutes ahead of schedule. The result set a world record for the longest single underground jacked structure ever driven.
A freight path threaded without boring
Once the concrete was in position, the work moved inside. About 2.5 miles of track went in along with signalling equipment, all without disrupting train services above. The junctions at the north and south ends were connected later that year, as was the signalling.
The logic is shared with other moves in heavy construction: immersed tunnel boxes are also built first and positioned second, though at Werrington the structure never touched open water. And unlike a bridge slid sideways on skid shoes, this move happened underground, beneath a railway that reopened days later.
What the ground under the main line demanded
The flat country around Peterborough is not simple digging. Ground conditions at Werrington are complicated by four confined aquifers, and dewatering wells were installed to manage base slab heave during excavation. Relic shear planes in the Oxford Clay can fail where slopes reach about one in three, a risk the design had to address directly.
During the push, clay was taken out by hydraulic excavators working at the front of the box. The excavated envelope was overcut by only 50 mm, so minimal pea gravel backfill was needed before the sides and base were grouted to transfer load into the guide tunnel foundations. That is about 2 inches of tolerance across 508 feet.
The dive under was completed and opened to freight traffic toward the end of 2021, and as a single structure with no joints or bearings it should be virtually maintenance free across a 120-year design life. Ground Engineering reported that successful delivery creates the potential for the patented technique to be used elsewhere, and the Royal Academy of Engineering confirmed it stands as the world’s longest single underground jacked structure. The expresses still run above; the freight now runs beneath them.
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