In the grand scheme of things, a pair of 1970s viaducts are hardly the oldest examples of infrastructure in a city like Glasgow, Scotland.

After all, Scotland boasts the world鈥檚 third-oldest underground metro after London and Budapest respectively (more on that later).

But after decades of carrying an ever-growing volume of traffic that now stands at an estimated 150,000 vehicles a day, the viaducts between junctions 16 and 17 of the M8 motorway in the centre of the city are in urgent need of attention.

In fact, intrusive inspections carried out in 2020 and 2021 revealed that the viaducts鈥� crossheads were in a much worse state than anticipated and led to traffic restrictions including lane closures.

It rapidly became clear that the viaducts required temporary propping. The eastbound viaduct is around 360 metres long with 12 spans, while the westbound viaduct is a little longer at almost 15 spans, explains Andrew Dickson, project director at infrastructure support services company Amey.

Illustration of the M8 viaduct. The lower nibs of the half joints require repair. The propping works support the suspended bridge deck and take the load of the deck and traffic off the half joints (Image: Amey)

Amey has been tasked with the temporary propping works, costing an estimated 拢126 million - 拢152 million (鈧�151 million - 鈧�182 million). The main aim of those works, which started in 2021, is to get all four lanes of the motorway across the viaducts running again 鈥� currently, lanes one and four are closed.

That involves installing temporary props to support the suspended spans of the bridge deck and the traffic passing along them so that the half joints in the original structure are no longer carrying a load from the spans, ahead of their repair.

鈥淭here鈥檚 a total of 25 existing supports between the viaducts, of which 23 have been identified as having some level of deterioration,鈥� Dickson explains. 鈥淭he investigation focused on the condition of the half-joints supporting the main spans and found some fairly severe spalling and heavy corrosion of some of the existing reinforcement within those parts of the structure.鈥�

Layer upon layer of infrastructure

Time was of the essence, then. But that鈥檚 where things got really complicated. That鈥檚 because the viaducts straddle a mass of other infrastructure, including a 19th century tunnel for the metro system, 36-inch Victorian water mains, critical pedestrian routes, oil-filled cables close to existing piers, major gas infrastructure, and surface streets that had to be closed.

That鈥檚 not to mention the fact that the viaducts carry the M8 itself, which is one of the busiest pieces of motorway in the UK. The viaducts can only be closed for short periods overnight, placing another significant constraint on the works.

鈥淓ach of those constraints in their own right aren鈥檛 unusual but the sheer volume of them all being concentrated within the location of this particular size is what creates the magnification of the challenge,鈥� says Dickson.

鈥淭he subway tunnel encroaches into what would ordinarily be an exclusion zone from any works being carried out, which led to a very careful process with Strathclyde Partnership for Transport (SPT) doing trials of much of the disruptive work like piling and proving vibration levels etc.鈥�

Image courtesy of Amey

To make matters even more complicated, hardly any of two of the viaduct supports are the same. 鈥淎ll 23 locations have been unique in their own right, based on different geometries of the foundations, different heights, and different constraints. At the outset, looking at it from ground level and in the absence of the investigative information, it may look straightforward and repetitive but it hasn鈥檛 been,鈥� he explains. Some of the as-built information dates back 50-60 years and hasn鈥檛 always proved accurate, just to add to the challenge.

Meanwhile, a substantial amount of work has been required to build new foundations or strengthen existing ones to allow steelwork to cantilever off it, although in a small number of locations, the existing foundations were robust enough not to require strengthening.

鈥淭his was primarily so that the jacking axis could be moved further out from the crossheads themselves so we could pick up the ends of the beams without interfering with the crosshead. That meant in future there wouldn鈥檛 be any restrictions around carrying out repair works to the deteriorated areas,鈥� says Dickson.

Amey started with some of the less complex locations first to get designs turned out and get groundworks happening as soon as possible to progress the works.

So far, propping has been installed to 13 of the 23 locations required and 10 of those are load bearing, with the load transferred from the existing crossheads onto the temporary works.

鈥淭he design work is largely complete and we are a little over halfway with the construction aspects of it,鈥� says Dickson.

Piling work involves going 12 metres below ground and so far Dickson estimates that there has been a total of about 10km worth of piling, along with 2,500 tonnes of steel and 4,500 cubic metres of concrete.

Innovative techniques

Amey has employed several innovative technologies on the project, including sensors on both the existing structure and the temporary propping to monitor for any movement or settlement.

The company is also considering the limited use of carbon fibre reinforced polymer (CFRP) as a means of strengthening the structure. Currently at the design stage, the intention is to use it to reinforce some of the crossheads that have been judged to be less able to bear loads to enable the reopening of all four lanes of the motorway without the need for a fully developed long-term solution.

That said, much of the rest of the work has been fairly traditional, Dickson adds: sheet pile excavations, cofferdams, and king post walls.

Image courtesy of Amey

Amey and its partners also had to give a lot of thought to how to protect vulnerable services around the viaducts, some of which lie close to ground level, from plant and machinery.

鈥淎ny damage to them would have been fairly catastrophic so we bespoke-designed a service protection mat across the whole working area,鈥� Dickson explains.

It has been a belt-and-braces approach to cushioning the ground and the services within it to the impact of machines. Instead of just a track mat, Amey opted for a dense polystyrene layer bedded in sand and then topped with an abrasion-resistant track mat.

鈥淚t was designed with all of those components in order to reduce the point loading sufficiently that we could be very confident that that there was going to be no risk to the services below ground,鈥� he adds.

Lessons from an unusual project

The complexity of the work, combined with the urgency with which it has had to be introduced, has made it an unusual project for both Dickson and M8 bridges special projects manager for Transport Scotland, Iain Ferguson.

鈥淭his is quite a unique project for Transport Scotland,鈥� says Ferguson. 鈥淎 major project like this would normally involve a traditional procurement process where you get all of your design done up front, you tender for the project, and it would go to a contractor fully developed.

鈥淏ut this approach is teaching us within the bridge team to use a more agile way of working where we鈥檙e developing the design at the same time as getting a contractor working on site. This has been a real change of mindset for us. And that鈥檚 all to do with the urgency of the project.鈥�

The M8 Woodside Viaducts in Glasgow (Image: Transport Scotland)

Dickson recalls how the start of the project involved getting multiple plates spinning. He adds, 鈥淓verything was running in parallel 鈥� we had site investigations going on, design commencing, construction commencing. We have certainly identified some more challenges than we would ordinarily expect.鈥�

That extends to having multiple interfaces between the different designers and contractors 鈥� at one point, there were in excess of 100 designers working on the project, he points out.

鈥淎s a design consultancy organisation in our own right, Amey has the capability to perform all of these roles but we didn鈥檛 have that scale of resource sitting on the shelf to be deployed because of the way this project evolved. To mobilise that level of resource and expertise quickly and effectively inevitably involved spreading that across multiple organisations.鈥�

How long the temporary propping remains in place before a long-term solution for the viaducts is designed by a separate team of technical advisors remains to be seen. The propping system, which should be complete by 2026, has a design life of 25 years but Ferguson is quick to stress that there鈥檚 no intention for it to be in use for anything like that long.

A critical moment for Germany鈥檚 bridges As Europe鈥檚 largest economy, Germany is renowned for having first-class infrastructure but even in one of the region鈥檚 wealthiest countries, the bridges are crumbling