Flanked by listed buildings, pedestrianised streets,and with limited access to bring in heavy plant and cranes, ISG engineers were tasked with taking down a former flagship department store on Edinburgh’s Princes Street.
The former British Home Stores Edinburgh flagship was a mammoth building on the city’s main shopping thoroughfare, Princes Street.
Constructed for the now-online high street name in 1965, the building was one of the city’s first, purpose-built department stores.
It was massively over-engineered, with 1.7m thick reinforced concrete columns, and huge concrete floor slabs 500mm thick.
Following plans drawn up by architect CDA Group, contractor ISG is demolishing much of the existing property and building a new hotel and retail complex, with much of the iconic Princes Street façade retained.
Taking down such a mass of concrete and steel would pose a challenge under any circumstances.
The site of the £20.5M scheme, owned by client LaSalle Investment Management, is formed of two buildings. The main retail space stands on Princes Street. The second building, which sits behind the main building, was used to hold stock for the store and faces out onto Rose Street – which runs parallel to Princes Street. The buildings are linked by a lower ground floor and basement level, but at ground level they are split by a goods access lane, called Rose Street South Lane.
Site workers are demolishing the existing Rose Street property and erecting a new hybrid concrete and steel frame building. The lower section up to the first floor is a reinforced concrete structure and composite decks will be used for the second to fourth floors.
An extra storey will also be added to the Princes Street building, to be physically linked to the Rose Street site via a new bridge across Rose Street South Lane. The basement levels are being retained, but stripped out and reinforced to support the weight of the new storeys above. The new building will be used as a hotel and retail complex.
The structure supports Rose Street, which has a footfall running into the hundreds of thousands each day. This meant engineers from main contractor ISG had to think “outside the basement” – its words – to complete the job.
The site location posed the first challenge, ISG senior project manager Alistair Lyon tells New Civil Engineer. Princes Street at the front of the site was completely off-limits, due to its high volume of foot, vehicle and tram traffic. Rose Street to the rear is another popular retail street that is pedestrianised for most of the day – so all site traffic had to be meticulously managed through the goods access lane, which could only be accessed from Rose Street outside its pedestrianised times.
The city also imposes two construction embargo periods during the Christmas and Edinburgh Fringe seasons.
“Without a doubt the single hardest thing with any project of this nature is the logistics aspect,” Lyon says.
“The challenge was working in proximity to the adjacent buildings. We are hemmed in on all four sides, and the key thing here is knowing before you go in what the core complexities are and developing a robust plan during the tendering stage.”
“That’s where the relationships ISG has built with the city come into play. We could get plans organised ahead of time so there were no surprises when we eventually got onto site.”
Demolishing the existing Rose Street property presented the engineers with their next challenge. The massive beams and concrete ground floor and lower ground floor slabs of the building were providing the lateral restraint for the basement retaining wall on the Rose Street side of the building.
It also supported Rose Street South Lane, between the two buildings. This meant that as demolition progressed, a second team of engineers was hard at work installing huge amounts of propping to keep both thoroughfares stable.
The propping of the Rose Street retaining wall transfers the load from the retaining walls through the lower ground floor slab, down to a system of vertical propping in the basement. Rose Street South Lane was shored up with vertical props running through both floors.
Lower ground floor of Rose Street, showing propping crop
Ensuring the street and goods access lane remained stable was vital as both have to accommodate heavy goods vehicles.
In total, ISG engineers have installed 94t of propping on the site. Elsewhere, steel removed from the buildings was repurposed into additional temporary works on site.
Lyon says that ISG had installed a “forest” of vertical propping under the Rose Street South Lane, as it was the only way in for the heavy plant and cranes needed for the job.
“We had to put a significant amount of propping in just to allow a 13t excavator to sit on the concrete slab on the goods access lane,” Lyon says. The excavator was being used for demolition work.
Forest of props
“There is a forest of props down there, I liken it to Sherwood forest with all the vertical shoring. We used over 144 props just to hold up the 13-tonner.”
Access to the site via the Rose Street South Lane goods access lane is further hindered by a 90° corner, restricting the length of the largest vehicle that could safely get to the site to 10.5m, limiting the size of heavy plant that could be brought in for the demolition phase.
“In any other, more open situation, the building could have been brought down with a 40t excavator or similar with a breaker or a muncher, and tippers to take away the rubble and the building would be down in four weeks give or take, whereas we didn’t have any of that,” Lyon explained.
Some of the concrete slabs ISG had to break down were over 500mm thick. This presented a challenge as the robot breaker jaws were unable to fit round the concrete.
Instead the concrete had to be manually broken down with the breakers finishing the job.
The massive 1.7m by 500mm beams in the Rose Street building were taken down with core drilling and hydraulic bursting, keeping dust and noise to a minimum because of the proximity to other retail, residential and office buildings.
“Respect for others is paramount in a job like this, hence the crunching, the core drilling and hydraulic bursting and the dust suppression. It’s all about quiet completion of the job,” Lyon says.
The challenges did not end with the Rose Street building. ISG is also tasked with redeveloping the top floors of the Princes Street structure, and adding a unique feature – a huge 10m by 15m light well running through the core of the structure – with the goal being to give natural light to all 137 hotel rooms no matter where they are located in the building.
Partial demolition of concrete slabs on the Princes Street building before the additional storeys were added presented another challenge.
The key thing here is knowing before you go in what the core complexities are
The existing gable walls required additional support during the demolition of the roof structure, as the wall heads were not tied back. ISG engineers had the ingenious idea to repurpose steel removed from the buildings during demolition into bracing for the gable walls, saving the need to bring in more heavy propping equipment to the top floor of the building.
With the demolition phases of the project complete, the next stage is to start construction of the new Princes Street building extension and the frame for the new Rose Street building, which will include 400t of new steel and 1,300m3 of concrete. With those works on the horizon,
ISG had to find a way to get cranes on site.
Lyon says that the preferred method would have been to put in a tower crane, but with such a confined site, the question was how to assemble it.
Rose Street South Lane was unsuitable for a tower crane because the tight 90° corner prevented ISG from bringing in a mobile crane to assemble one there.
Alternative locations were ruled out because of doubts about the integrity of basement slabs and retaining walls.
“We thought about a 200t tower crane on Rose Street, but there were too many retaining walls that no-one could vouch for there,” says Lyon. “Another option was a 500t crane in Hanover street, which is at a right angle to the site – but there we had vaulted basements under the road that again no one could vouch for.”
The ISG team eventually opted for a 60t crawler crane that it could get through narrow Rose Street Lane South, although this meant further propping was needed under the lane in addition to the 144 props already in place to carry the weight of the crane as it entered the site.
Extra propping for crawler crane
“The crawler crane took 54 additional props, for each of the crane’s tracks, per floor to hold it up,” says Lyon.
“There are three bays the crane needs to sit in for the project, meaning over 300 extra props in total had to be installed to hold the weight of the crawler crane alone.”
But even with a full fly jib the crawler crane cannot reach the front of the Princes Street building, meaning Lyon’s team is using additional spider cranes for tandem lifts.
“Because we couldn’t get the tower cranes in, the crawler lifts the steels on to the roof and then we are using spider cranes to assemble the steel,” says Lyon.
“We are actually having to do several tandem lifts with both the spider and crawler cranes to get these beams up and in, it’s quite a challenge.”
Positive working relationships
However, it is by working in such challenging conditions that you foster positive working relationships, that are vital to the success of the project, explains Lyon.
“Through these city centre jobs we have built a really good working relationship with City of Edinburgh Council as well as its mental health department, so that in advance of these kinds of jobs we are contacting them telling them what is going to be happening, or explaining that sometimes there is no other way around noisy jobs, like getting concrete out from around rebar
on the columns of the Rose Street building without using a noisy hydraulic breaker, even bringing them on to site to show them what we are doing and how we are doing it.”
“I would liken what I do to conducting a symphony orchestra. It’s all about communication and coordination to overcome the complexities of a job like this. It has been a cracking challenge, and really interesting work.”
ISG started work on the site a year ago and is due to deliver the project in July 2020.
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