Brokk demolition robots make a splash in Scotland
11 August 2009
A trio of versatile robotic concrete-breaking machines has successfully demolished a 100 year old Scottish swimming pool quicker, cheaper and safer than any other possible method. David Hayward reports
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 330 - to 'delicately' remove the structure at a combined rate of up to 22 m3 (776 ft3) per eight hour shift.
"While demolishing the 1.3 m (5.5 ft) thick pool walls, we had to carefully remove concrete less than 100 mm (4 inches) away from the original columns", said 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 amenity for the residents of Dunfermline. The impressive town centre building has been enlarged and improved internally several times over the last century and now boasts three varying sized pools, plus gymnasium and sports hall (see box).
The main focus of the current two-year £17 million (US$ 27.7 million) upgrade, for owner Fife Council, is the original and largest of the three pools, which is being totally replaced.
In daily use until last year, this 23 m (76 ft) long rectangular pool, surrounded by a suspended 2.4 m (7.9 ft) wide concrete walkway, is enclosed beneath its own glazed, arch-shaped steel roof supported on 14 ornate cast iron columns. These 180 mm (7 inch) diameter columns run down, close to the poolside, and through the suspended walkway to footings beneath.
Pool removal
The old pool was leaking and its replacement is a full 2 m (6.6ft) longer, creating an Olympic sized facility. The original roof and columns are however central to the building's Grade B listed status so could not be damaged or even touched during the refurbishment contract.
Main contractor A.C. Jardine Demolition contracted G & I Diamond Drilling to oversee removal of the swimming pool. "To remove 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 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. It brought in three different sized models, each suited to a specific challenge thrown up by the demolition operation.
The smallest, a Brokk 90, weighs just 950 kg (2,090 lb) 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 330 mm (13 in) 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 4,500 kg (9,900 lb) and boasting impressive reach and hammer force, was to be the versatile workhorse tasked with main wall demolition. And a specially modified Brokk 330 overcame one of the project's most testing problems - difficult initial access into the pool basin itself.
First came removal of the walkway alongside the pool and here access for the Brokk 90 proved easy. "We simply walked it in though 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 the 28 m (92 ft) length of walkway down one side of the pool in just four days, G & I Diamond Drilling was saw-cutting the other side into 1 m (3.3 ft) square blocks. These were then lowered to the ground for the Brokk 330 to percussive break into removable pieces.
Mr McGhie acknowledges that it took over twice as long to remove this second walkway section, proving the efficiency of the Brokk operation. Mr Turnbull, however, emphasises the mutual advantages of his robotic breaker working in conjunction with a drilling and sawing contractor.
Before Gnat could begin pool wall demolition, the two larger Brokks had to be positioned inside the pool itself. Here access was to prove the exact opposite to the easy journey taken earlier 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 planned to break a hole through this end wall to gain access to the pool basin.
The problem was that the narrow 4.5 m (14.7 ft) 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 7 m (23 ft) working area.
The solution proved innovative though relatively inexpensive. Gnat modified its 330 Brokk by shortening the main arm by 1.2 m (4 ft), so reducing the machine's operation length to just 4.3 m (14 ft)
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 demolish the heavily reinforced 1.3 m (4.3 ft) thick wall.
Working in tandem
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 7 m (23 ft) 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.
Where the wall ran just 100 mm (4 inches) in front of the row of cast iron roof columns.
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 the column - close to the pool wall beneath - the contractor drilled a row of eight 150 mm (6 in) diameter holes to form a slot.
The role of these slots was to provide a physical gap in this retained circle of walkway slab while Gnat's robots collapsed the adjacent pool wall. This 150 mm (6 inch) gap eliminated any structure borne vibration or movement during demolition, negating the possibility of damage or stress to the columns.
The two machines demolished the entire 553 m3 (19,508 ft3) 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 at least a third quicker than any other comparable method.
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 2.5 m (8.2 ft) diameter tunnel face with the shortened boom then being used to lift and position concrete lining segments.
