A version of this article was published in RAIL Magazine issue 1040
Some of the viaducts will be engineering marvels in their own right, such as the 3.4km (2-mile) long Colne Valley Viaduct, while others, which have been designed to blend into the landscape, will be modest in comparison.
Given that so many bridges and viaducts are being constructed, it's unsurprising that some will share the same or similar construction techniques. Whilst others required a more bespoke solution to overcome certain challenges and meet a particular set of criteria.
It would have been ideal if it were possible to use a standardised method of construction across the whole route. However, the construction methods for each structure had to be decided upon based on several factors. They included length and height, the space available for construction, ground conditions, and nearby infrastructure.
In addition, the DfT specified that each structure must be designed to last 120 years without requiring significant maintenance. Many of the viaducts have also been designed to withstand immense lateral forces. The viaducts on the mainline, for instance, have been designed to resist the force that would be imparted if full braking were applied as a 400m long train travelling at up to 360km/h (225mph) was on or approaching the viaduct.
So, whilst using standardised methods would be preferable, it is not always possible to create just one or two viaduct designs that would meet all of the criteria.
That’s not to say that the contractors delivering HS2 haven’t tried to use as much standardisation as possible, and many of the viaducts share the same engineering principles. One technique which has almost universally been adopted is the use of as much off-site construction as possible to manufacture structural elements in a controlled environment.
Colne Valley Viaduct
In the case of the Colne Valley Viaduct, engineers had to determine how to build “the UK’s longest bridge” over a series of lakes, waterways and roads whilst keeping local disruption to a minimum.
The viaduct is being constructed by the Align Joint Venture, made up of Bouygues Travaux Publics, Sir Robert McAlpine and VolkerFitzpatrick, which is delivering a 22km long C1 HS2 section between the northern Northolt Tunnel portal and northern Chiltern Tunnel portal
Colne Valley Viaduct just prior to completion in 2024
Due to the uniqueness of the viaduct and the surroundings, it was decided to utilise a bridge-building leviathan imported from Hong Kong. The so-called launching girder christened Dominique was used to build the East Tsing Yi Viaduct, constructed by Bouygues Travaux Publics in 2008.
The 160m long, 700-tonne launch girder was used to lift 1000 precast concrete segments into place, which weighed between 60 and 140 tonnes. As each bridge span was completed, Dominique was able to move forward to construct the next.
The segments were manufactured close to the northern abutment at a purpose-built factory in West Hyde. Assembly of the viaduct began in 2022, and the final segment was lowered into place in September 2024, which is an incredible achievement.
The Colne Valley Viaduct is a landmark structure and is noticeably larger in scale than most of the viaducts along the route. But despite its sheer size, the viaduct looks graceful. This is thanks to the design, which was intended to mimic a stone skipping across water.
The Final segment being lowered into place in September 2024
Thame Valley Viaduct
Many of the viaducts on the route are more modest in scale, but construction has been no less challenging. A floodplain to the west of Aylesbury, for example, posed a unique challenge which drove engineers to use as much pre-fabrication as possible. So much prefabrication, in fact, it is thought that the construction of the viaduct is a UK first.
The viaduct crosses a flood plain, which meant there is little space to construct the deck, and a lot of preparation had to be carried out before assembly of the viaduct could begin.
First, a haul road had to be constructed, as well as pads used to support cranes. The temporary road, which had to be constructed above the floodplain, also had to be built in such a way that it did not contaminate or block the flow of water.
Completion of the haul road allowed workers to assemble the bridge piers and deck relatively quickly. Assembly of the deck structure began in October 2023 and were installed by the following year. Work then focused on pouring the final layer of concrete to permanently stitch the precast deck elements together.
One of the bridge beams being lowered into place in October 2023
Each span was constructed using 25m long, 100-tonne bridge beams manufactured by PACADAR on the Isle of Grain. The beams were transported to the site by road and lifted into place using two crawler cranes. The beams were then topped with bridge deck sections, which were also manufactured off-site. It is the use of prefabrication for the bridge piers, beams and deck, which makes this a UK first.
Wendover and Small Dean Viaducts
The Thame Valley Viaduct, which is only 3m high, is a relatively low structure, which meant that it was possible to assemble the viaduct in situ using cranes. The deck for the Wendover Dean Viaduct in Buckinghamshire, however, is much further off the ground. It would therefore have been difficult to utilise cranes to move large bridge sections into position.
For this reason, the viaduct was constructed using a method known as a deck slide, otherwise known as a “push launch”. This method quite literally involves pushing (or more accurately pulling) the bridge deck out onto pre-built piers.
Using this method meant the deck could be assembled in a single location rather than having to reposition cranes as construction progressed.
Wendover Dean Viaduct structural steel following the third and final slide
The nearby Small Dean Viaduct, which is 345m long, was also constructed using the same method. In this instance, however, the deck slide was used in part to keep disruption to the busy A413 and Chiltern Mainline to a minimum. This is because both had to close for a period of time during the slide, which took place in February 2025.
The Wendover Dean Viaduct deck was pushed into position over three separate slides, with sections of the deck assembled before each launch. The 4320-tonne deck for the Small Dean Viaduct, on the other hand, was pushed into position in one go to reduce disruption to the main road and railway.
The Wendover and Small Dean Viaducts are just two of five viaducts which are being delivered using the same structural design and construction technique. They are being delivered by the EKFB Joint Venture, formed of Eiffage, Kier, Ferrovial Construction and BAM Nuttall, which is constructing the central C2 and C3 section of HS2.
Small Dean Viaduct "deck slide" January 2025
The decks have been constructed using a double composite construction method, which involves placing precast concrete panels at the top and bottom of the steel beams. The top and bottom panels are then covered with a layer of in-situ poured concrete. This produces a hollow deck which is relatively lightweight but is also incredibly strong.
Delta Junction
Delta Junction, which is being built to the east of Birmingham, will comprise a series of viaducts, forming a three-way grade-separated junction. Such is the scale of the junction and its location that several techniques are being employed to construct several viaducts across the sprawling site.
Some methods are more straightforward than others, such as that used to construct the River Cole Viaducts built on the southern section of the Junction. The two 160m long spans were constructed using steel beams, which were lifted into place onto pre-built concrete piers. Installation of the beams was completed in December 2024, after which the process of lifting precast concrete deck segments and parapets began.
Although several construction methods are being employed across Delta Junction, the scale of the junction has allowed the Balfour Beatty VINCI joint venture (BBV JV) constructing N1 and N2 section in the West Midlands to use a standard method for the vast majority of the viaduct spans.
Delta Junction, River Tame Viaduct. The Blue tower forms the mast for the "temporary deck erection system"
Nine of the viaducts will be constructed using precast concrete deck segments, much like the Colne Valley viaduct; however, the assembly method used differs somewhat.
Although the Delta junction site as a whole is enormous, the individual site locations for the viaducts are, in some cases, incredibly constrained. The Water Orton Viaduct, for example, must cross the busy M6 Toll / M42 interchange, which is 11 lanes wide. Whilst the River Tame Viaduct, which is sandwiched between the M6 Toll / M42 Interchange and sprawling Hamms Hall Distribution Park, must cross the River Tame.
The challenging conditions mean that the viaducts, whilst sharing the same segmental design, will be assembled using three distinct construction methods.
The segments for 153 individual spans are being assembled off-site at a purpose-built facility established on the outskirts of Marston in Sutton Coldfield. The Kingsbury site, as it is known, is being used to manufacture 2,742 concrete segments, each weighing between 50 and 80 tonnes.
136 out of 153 individual spans will be constructed using a cantilever method, which utilises a so-called temporary deck erecting system. During construction the system resembles a cable-stayed bridge with stay cables attached to a temporary steel tower, which is installed above one of the pre-built piers. The stay cables are attached to the previously constructed deck span, whilst further stay cables are attached to concrete segments for the next span as they are installed.
The system also consists of a swivel crane, which lifts the segment, swivels it into position and holds it in place whilst temporary steel bars are installed within the segment. Once the steel bars are installed, the stay cables are attached, after which the segment can be released from the swivel crane. The crane is then moved into position on top of the segment, which has just been installed.
River Tame Viaduct, inside the concrete box girder
The segments are transported to the crane from a holding area and across the previously assembled using an SPMT (self-propelled modular transporter).
Due to the number of spans being assembled using this method, BBV JV ordered four temporary deck erection systems designed and manufactured by BERD. Although this is not the first time that a bridge has been constructed using a cantilever method in the UK, it is the first time that this particular system has been used to construct a viaduct in this country.
Although the majority of spans will be constructed using this method, 11 spans must first be constructed using a so-called “side span” method. This utilises a temporary steel truss, which is placed between two piers and is used to support the segments during assembly. This method is used to construct each initial viaduct span, as the cantilever method requires a completed span to which stay cables are attached.
"Side span" system being lifted into position. Image credit HS2 Ltd
A further six spans will be pre-assembled and then moved into position using SPMTs. This method involves fully assembling and post-tensioning the spans close to the viaduct site. The spans will be moved into position above the M6 Toll / M42 Interchange during a series of weekend-long closures, which are due to take place in mid 2026.
The reason for using pre-assembly is to reduce disruption to a key motorway interchange. Whilst it may have been technically feasible to use the side-span or cantilever method, this would involve working above the motorway for extended periods, which is best avoided.
Despite using three different methods to assemble the 153 spans, they will all share the same structural design. Which means the deck sections will all take the form of hollow post-tensioned concrete box girders.
Curzon No.3 Viaduct
Heading towards Birmingham, a series of bridges and viaducts will bring trains into the city from Bromford Tunnel. The techniques used for some of the bridges will be somewhat similar to those described so far.
However, Curzon No. 3 Viaduct is unusual in that it is being constructed entirely in situ, rather than being assembled from pre-manufactured elements.
In situ construction first involves assembling formwork on top of a scaffold structure. Rebar (reinforcing bar) is then placed inside the formwork before concrete is poured to form the structure for the deck.
The dense scaffold structure used to support the formwork into which the concrete is poured.
This method was chosen due to the size and shape of the viaduct, which fans out from three tracks leading from Curzon No. 2 Viaduct to seven tracks which form the throat of Curzon St station.
The dense scaffold structure used to support the formwork into which the concrete is poured.
Work to build the first span appeared to progress relatively slowly, with construction of the span continuing throughout 2023 and was only completed in April 2024. However, the first span closest to the station consisted of three individual spans, which together will support the seven tracks.
The mostly complete section of Curzon No.3 Viaduct in August 2025
Given the scale of HS2, it is unsurprising that so many different construction methods are being used for the various structures. However, even though the route is being delivered by three joint ventures that have been tasked with delivering 50 significant viaducts, it is perhaps more surprising to learn that so many of the viaducts share the same design principles and broadly similar construction methods.
Despite many of the viaducts appearing to be different, they are or have been built using only a small number of different techniques. A large number of the viaducts also share the same segmental concrete design, which has been widely adopted by the three joint ventures.
No comments:
Post a Comment