A bridge so far: HS2’s Colne Valley viaduct

Crossing a busy road and several waterways, building HS2’s Colne Valley Viaduct has prompted some novel approaches. Ben Vogel finds out how the Align JV is taking on the challenge

One of the trickiest construction and engineering aspects of the HS2 megaproject is the 3.4km-long Colne Valley Viaduct, whose main structure is scheduled for completion later this year. Standing tall above the Colne Valley Regional Park and Grand Union Canal in Hillingdon, west London, the bridge faces tricky terrain, needing to cross a series of other water features as well as the A412 road, which runs between Slough and Watford.

The Align JV (Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick), which is building the viaduct as part of the 21.6km Phase One Central 1 portion of HS2, is tackling the difficulties with methods including offsite manufacturing and smart engineering.

Construction work on the viaduct began in early 2021 when the first of 292 foundation piles was constructed. By the time Construction News visited the site in mid-January, the first 480-metre stretch of the viaduct was complete. Work on this included installing 144 precast concrete deck segments — out of approximately 1,000 needed to complete the viaduct — using a 700-tonne, 160-metre launching girder dubbed ‘Dominique’. Work had also been completed on all but two of the foundation piles, although those were completed by the end of January.

V-shaped piers 

The viaduct’s land piers are built with faceted formwork above bored piles. The complex V-shaped water piers require pile caps to be cast within temporary cofferdams. The piers follow a pile base structure in 80-metre spans. “We’ve got four of [the piers] under construction at the moment. One of them is more or less complete. There will eventually be 11 and on average they take between seven to nine months to make,” says HS2 senior project manager Billy Ahluwalia.

Because the viaduct curves to follow the contours of the Colne Valley, each deck segment has a unique shape and is cast in a 105,000 cubic metre temporary factory close to the north end of the construction site. The piers are cast on site along the trace of the viaduct across the valley.

When completed, the viaduct will feature 56 pier supports at an average height of 8.7 metres plus an abutment at either end. Each pier weighs approximately 370 tonnes; Dominique installs the 140-tonne deck segments sequentially from one pier to the next.

Factory work

“The [deck construction] method here is what we call segmental balanced cantilever construction,” says Ahluwalia. “We make individual segments in the factory, and transport them from the factory onto the built viaduct.

“As we build the viaduct progressively forwards, we use an overhead launching girder to pick the [new] segment up. We locate them next to the previously constructed segment, glue them together and temporarily stress them. Then we do the permanent stressing.”

A pair of segments is installed every 36 hours. “We’re getting a pair of segments every shift at the moment, and we’ll progress to two pairs of segments so the production efficiency is increasing,” Ahluwalia says.

He is confident that as the “learning curve” continues, the pace of the project will speed up. Segments are added progressively from either end of the structure, so that eventually “we’ll meet in the middle to stitch the final pieces together”.

The 480-metre stage of the deck is “the first fixed point of the viaduct”, Ahluwalia adds.

Road crossing

Work is already in progress on the next major milestone in the project — the challenge of bridging the A412.

“This requires our longest span [80 metres] because it crosses the road at an oblique angle,” Ahluwalia says. “We’ve planned that span to be completed over a two-month period with a road closure every night for those two months, except weekends. That was carefully planned to ensure that we can minimise the disruption that is caused to road users.”

Installation of the deck segments in the part of the viaduct that crosses the road is scheduled for completion in late 2024. After that, Ahluwalia says, “we have work to do on top of the deck and elsewhere that will take us into late 2025 before we hand over to the rail systems contractors for track laying”.

Tunnel vision

To the north of the viaduct site, just inside the M25, the JV has also started building 16km of twin-bore tunnel through the Chiltern Hills.

The work is being delivered through a combination of traditional tunnelling methods and a pair of 170-metre long advanced tunnel boring machines (TBMs) named Cecilia and Florence, which have covered about half the distance so far.

Tunnelling machine manufacturer Herrenknecht made the two variable-density TBMs specifically for HS2. They had to be designed to deal with the chalk ground conditions and the fact that two-thirds of the tunnelling will take place below the water table.

“The [chalk] aquifer is very sensitive,” notes Align JV underground construction director Didier Jacques. He adds that tunnelling techniques have been selected to minimise the risks of aquifer contamination and they test regularly for potential groundwater contamination as the machines progress on their journeys.

Each TBM operates with an excavation diameter of 10.26 metres to carve out a 9.1 metre internal-diameter tunnel, excavating vast volumes of material at a rapid pace.

“A lot of innovations have been implemented on the [tunnelling] machines, not only to improve performance but also to improve safety and change the way we work,” Jacques adds.

Traditionally, a TBM would excavate for 2 metres, stop, put rings in for 2 metres, push on those rings and excavate the next 2 metres. However, Florence and Cecilia are capable of semi-continuous boring, which is intended to increase speed and improve efficiency.

Semi-continuous boring “can improve performance by 5-15 per cent depending on the speed of operation of the TBM… Just last week [mid-January] we beat our record by building a 220-metre stretch of tunnel in one week,” Jacques remarks.

Re-establishing the grasslands

A particular challenge is to ensure that chalk spoil is processed on site. When the tunnel is complete, the two TBMs will have generated a total of 3 million cubic metres. Align JV calculates that about 1.4 million m³ has been mined so far.

The chalk is pulverised by the action of the TBM’s cutterhead and then separated from the slurry. Water is pumped to the TBMs and the excavated chalk and flint is pumped out of the tunnel to a slurry plant, where the flint is extracted and filter presses take water out of the chalk.

Water is pumped back into the tunnel for re-use and the chalk is taken to the nearby Pynesfield Quarry, where it is being landscaped as part of a rewilding programme.

“But we have a challenge to place that into the ground because you can’t really drive on it,” explains Align JV surface operations director Derek van Rensburg. “It’s like building a mountain out of toothpaste, and it was challenging in the first winter to do earthworks.”

Jacques’ team is constructing ventilation shafts at Chalfont St Peter, Chalfont St Giles, Little Missenden and two in Amersham. For this they are using diaphragm wall techniques, and they need to make sure they are complete before the TBMs reach them.

Align JV is also building 38 cross-passages, each 15-20 metres wide, to connect the twin bores in the tunnel, with the first passages already completed.

The TBMs have already passed the shafts at Chalfont St Peter and Chalfont St Giles and are now approaching Amersham and the third shaft.The shafts are designed so the TBMs can cut through their sides.

Uniquely, the tunnel invert where the rail track will eventually be laid is being prepared at the same time as the TBMs move forward, instead of being done once the tunnels are complete.

Two precast factories produce the concrete tunnel segments for the TBMs. A total of 112,000 segments are needed to complete both bores in the tunnel, with 58,000 made to date. Each segment weighs about 8.5 tonnes, and there are seven segments in each ring.

Asked about the difficulties of running a complex construction operation at a time of material shortages and price hikes, van Rensburg notes there has been “considerable pressure” on the supply of cement and cement replacement products such as certain types of ground granulated blast-furnace slag.

“We have to work extremely closely with our suppliers to almost ringfence our materials,” he says. “A lot more planning and foresight is needed. You can’t just ring up and order materials for the next day — those days are gone.”

The TBMs should reach their destination (the North Portal) by early 2024, but as Jacques warns: “We still have a long journey in front of us.”