The Viadux

The monumental challenge of building a 40-story high-rise through the Castlefield viaduct

The Viadux project is a ground-breaking mixed-use scheme taking place in the Deansgate district of Manchester City Centre. Delivered for client Salboy, the £300m development will provide premium apartments and stunning office space above the historically significant Castlefield viaduct.

The viadux – project specifics

In the phase one build, a 40-storey skyscraper will provide over 370 one and two-bedroom luxury apartments with panoramic city views. Standing proudly above Castlefield Locks, the apartment block will provide a unique opportunity for those looking to take advantage of a prime city-centre location.

The second phase of the project will deliver 270,000 sq ft of modern office space above the Castlefield Viaduct. This phase of work will also encapsulate the nearby tramline to protect it during construction, becoming a permanent works when the project is complete.

The challenge

The Grade II listed Castlefield Viaduct has been a permanent fixture in the centre of Manchester since its creation in 1892. Although the railway hasn’t been in use since 1969, it forms an important part of the city’s history and is emblematic of its industrial heritage.

With that in mind, any development that looks to encompass the viaduct needs to do so while preserving and protecting it. The plans by Salboy are elegant in their treatment of the historic location and will provide a beautifully modern mixed-use scheme that highlights the period features while pushing the envelope in modern construction methods.

Our role as engineers has been to navigate the complexities of such a scheme, ensuring that the project is accomplished safely, efficiently and with minimum effect on the existing viaduct structure. This has required much careful consideration and all of our engineering prowess and know-how.

Many site constraints go hand in hand with working on such a project. The challenges that needed solving included:

  • Finding the best option for the piling within the confines of the site
  • Managing the extra load on the ground that comes from building a skyscraper without a basement
  • The process of punching through the listed arches without jeopardising their integrity
  • Working within the confines of a tight inner-city site and close proximity to a nearby substation
  • Working through an active service yard for Manchester Central that needs to remain operational for the duration of the project

A solution to the piling challenge

To begin with, we needed to tackle the piling issue and find the optimum solution. We worked with a Geotechnical specialist to undertake the site analysis, examining how both the temporary condition and the finished building would affect the integrity of the existing viaduct structure.

Whatever solution we arrived at, we needed one that allowed for a piling rig that would fit within the confined arches of the viaduct. Ultimately, this meant the diameter of the pile was limited to 600mm given the limitation on the size of rig we could get through. With these new limitations outlined, we had to establish what load each pile could take.

We established the load capability of the piling by working with piling contractor Van Elle. Owing to their experience on the nearby Tower of Light, Van Elle had pre-existing knowledge of the ground conditions and along with load testing the 600mm piles, we established that this piling option was deliverable.

A solution to the piling temperature challenge

Because we were not in a position to build a basement for the tower (a standard industry practice for alleviating the load on the underlying strata), the loads applied to the ground in real terms were actually at a magnitude of sixty storeys rather than the actual tower size of forty.

To work around this, the pile caps needed to be of a suitable depth, going to 3.6m instead of 2.6m. The challenge with this is when you install the pile cap so deep, you’ve got to control the temperature differential to manage the curing of the concrete. Where you have hot concrete meeting a cooler outer temperature, you run the risk of the concrete cracking and potentially cracking in a place where you can’t see it.

A workaround was required. Thermal couplers placed within the pile cap allowed us to track temperatures in real-time, helping us ensure the temperature differential never reached an unsafe level . We also used a layer of insulating sand which acted as a thermal buffer, preventing the pile cap from cooling too quickly and inducing cracking.

Another simple solution we employed to prevent extreme temperature differentials was the use of ground granulated blast furnace slag (GGBS). Essentially a bi-product of quenching molten iron slag, which when added to concrete, facilitates strength gain with the upshot being that you need less cement. Less cement means less heat, reducing the chance of unsafe temperature differentials and reducing embodied carbon output.

These rather low-tech solutions prove that the most efficient solutions are often the simplest – a tenet that guides much of the work we do at renaissance.

A solution to punching through the viaduct arch

Essentially, we were constrained by how much masonry we could take out of the arches to avoid any movement or potential damage. The preservation of the listed asset was of the utmost importance throughout the project. When you combine that with the fact that we were working within the confines of the Manchester central service yard, the need for a compact solution was apparent.

Drawing on historical precedents, we developed a strategy of using ‘ring beams’ or ‘picture frames’, generating openings through the arch structures for the columns and the building core. Along with some well-thought-out temporary works, this strategy allowed us to penetrate through the arches safely and without disturbing the operations of the service yard above.

In particular, the tower columns that were punching through the viaduct could in no way support or even touch the historic masonry arch. Complete structural independence was required.

By adopting super-stiff ‘picture frames’ or ‘ring beams’, the load in the arches can pass around the holes formed for the mega columns. In this way, the columns can perform independently without impacting the existing brickwork. In other words, the heritage viaduct structure is completely unaffected by the load of the skyscraper above.

Essentially, the load of the building is funnelled via bifurcating (y-shaped) columns into a 11mx6.8m column grid. The benefit of this is that we only needed to punch through the historically significant arches 11 times as opposed to the 22 times we would need to with a straight up and down column structure.

With the work to punch through the listed arches and the transfer slab formed, the complexity of the project is much reduced and the rest of the build can be achieved much more easily over the coming months.

The client’s view

“The Viadux development is pushing boundaries in its approach to building within historic sites of cultural significance. It’s always been a top priority that the heritage of the Castlefield viaduct be preserved throughout the project and Renaissance has enabled us to do that with their innovative approach.

Their extensive expertise and solution focussed work have delivered the results we need to ensure the Viadux takes its place as one of Manchester’s most iconic new buildings.”  Nick Russell – Salboy

Client: Salboy and Domis
Location: Castlefield, Manchester
Value: £300 million