Engineering one of Manchester’s most ambitious SKYSCRAPERS
It seems like we’ve been talking about the Viadux project for years… and that’s because we have! With construction starting in 2021, the project has now reached practical completion and is fully occupied.
From start to finish, the Viadux project has been ambitious to its core. Not only is the development cutting edge in its facilities and quality but it’s also built in a historically sensitive area and located on an incredibly tight site. These elements added a level of complexity unique to the project and one that required the most considered of engineering approaches.
With much of the work now done on phase one and phase 2B underway, we wanted to write a retrospective of some of the major engineering achievements of the project so far, outlining some of the unique approaches taken to achieve such a ground-breaking project.
Let’s get stuck in!
Building through the Victorian viaduct structure
We should be start by getting the obvious out of the way.
One of the biggest challenges of the project was always going to integrating the tower with the heritage viaduct. With a storied history intimately linked with the city’s industrial past, the viaduct structure is to be preserved and respected, forming an important part of Manchester’s DNA.
Whatever engineering intervention we carried out had to be utterly deferential to the structure of the viaduct and as noninvasive as possible. How can that be achieved while building a skyscraper through this historic asset?
The question is, how can you build a skyscraper around/through a building that was designed to support railway traffic?
Supporting the tower independently of the viaduct arches.
The foundation of our whole approach to the Viadux project is the table-like structure we designed. Eleven “table legs” were built on foundations which in turn gently punched through the viaduct structure until, on the other side, a reinforced flat “table surface” could be constructed that distributed the building’s weight. It was on this table-like structure that the tower would be built and supported. Ultimately, the existing viaduct takes no load-bearing role.
As the columns or “table legs” continue through the viaduct structure, they bifurcate as they rise up[ the transfer structure. You’ll notice the column profiles are reflective of the magnitude of load they support, only providing material exactly where its needed.
Punching through the viaduct structure
When it came to the 11 columns gently punching through the viaduct structure, great care was warranted. From a heritage aspect, it was perhaps one of the most important elements to get right. While the design was engineered to be minimally invasive, you couldn’t get around the fact that some level of intervention was inevitable to enable the development.
For the columns to pass through the viaduc, it had to be achieved in such a way that no load was transferred to the viaduct itself or for the structural integrity of the arches to be affected in any way.
We achieved this with the design of “picture frames” through which each column would pass. This method required casting in-situ reinforced concrete ring-beams around openings made in the arches. These ring-beams were designed to distribute the loads and limit the movement of the arches to less than 5mm, ensuring their integrity is protected throughout the construction process.
There was a lot to consider to ensure this was done right:
- Assessment and planning – We first conducted a thorough analysis of the arches to determine the optimal locations for the openings, ensuring minimal impact on the structure.
- Precise cutting – Openings were carefully cut into the brickwork of the arches, with precision being paramount to avoid unnecessary damage to historic fabric.
- Reinforced concrete casting – Reinforced concrete was cast in situ around these openings, enveloping the temporary supports and creating rigid ring-beams.
- Load redistribution – These ring-beams redistributed the load evenly around the openings, maintaining their status quo and preventing any concentration of stress that could lead to cracking or movement in the arches.
- Movement limitation – Throughout the build process, high-precision instrumentation monitored the movement of the arches. The goal was to keep any displacement to under 5mm, both ensuring that the structural integrity of the arches remained intact.
- Temporary supports –Temporary supports known as Pymford Stools were inserted into the openings, which stabilised the arches during the formation of the openings.
Laying the foundations
As well as working sensitively with the heritage aspects of the viaduct arches, the constrained space of the project meant we had to think intelligently about the design and its buildability. The first and most obvious place this manifested itself was the piling of the foundations.
Typically, for a tower of this scale, it would be beneficial to introduce a basement in order to distribute loads evenly across the building footprint. On Viadux, this wasn’t possible due to the arches and exacerbated further by having concentrated loads from the bifurcating columns. Therefor, the loads applied to the ground were at a magnitude of 60 storeys rather than the actual tower of 40 storeys high.
Additionally, we were limited on the size of the piling rig we could fit within the Viadux arches space. The biggest piling rig we were able to get into the setting would allow a maximum pile diameter of 600mm.
Working with piling contractor Van Elle, we established a workable solution for the 600mm piles, and subsequently undertook a detailed assessment of the soil-structure interaction with geotechnical consultants — A-squared — to limit the settlement of the tower throughout construction and equally ensure that the existing arch foundations were not affected.
Managing deeper-than-usual pile caps
One of the challenges to overcome here given the depth of excavation and foundation, is you have to control the temperature differential during the curing of the concrete, otherwise, you start to run the risk of the concrete cracking and potentially cracking in a place where you can’t see it.
To overcome this, we suggested thermal couplers be placed with the pile cap to allow us to track temperatures in real-time, and ensure the temperature differential never reached critical levels. In the end, we opted for a more low tech solution to control heat loss — placing a sand blanket on top of the foundation, preventing the cap from cooling too quickly.
What’s more, we also used GGBS (ground granulated blast furnace slag) in the concrete mix to reduce our dependence on cement. Less concrete meant less heat and a smaller temperature differential. The mix also had a longer curing period.
Working within an active service yard
On top of the viaduct structure sits Manchester Central’s, busy service yard, adding further constraints to the design construction sequencing. Our project partners Domis and Mayo Civils programmed their works around the activities of the service yard, ensuring all onsite works kept interferences to a minimum.
This included the temporary works installations required to temporarily support and transfer the weight of the wet concrete onto the existing viaduct masonry piers until the concrete had cured enough to be self supporting.
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Every aspect of Viadux Phase One presents an interesting engineering challenge, it’s difficult to hone it down to a shortlist. It has everything: an intimate relationship with a heritage structure, close proximity to third party assets like the Metrolink and Manchester Central service yard, and a large skyscraper structure that has taken its place amongst some of the most notable in Manchester’s burgeoning skyline.
With phase two on the horizon, the work we did on phase one lights the way forward for its success.
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Got a groundbreaking project that needs a considered engineering approach? We’d love to hear what you’ve got going on. Contact us today to talk it through.
