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Case Study - Carnegie Swimming Baths

Gnat UK’s BROKK team in at the deep end

A trio of versatile Brokk robotic concrete-breaking machines has successfully demolished a one hundred year old Scottish swimming pool quicker, cheaper and safer than any other possible method. David Hayward reports from Dunfermline.

Overcoming difficult access, confined working spaces, and the need to demolish a swimming pool’s concrete sides - located just millimetres away from listed fragile cast iron roof columns - headed the list of challenges faced by specialist demolition company Gnat UK during refurbishment of a leisure centre in Dunfermline.

The solution was to use three of the contractor’s electrically powered Brokk robots - including its top of the range Brokk 330 - to 'delicately' remove the structure at a combined rate of up to 22 m3 per 8 hour shift. "While demolishing the 1.3m thick pool walls, we had to carefully remove concrete less than 100mm away from the original columns" says Gnat Managing Director Nick Turnbull. "If we had touched or even stressed any of the columns there could have been serious structural damage."

Opened in 1905, Carnegie Leisure Centre has long been a major local amenity for the residents of Dunfermline in Fife. The current £17million upgrade, for owner Fife Council, includes a rebuilt gymnasium and entrance area, plus a new café and 'health village'. But the main focus of the two year project is the original and largest of the three pools which is being totally replaced.

The 23m long rectangular pool, surrounded by a suspended 2.4m wide concrete walkway, is enclosed beneath its own glazed, arch-shaped steel roof supported on 14 ornate cast iron columns. These 180mm diameter columns run down, close to the pool side, and through the suspended walkway to found in footings beneath. The old pool was starting to leak and its replacement is a full 2 metres longer, creating an Olympic sized facility. The original roof and columns are central to the building's Grade B listed status so could not be damaged or even touched during the refurbishment contract.

Main demolition contractor A.C. Jardine Demolition contracted G&I Diamond Drilling to oversee removal of the swimming pool. "To demolish concrete so close to the columns in this enclosed building, we would have had to saw-cut it into sections and use hand drill tools" says G & I Diamond Drilling Director Gordon McGhie. "So, after carrying out preparation works, we brought in specialists Gnat UK to demolish the pool’s walls and floor."

Gnat claims to have the largest fleet of Swedish built Brokk demolition robots in the UK, possibly Europe. Its solution was to bring in three different sized models, each suited to a specific challenge thrown up by the demolition operation.

The smallest, a Brokk 90, weighs just 950kg but packs an impressive 255 joules hammer power. This was to prove ideal for sitting on the weight-restricted suspended walkway while the machine broke up the 330mm thick slab beneath its tracks.

But the more challenging task - demolishing the pool's perimeter sides - demanded two more heavyweight machines. The Brokk 250, weighing in at 4500kg and boasting impressive reach and hammer force, was to be the versatile workhorse tasked with main wall demolition. And a specially modified Brokk 330 – one of Gnat's largest robots - was brought in to overcome one of the project's most testing problems; difficult initial access into the pool basin itself.

First though came removal of the walkway alongside the pool and here access for the small, highly manoeuvrable, tracked Brokk 90 proved easy. "We simply walked it in through a side door, across the gym and reception area and onto the walkway" recalls Gnat Project Supervisor Doril Peake. "With its excellent power to weight ratio, the robot proved very efficient at tracking backwards along the walkway as it broke up the slab beneath."

Concrete debris dropped down into the shallow void beneath the suspended and propped slab for removal by skid-steer loaders.

While Gnat was demolishing, in just four days, the 28m length of walkway down one side of the pool, G & I Diamond Drilling was saw-cutting the other side into 1m squares blocks. These were then lowered to the ground for the Brokk 330 to percussive break into removable pieces.

Gordon McGhie acknowledges that it took over twice as long to remove this second walkway section, proving the efficiency of the Brokk operation. Nick Turnbull however emphasises the mutual advantages of his robotic breaker working in conjunction with a drilling and sawing contractor.

Further evidence of this came in the next and most intricate challenge - demolishing the 2.3m high pool walls, especially where they ran just 100mm in front of the row of cast iron roof columns.

To ease the Brokks' task, G&I Diamond Drilling first saw-cut the walkway concrete in a tight circle around each column, while the rest of the slab was being removed. Then, directly in front of each column - close to the pool wall beneath - the contractor drilled a row of eight 150mm diameter holes to form a slot.

The role of these slots was to provide a physical gap in this retained section of walkway slab while Gnat's robots collapsed the pool wall just 100mm away. This 150mm gap would eliminate any structure borne vibration or movement during demolition, negating the possibility of damage or stress to the adjacent iron columns.

Before Gnat could begin the pool wall demolition however, the two larger Brokks had to be positioned inside the pool basin. Here access was to prove the exact opposite to the easy journey taken by the smaller robot onto the walkway above.

The only way in at this lower level was to snake the machines through a labyrinth of corridors to reach a small chamber behind the now exposed end wall of the pool. Gnat then had to break a hole through this end wall to gain access to the pool basin.

The problem was the narrow 4.5m wide chamber gave insufficient clearance for the extending booms of either the Brokk 250 or 330 to attack the wall. These standard three-section arms would normally need at least 7m working area.

The solution proved innovative though relatively inexpensive. Gnat modified its 330 Brokk by shortening the main arm by 1.2m so reducing the machine’s operation length to just 4.3m.

The Brokk’s electro hydraulics were adapted from single to double cylinder operation, and an additional ram added giving it five in total. The shortened arm could now be fully extended horizontally, yet retain the same breaking power to break down the heavily reinforced 1.3m thick wall.

Once into the pool basin, the two machines worked in tandem. The versatile Brokk 250 could operate in tight corners and was narrow enough to work in the small walkway void behind the wall. This would have allowed it to collapse the wall forwards, well away from the columns.

Its 7m arm would also have enabled the machine operator to reach above and over the wall 'pulling' it down inwards from the top.

In practice, neither of these options was needed, as both machines worked from the front demolishing the wall conventionally top-down. In areas close to columns, the Brokks changed their technique and nibbled the wall from its exposed sides to minimise the risk of debris falling against the ironwork.

The two machines demolished the entire 553m3 volume of walls and slab in just five weeks - twice as fast as programmed.

"It proved one the most difficult access jobs we have faced, but our extensive planning paid off and the operation went without a hitch" says Mr. Turnbull. "We achieved demolition rates of 22m3 over an 8hr shift; at least a third quicker than any other comparable method."

His claim is supported by the main demolition contractor: "Gnat proved it had the experience we needed and worked both efficiently and quickly" says A C Jardine Demolition’s Managing Director Cunningham Thomson.

Mr. McGhie also endorses Gnat’s achievements. "Their method was significantly more cost effective than any other technique possible, and we would definitely use them again" he says.

Even the overall client, Fife Council, noticed the versatility of the Brokk machines: "I watched them like a hawk to ensure no damage occurred to our columns" comments Clerk of Works Andy Ross. "The contractor would have been in a lot of trouble if any had been touched, but none were and Gnat did a good job."

Modifications to the Brokk 330's boom has proved to be cost efficient; as Gnat has now sent the machine south to London for use on a tunnelling contract. Its compact working length is proving invaluable in both excavating the 2.5m diameter tunnel face with the shortened boom then being used to lift and position concrete lining segments.