The High Speed 2 rail project’s first cut and cover “green” tunnel built with precast segments has started to take shape.
At High Speed 2’s (HS2’s) site in Northamptonshire a crane lifted and then lowered a huge concrete tunnel segment into place onto an M-shaped arch in early June. The operation marked the start of work to construct HS2’s first cut and cover tunnel.
HS2 main works contractor EKFB, a joint venture between Eiffage, Kier, Ferrovial Construction and Bam Nuttall, is building the 2.5km long structure in a shallow excavation near the village of Chipping Warden.
What makes the tunnel novel is that it is made up of 5,020 individual precast concrete segments. They are brought to site where a crane slots them together like giant jigsaw pieces.
This “green” tunnel will eventually be covered with backfill and landscaped to blend the high speed railway line with the surrounding countryside.
It will also have specifically designed “porous portals”, which are 100m long V-shaped sections at either end of the tunnel that have openings to the outside air. They will dissipate the pressure and reduce the noise made by the 360km/h trains entering and exiting the tunnel.
Overall, five cut and cover tunnels are being built on HS2 phase one. These tunnels will also be built at nearby Greatworth, at Wendover in Buckinghamshire and at Burton Green in Warwickshire. They will have a combined length of 6.4km.
A total of 13,290 concrete segments for Chipping Warden, Greatworth and Wendover are being made by Derbyshire-based Stanton Precast.
French specialist engineer Matière was subcontracted to install the segments for the tunnels. It in turn appointed Hewson Consulting Engineers to prepare inspection test plans and risk assessment method statements and to carry out temporary works design.
Shallow tunnels are often built by casting a box structure insitu in an excavated trench. The tunnel at Chipping Warden, with its M-shaped double arch with separate halves for southbound and northbound trains, is a departure from this.
Its concrete segments are manufactured offsite at Stanton’s Ilkeston factory and then brought to Chipping Warden in batches.
Five different precast concrete segments are slotted together to form the double arch tunnel structure which comprises a central pier, two side walls and two curved roof slabs. All segments are steel reinforced. The largest segments are for the middle central wall and weigh up to 43t. The internal height of each arch is 8.4m, and the total width of both arches is 20.3m.
EKFB project manager Jérémie Martin explains that the tunnel design was influenced by French high speed rail projects. Most notably, Eiffage delivered the 200km LGV Bretagne-Pays de la Loire high speed project in France, which featured a cut and cover tunnel of the same design.
Stress testing is carried out on the concrete segments to ensure their durability. “When we cast the concrete, we take some samples of the concrete that we are casting in order that we can test that the strength is what is expected,” says Martin. He adds that the segments are designed to fit together like “a pin joint – so it’s like a knuckle basically”. This allows for some movement of the structure.
Crucially, the backfill has also been developed to work with the structural design and keep the tunnel in place. After waterproofing is installed on top of the concrete segments, around 2m of technically designed backfill and 2m of normal landscaping earth will cover the tunnel.
“The earth is going to put some pressure on the roof, so the arch will tend to expand. But because of the backfill it can’t, so the whole structure has been designed to work together,” notes Martin.
He explains that when you apply pressure to the top of the arch – in this case the weight of the ground above – it transfers out to the sides. On a simple arched bridge this is absorbed by the abutments on either side. In this case, it is absorbed by the counterpressure of the backfill.
With a conventional double box, the load bears down straight onto the slab so only the reinforced concrete takes the load, meaning that the slab must be much thicker. EKFB’s design of the tunnel enables the loading to be spread out evenly across the structure because of the double arch shape as well as the technical backfill, which takes part of the load.
Because of this the structure uses significantly less concrete. Indeed, the two concrete roof slabs are only 350mm thick – compared to 2m thick in HS2 Ltd’s original reference design.
By reducing the amount of concrete and steel needed for the tunnel, this lighter-weight modular approach is expected to more than halve the amount of carbon embedded in the structure, says HS2 Ltd.
Other elements of the design have been similarly value engineered. The tunnel segments will sit on a 125mm blinding. This reinforced concrete slab has been designed to be used as a working platform for the crawler crane, a level platform for the segments and as protection for the waterproofing under the structure.
The M-shaped double arch tunnel sits in a 10m deep excavation
The tunnel is being constructed in a 10m deep excavation. To the north the tunnel route progresses through a hill so at this point the excavation becomes 20m deep.
HS2 Ltd project manager Suhel Uddin says that constructing the steep slopes has been made much easier given that the cutting runs through Oxford Clay, which provides good ground conditions for excavation.
EKFB has already carried out borehole and ground investigations for the earthworks. Uddin adds that the programme has been designed to ensure that the earthworks can happen in “earthworks season – April to October – because that’s the dry period. When clay gets wet, it is messy and sticky.” As such, excavation works started in spring last year. The contractor has also installed dewatering wells in the excavation to provide drainage and water management.
To carry out the excavation, material from the top of the ground is skimmed off in layers. Heavy mining plant takes between 4t and 5t of material per bucket or up to 40t per vehicle off site. The excavated material is being reused for the haul road and then to cover the tunnel.
The team has installed slope monitoring points along the excavation. “We monitor the movement [of the slopes] at all times, to make sure that the ground is moving within the limits of the detailed design. And to make sure that we are working in a safe environment,” notes Martin.
Excavated material being removed further down the tunnel alignment.
To save time, EKFB is working on different parts of the tunnel simultaneously. The south end has progressed furthest and tunnel segment installation has begun. In the middle section the team is one step behind, and is laying the blinding. At the north end the team is still excavating the cutting.
The team has designed the programme to limit disruption to the local community. Most notably, it has avoided closing the A341 road, which runs across the tunnel’s path, by starting with constructing a mid-section of the tunnel just to the north of the road.
Once built, this section will be covered and a new road will be installed over it, freeing up land to its south for the remaining stretch of the tunnel. Martin adds: “[On this section where the road will run] the design is different. We have more reinforcement to cope with the dynamic loading. And because of more reinforcements, the section is a bit thicker.”
The 150m section where the road crosses the tunnel was due to be completed by the end of July. The next step is backfilling, which is scheduled to be finished by the end of the year.
By mid-2023 the new road is expected to be up and running. The tunnel itself is scheduled to be finished by spring 2024.
After quality issues were recently identified with some of the precast segments EKFB is now carrying out quality assurance tests.
An HS2 Ltd spokesperson said that the initial test results have been positive. “We still need to get the results back from a few more tests before assembly can restart, but excavation, foundation work and offsite manufacturing of the segments are all full steam ahead,” they added.
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