An unstable gable wall and a retained façade that needed steelwork threaded into the exisiting building has forced the contractor into repeated resequencing.
Project: The Mint Building, Edinburgh
Client: Chris Stewart Group
Main contractor: ISG
Steel frame subcontractor: GR Contracts
Demolition subcontractor: Central Demolition
Start date: April 2017
Completion date: January 2019
While much of the construction industry focuses on the capital to the south, the Scottish capital of Edinburgh is also home to a large number of tower cranes, dotting the skyline in all directions.
One of those cranes is helping to put up the Mint Building on West Register Street just to the north of Haymarket station.
ISG is main contractor for client Chris Stewart Group on the site, which will see more than 61,000 sq ft of Grade A office accommodation built alongside 11,000 sq ft of restaurant and leisure units.
Part of the development will sit behind the retained façade of a Venetian Gothic building that dates back to the 1860s, originally designed by George Beattie & Sons for papermakers Cowan & Co.
This part of the site has posed the biggest challenges for the project team, with this and other unforeseen factors leading to the programme’s resequencing on more than one occasion to keep it on track.
Chris Stewart Group is developing two sites simultaneously on either side of West Register Street, which together will form a development known as The Registers.
The more northern of the two buildings at 42 St Andrew Square, which will be known as the Edinburgh Grand, will contain 50 serviced apartments with more leisure and retail space at ground level – and is being constructed by a different contractor, Thomas Johnstone, on a separate contract.
ISG McAteer West Registers The Mint 2
ISG is working on the southern side of West Register Street on the Mint Building, with access to both sites shared on the now-closed street in between, with each contractor using their own end of the road. The external roads around the site, including the busy South St Andrew Street to the west which carries trams, have had to remain open throughout the build.
The project started on site in April this year. Central Demolition was already carrying out demolition works in contract with the client, before ISG was brought in to oversee the demolition of the interior of the Venetian building as principal contractor due to its complexity.
“[Central Demolition had] also exposed asbestos in various areas, which delayed their progress,” says ISG senior contracts manager Allan Currie. “Those issues delayed what was going to be our planned start.
“There then came a point where we went into negotiations with the client, and we agreed that we would start before the demolition contractor had fully completed. That was a risk and we took a view based on how they were progressing – we’d worked with them in the past.”
“We’ve been working closely with Scottish Power and it’s proved beneficial to both sides. We’re helping them to help us”
Alan Currie, ISG
At the time of CN’s visit, most of the rest of the site’s footprint had been cleared, leaving the Venetian building in the south-east corner – apart from a small existing building on the opposite north-west corner.
“There’s a live substation here at the basement level, which services most of the city centre within 250 yards of the site,” Mr Currie says.
The critical path of the project partly hinges on this area: the team needs to demolish the substation to make way for the new structure, but cannot do so until it provides a watertight and secure room for it elsewhere on the site, inside the Venetian building.
“We’ve been working closely with Scottish Power and it’s proved beneficial to both sides,” Mr Currie says. “We’ve agreed that we’ll put the ground slab in, and then we’ll lift it [the substation equipment] in with the tower crane, right onto the slab.
“We’ll have to take some of the new steelwork [in the Venetian building] out to let it in, then [Scottish Power will] protect it and we’ll put the new roof over the top of it and concrete it. Normally they won’t even talk to you until you’ve got a watertight, dry and secure space [for them]. We’re helping Scottish Power to help us.”
It’s the Venetian building, however, that has provided the stiffest challenge for the team.
ISG McAteer West Registers The Mint 7
Here, ISG is retaining the façade of the 150-year-old Grade B-listed building on three sides – the fourth side was hidden from view initially, as it butted against the other existing building on the site.
“We couldn’t see it at the time of us pricing the job,” Mr Currie says. “And as the demolition progressed, we thought ‘uh-oh’ – this isn’t as straightforward as we had hoped. The existing gable was not very stable, as there had been lots of break-ins and alterations which made it structurally unsafe once the other building came away from it.
“This was the single biggest challenge. Once we opened the building and realised what was really there, [we had] to find a solution that would still allow us to deliver the job.”
All of this was complicated further by the façade retention. As the external roads could not be compromised, all of this had to be done internally, from inside the building’s existing footprint.
“The only way we could do that was to actually install the new steel frame down through the existing structure,” Mr Currie says.
“Once we opened the Venetian building and realised what was really there, [we had] to find a solution that would still allow us to deliver the job”
Alan Currie, ISG
“We went into the basement first and put the new foundations in there, working in very confined spaces, and then we knocked holes through the floor all the way up through the building to let us drop the new steel frame down inside, threading it down through the building. We couldn’t underpin – we just had to stabilise the building by putting a frame inside it.”
The instability of the gable wall also posed a problem for the new foundations.
“It came out in the wash that, because of the risk associated with the gable wall, we couldn’t put in new foundations within 2-3 m of it until it was away – but we still needed the steel frame up,” Mr Currie says.
ISG’s solution was to install sacrificial foundations at the existing base level, allowing the contractor to put in a new column which reaches up five storeys, right up through the building. Then, 8-10 weeks later, the team will support the existing beams around the columns at ground-floor level, excavate down and take the temporary foundations out.
“We’ll then put a new foundation in below it, and splice an extension of the column at the base level,” Mr Currie says.
“The key for us to maintain progress on the main works is to get the steelwork up and get the elevations moving, then we can deal with the basement and the lower-level stuff, and basically run them in parallel for as long as it takes.”
A different kettle of fish
The building also contained a number of iron columns that date back to its original construction, estimated to weigh 850-1,000 kg each, topped by 300 x 250 mm timber beams. Remarkably, these all sit on top of each other with no fastening, with the iron columns dropped vertically on top of each other, in perfect alignment.
ISG McAteer West Registers The Mint 12
“When you look at the stone and what else they put in 150 years ago, without a machine – it’s fantastic. So much of it just sat on top of each other, because it’s in exactly the right place,” Mr Currie says.
There were also “massive” existing foundations under the site, so ISG adapted the basic foundation design with its structural engineers to allow it to re-use some of that existing concrete, including using some of it in place of temporary retention that might have been needed as it digs a new basement.
Mr Currie says the team was surprised to find a relatively large volume of concrete in the ground. “There’s been about 65 cu m that we never expected to find – you’d think it was a new invention at the time,” he says.
The project is progressing rapidly now, with a finish date of January 2019 that is “sancrosanct”, according to Mr Currie. The team completed the structural steel frame inside the Venetian building in October, after CN’s visit, and will reinstate its mansard roof along the same building lines that were there before.
It’s the continual surprises that have made this job a challenge for ISG. “The key date for us was to [hit] the new structural steel frame [installation], and as long as we did that, by the middle of October, that’s the critical path,” Mr Currie says.
“Once we exposed the gable and realised the risk associated with it, though, it was a whole different kettle of fish.
“We’ve had to think on our feet.”