Few people realise just how crucial concrete has been to almost every aspect of Crossrail. Lucy Alderson examines how the £14.8bn project has pushed the boundaries of what the material can do.
Which project do you think uses more concrete reinforcement: a nuclear reactor or Crossrail?
According to Crossrail delivery director Bill Tucker, the concrete needs of even a nuclear power station pales when compared to elements of a Crossrail station.
“One of the first projects I was involved in when I got out of college in the 1980s was the pouring of a basement underneath a nuclear reactor in the US,” he says. “I can tell you that we have more concrete and more reinforcement at the bottom of our Crossrail station shafts than at the bottom of that nuclear basement.”
Concrete has been a fundamental part of the scheme. Without it, as Mr Tucker points out, Crossrail could not have been built.
More than three million cubic metres has been used to deliver the £14.8bn project. From the 26,000 concrete rings used to build the tube tunnels to the 50,000 cu m to create a typical station, it has been the backbone of this scheme.
And it has given contractors an opportunity to develop innovative new methods of construction.
The tunnel boring machines used to create Crossrail’s tunnels use rotating cutterheads at the front to cut through the earth. Once the machine has drilled through a section of the earth, a concrete ring is assembled to create the internal lining of the tunnel.
Each ring comes in eight precast concrete sections, and are assembled by being locked together piece by piece. Grout is injected to fill the void around the outside of the concrete ring, while concrete is also used within the reinforced steel fibres sprayed to create the tunnel lining.
The TBM process has been completed and 26,000 concrete rings have been used to build 21 km of tunnel.
Unique concrete method
However, over at Farringdon, a different method was used.
Because the ground conditions were wetter and at a lower level, the station tunnel was constructed using a cast in-situ method.
Construction company Kearn designed and manufactured a shuttering machine to allow BFK (the joint venture between Bam Nuttall, Ferrovial and Kier) to pour concrete against a shutter. The machine allowed the shutter to be removed (leaving behind the concrete shell in its place) and re-used. The same shutter was used 60-70 times to create the Farringdon station platform.
“We found it was a lot more efficient to use the cast in-situ shutter, as you could pour a bay of concrete every day,” BFK tunnel engineer Simon Webber explains. “However, we came up against an issue.”
This concerned the logistics of working on the inclined ‘barrel’ shafts that house escalators, the construction of which was taking place at the same time as that of the station platforms. Only a certain amount of weight could be placed on the station platforms, which meant a limited amount of equipment could be brought down to the site.
“We completed the escalator barrel within a month; we could also do other works concurrently and saved two months [off the expected project delivery date]”
Simon Webber, BFK
There were two options: simultaneously build the escalator barrels and station platforms on a smaller scale; or build one section at a time. However, both options would increase the time needed for construction, meaning Crossrail would miss some important milestones.
This wasn’t an ideal choice, so a new solution was developed instead.
The shuttering machine previously used to build the station platforms was adapted so it could be pushed up a slope, pouring concrete at a 30-degree angle to build the escalator barrels. This meant the team could carry out a pour every two days for the barrels, while delivering other works concurrently.
Two large legs fitted with jacks were attached to the shuttering machine. These jacks could push into the concave-inverted concrete, providing enough support for the 30-tonne shuttering machine to travel up a 30-degree incline. BFK’s team could also control the process remotely, meaning no workers needed to be on the back of the shutter while construction was taking place.
“We completed the escalator barrel within a month; we could also do other works concurrently and saved two months [off the expected project delivery date],” Mr Webber says.
Suspended diamond ceiling
Heavy-duty structural work such as this is what concrete is most closely associated with. Many will be surprised to learn, however, that it has also been a key ingredient of the showstopping ceiling above Farringdon’s new ticket halls.
Crossrail project manager Mujahid Khalid explains: “In the western ticket hall, we have a precast suspended ceiling weighing 450 tonnes in a diamond structure to illustrate the [jewellery business of] Hatton Gardens nearby,” he says.
Crossrail tunnelling machine Victoria breaks through into Farringdon May 2015
A massive steel temporary works structure was built to support the ceiling’s construction. The steel frame could only be removed when a permanent structure was built and could carry the weight of the ceiling.
Concrete also forms a Brutalist ceiling over the eastern ticket hall. “With concrete finishes you try to get a polished finished,” Mr Khalid says. “But this concrete has been left raw. It’s very similar to the Barbican arts centre.”
As well as tunnelling and design, another vital consideration throughout the Crossrail project has been logistics.
To ensure work never had to stop, a concreting train played a particularly crucial role in constructing the track. The concreting train is essentially an underground concrete batching factory, weaving its way underneath the capital from the east of London to the centre.
It created the standard track slab (the type installed on the majority of Crossrail), travelling from its base point in Plumstead to the south-east into central London. Dry materials are loaded onto the concreting train and are batched into concrete while the train travels underground.
This train operated every night and could install up to 300 cu m of concrete, before stopping during the day for maintenance. “The concreting train was a very quick installation method,” adds Crossrail deputy project manager Camilla Barrow.
“With concrete finishes you try to get a polished finished. But this concrete has been left raw. It’s very similar to the Barbican arts centre.”
Mujahid Khalid, Crossrail
Standard track slab was built between Royal Oak Portal in west London and central London using another method: concrete shuttling.
Instead of mixing concrete on-the-go as with the concreting trains, pre-mixed steel fibre reinforced concrete was transported from the concrete shuttle base pads in Paddington New Yard across to central London.
“We didn’t want the concrete to go off, so timing was important,” Ms Barrow explains. “It was a bit more of a challenge using the shuttles to make sure we timed it just right, so we had the right concrete mix.”
Precast innovation at Custom House
Innovation along the Crossrail route was not limited to simply the track, however.
The construction of Custom House station, for example, offered a real opportunity to “do things differently”, according to Mr Khalid.
Most of Crossrail’s stations are underground, but Custom House is an island station, located above ground and encircled by track. The team therefore decided to use precast concrete units as a more innovative way of building the station’s superstructure.
Crossrail concrete is pumped through 300 m long pipe
Trucks transported a total of 887 precast units from an offsite factory in Nottinghamshire down to the Custom House site. Only three people needed to be on site to piece the units together “like a big jigsaw”, Mr Khalid says.
It took a year and a day to build the superstructure. In contrast, Mr Khalid estimates that using traditional methods of construction would have taken a year-and-a-half and required 30 people on site.
Using prefabricated concrete units meant the quality of finish was higher and improved health and safety, he adds, as only three people were needed to put the units together on site.
“It’s demonstrated that it [using precast concrete construction methods] does and can work, and that there are a lot of benefits from using this,” Mr Khalid says.
Not only has this £14.8bn project showcased the best of British engineering, it has also allowed contractors to explore the outer limits of concrete’s potential.
What concrete has been used on Crossrail’s lines?
Crossrail has used five different types of track in its tunnels:
- Standard track slab – this forms 80 per cent of the track in Crossrail’s central section
- Direct fixed track – used in the Victorian-engineered Connaught Tunnel
- High attenuation sleepers – similar to standard slab, used only in a few small areas to reduce noise/vibration
- Floating track slab light – used to reduce noise and vibration underneath Soho
- Floating track slab heavy – used to reduce noise and vibration underneath the Barbican