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Deadline demolition

The big guns were out at Woolwich last month to get to grips with some heavy duty foundation demolition. Margo Cole reports.

Some of the world’s most powerful excavation and demolition equipment made a brief appearance on site in south east London last month to help dismantle some heavily reinforced concrete foundations ahead of construction of a massive £90M-plus retail and residential development.

The previous occupant of the site in Woolwich was a student hall of residence and an apparently unremarkable 1960s civic building.

Despite only being four storeys high, however, this government-commissioned building was built to withstand extremely heavy loads - possibly as a result of it being designed at the height of Cold War paranoia.

Small spans

“Most 1960s frames were designed with very small spans - about 6m - which is why a lot of them had to come down,” explains Patrick Hayes, a director at consultant Walsh Associates, the structural engineering consultant for the development. “This is very long span - up to 16m or 17m - so the floor slab was very deep and the columns were up to 1m square.

“It was also designed for the possibility of a six storey extension, so as a result of these loads the foundations are very large and very deep.”

“I’m used to seeing 30t or 40t excavators on site as the maximum, but here the minimum is between 45t and 75t”

Sabah Abed, Willmott Dixon

In fact, the pads measure up to 2.5m deep and 8m wide and are very heavily reinforced. “We think of 1960s building quality as being poor, but a lot of civic buildings were really well built,” says Hayes.

“The mix design is standard, but it is extremely well compacted concrete.”

Woolwich_36

Sabah Abed, project director for the development’s contractor Willmott Dixon agrees: “When we break the concrete it is hard,” he says. As a result, the usual practice of “nibbling” out the foundations with excavator-mounted hydraulic breakers was just not feasible, both for performance and programme reasons. “If I can park cost to one side, what was driving this was deadlines,” says Abed.

2012 opening date

The developer for Woolwich Central, as the scheme is known, is Spenhill, the urban regeneration arm of Tesco, and the development includes a massive 7,400m2 store that is scheduled to open in November 2012. “We are dealing with a tight schedule,” says Abed, who realised early on that some heavy artillery would be needed to break up the old foundations so that construction could start on the new buildings.

“We were here first hand to witness when the superstructure of the old building was being demolished, so we had an appreciation of what the construction was like. “I knew we had to tap into the right side of the construction industry, so we went to a specialist who has experience of that kind of work, and emphasised to them that constraints we were under when it came to the timescale.”

“Right at the outset we engaged the right people and tap into the right resources and that’s the key issue that helped us optimise “

Sabah Abed, Willmott Dixon

 

That specialist was 777 Demolition, which owns the UK’s largest demolition excavator, the Hitachi EX1200, which weighs in at a massive 120t, and has to be partly dismantled when it is transported between sites.

Woolwich_42

The tracked machine is more commonly used in conjunction with a jaw crusher to demolish building superstructures, but for the Woolwich job 777 invested in a new Atlas Copco HB 10000 hydraulic breaker - the largest ever used in Europe.

Huge drill bit

Everything about this equipment combination is big. An 18t counterweight had to be mounted on the back of the excavator; the breaker itself stands taller than a person and weighs an astounding 10.5t; and the drill bit attached to the end of it weighs 670kg.

The hard concrete of the foundations on this site caused two of these bits to break during the course of the job - at a cost of £4,500 each.

“I’m used to seeing 30t or 40t excavators on site as the maximum, but on here the minimum is between 45t and 75t, and we’ve had the 120t machine as well,” says Abed, who admits to being unable to resist having a go on the giant machine himself.

And, as well as the 10.5t breaker, 777 also brought in a 7.5t unit - the biggest previously used in the UK.

The result is that more than 10,000m3 of reinforced concrete was excavated and broken up at a rate of between 1,000m3 and 1,500m3 a day, and with less noise and vibration than using smaller breakers.

Engaging right people and resource

“What we did right at the outset was to engage the right people and tap into the right resources,” says Abed. “That is the key issue that helped us optimise.”

The contractor set up two crushers on site, and, as material was excavated and crushed, a geotechnical engineer based on the project full time, inspected it and determined that it was suitable for backfill.

The site is actually being developed in three phases, with the first phase consisting of the massive Tesco store, a further 900m2 of non-food retail, and 259 apartments, built in five blocks on top of the superstore. Willmott Dixon has until September 2013 to complete the apartments - 10 months after the store is due to open.

Later phases include a 28-storey hotel/residential tower, another 3,700m2 of shops, and up to 400 more apartments, with the whole thing scheduled for completion in 2018.

At peak Abed estimates there will be between 500 and 600 people working on the site, with 300 of them employed on the concrete frame alone.

They will be assisted by five tower cranes, ensuring that interest in the site equipment on this job does not end as the demolition machines head back to the yard.

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