Building a skyscraper with minimal working at height
24/2/20
Wates set out to eliminate the need for scaffolding or mast climbers on the 33-storey Anthology Hale in north London – and saved on costs by doing so.
Commuters or football fans travelling to Tottenham Hale underground station on London’s Victoria line are unlikely to be thinking too much about what lies above their heads as their train pulls in. They are unlikely to have any idea that above the tunnel leading into the station are the bases of two tower cranes whose presence leads to regular assessments by engineers to ensure their loading is not causing track movement.
Indeed, such is the lack of space around Wates’s 33-storey mixed-use development, Anthology Hale, that a range of innovative solutions – from engineering to technological and logistical – have had to be introduced.
The £83m development, which will feature 279 apartments and three retail units on the ground floor, is surrounded by other buildings, a busy road bridge and streets thronging with pedestrians, with London Underground assets running underneath. From the outset, the project has required a huge amount of planning, with Wates’s team having to rent the basement car park from affordable-housing charity the Newlon Group – based next door – to set up offices because no other space was available.
That was a small step, but a mark of the logistical demands that lay ahead. Installing and securing two tower cranes was a different challenge altogether.
The cranes needed to be situated on one side of the building to enable deliveries into the site’s pit lane. However, the side on which they needed to be placed is over the underground line. The weight of the two cranes on the tunnels could have prevented the work from going ahead, but the team came up with a solution: to cantilever the cranes off the main tower base, meaning no load is applied to the ground over the tunnel.
“We’re trying to eliminate working from height. You can’t do that [completely] when you’re building high-rise, but we’ve limited it" Richard Tisdell, Wates
Project director Richard Tisdell explains: “We had to get approval from London Underground because we're sitting on top of their tunnel and we have to do monitoring every three months.
"So far there’s been minimal movement in the tunnel so the strategy’s worked. At any time, we can send engineers down to see if there’s been any movement.”
The monitoring establishes whether there has been any horizontal or vertical movement to the rails below, with third-party engineers checking Wates’s assessments on Transport for London’s behalf.
“Because they’re cantilevered, we can't build them to their full height,” he says, explaining that this would overload the tower’s base. “So we put them up to a certain height and then climb them every so often. As we do that, we put ties into the building’s main tower,” he adds.
The BIM modelling behind the project enables the team to work out the most effective sequencing. This identifies spaces where windows will later be fitted for the crane ties that can then go back to the stanchions. By doing this, the process doesn’t overly interfere with the cladding work on the building. “We've tried to make sure that as little as possible of the temporary works disrupt the permanent works,” he says.
The proximity to rail lines also meant continuous flight auger piling was used to minimise vibrations on the underground network, despite it being a little more challenging than alternatives in saturated ground conditions that also necessitated the installation of a cofferdam to protect basement works from water ingress. There is a 5-metre exclusion zone around the tunnel and the closest pile is about 1 metre outside the zone.
The pace of the job has also meant that getting the planning right has been fundamental to the project – which is due for completion within just 130 weeks of its October 2018 start date. The bottom 10 floors are due to be handed over even earlier, on November 30 this year, with fit-out and finishing on the floors above continuing afterwards.
The core and concrete frame is being delivered by a jump-form method. Installation of shear walls follow two floors below, after which floor plates go in. The method, the project director says, enables the build to rise at the rate of about one storey per week. When Construction News visited the project in late January, the core had reached two-thirds of its eventual height, at level 22.
Anthology Hale is set to be the largest building in an area that is undergoing a construction boom. In the immediate vicinity, several residential buildings are under construction in a place dubbed Hale Village, while Tottenham Hale station is also being redeveloped. The project is less than two miles from the major Meridian Water development in Enfield. One of the selling points, from a real estate perspective, is the view from the apartments, with the skyline of the City of London, Docklands and Stratford all visible, as is one of the nearest landmarks: the £1bn Tottenham Hotspur Stadium, a reminder of the start of the area’s recent growth.
Despite the building being 33 storeys high, working at height on the project is being avoided wherever possible. There are no scaffold towers or mast climbers in use currently; instead, innovative alternatives are getting a look in.
“Pretty much everything we’ve done on this job has been pre-planned and has worked” Richard Tisdell, Wates
“We’re trying to eliminate working from height,” Tisdell says. “You can’t do that [completely] when you’re building high-rise, but what we’ve limited it to is the concrete-frame guys, and the next operation [in the sequence] is putting the cladding on, which is quite unusual. Normally work [continues] on the floor plates while you’re waiting for things to happen.
“There are people walking close to the site and one of the things we were desperate to avoid was dropping things. Under other circumstances, where you’ve got more room, you can create exclusion zones, but we don't have the ability to do that because of where we are – we've got people at ground level in close proximity.”
This ambition has led to the purchase from Spain of two mobile jibs for the project, each with an articulated arm. These are lifted in on a loading platform, rolled into position and put on a fixed running rail to guide them and prevent them from being driven off the building. Telescopically extended, they lift panels of unitised cladding into position two floors down. As this takes place, an operator stands next to the jib, communicating with people two floors below, who are making sure the panels go in correctly (see video below).
Tisdell says: “It’s eliminated the need for any scaffolding or mast climbers and the need for anyone to go outside the building line, whether it’s in baskets or cantilevering bits of scaffolding out there.”
The equipment is not cheap, though. Each machine, made by manufacturer SkyGondola, costs nearly £100,000. “But the time they’ve saved is significant and the alternative would have been a lot more expensive, so it’s cost effective,” he adds.
Tisdell says that Brexit-related uncertainty has required contingency planning for supplies, such as stockpiling of cladding (see box below). But he adds that bathroom pods are also often hard to obtain or are subject to delays, so he took a similar approach in ordering early and building a stockpile. The pods are manufactured offsite in Somerset and brought into the building just before the last cladding panel is installed on each floor.
“The success of this job has been meticulous planning,” he reflects, adding that the work has benefited from the ability to engage with the client early. “Pretty much everything we’ve done on this job has been pre-planned and has worked.”
He points out that while not everything always goes fully as expected, contingency planning has been key. Tisdell says: “We’ve always thought about backup plans; we’ve taken the time to do that.”
Source: Construction News