John F Hunt Demolition keeps the peace at Great Ormond Street

When children are lying sick in close proximity to a demolition site, the last thing anyone wants is for works to disturb their recuperation. So when John F Hunt went on site at Great Ormond Street Hospital for Children, it was made well aware of the site's proximity to sensitive treatment areas within the world-famous complex.

The company has been prevented from using traditional hydraulic breaking methods instead it is employing a combination of noise, vibration and dust-free techniques to effectively dismantle the various structures piece by piece.

The firm's site team is using a variety of track and wheel-mounted wall and floor saws, core drills and excavators fitted with hydraulic concrete crackers to dismantle the reinforced concrete framed buildings, all supported by a 20t capacity Wolff tower crane. The contract involves the removal of six of the hospital's older buildings, with two of them up to 10 storeys in height, and includes the Barrie and Southwood 'A' Wing, the Nurses Home Annexe and Cardiac Unit Link Bridge.

The demolition, at the rear of the hospital, will make way for the new 10-storey, £70m state of the art Morgan Stanley Clinical Building. It is part of the second phase of a major redevelopment programme, which is largely being funded through donations to Great Ormond Street Hospital for Children's Charity.

"Our contract dictates that we cannot use any methods that would create noise, vibration and dust, as we have to demolish some structures that are physically joined to the hospital's other occupied buildings," says John F Hunt Demolition site manager Scott McCue. "This restriction prevented us from using the normal method of hydraulic breakers on excavators. We opted instead to use wet cutting saws and hydraulic munchers and crackers, but needed to prop between the floors and take the buildings apart one floor at a time working top-down."

Forest of props

Each building is in turn first cocooned in scaffolding and clad in monarflex sheeting, prior to installing a forest of props from floor to ceiling at each level.

Additionally, to lessen and or completely eliminate the structural transmission of any noise and vibration from one structure to another, the project buildings were physically separated from the occupied buildings, and retained prior to any works commencing.

This separation was achieved predominantly by track sawing using 1,000mm-diameter diamond blades lubricated with water and running on tracks bolted in turn to the floors and walls. The cycle of fixing and sawing a 2m section takes about 45 minutes and is continued and repeated round the vertical periphery of each storey until complete separation of the two parts of the entire building from top to bottom is achieved. The cantilevered floor slabs left at the separation interface are temporarily propped.

The project has been additionally complicated by the necessity to remove the previously redundant heating system, which comprised panels of asbestos encased heating coils that had been integrated into the underside of the 300mm floor slabs.

Conventional hand-held breakers are obviously out of the question, so the team has devised a simple system to remove the panels intact, complete with the slab, again using a combination of sawing, propping and lifting. The layout and strategic positioning of the maze of props has been designed in-house by John F Hunt's temporary works team to accommodate the location of the heating coils. Critical positioning of the saw cuts enables the complete panels to be lifted clear and disposed of whole.

Depending on the location and area covered by the heating ducts, floor sections up to 4m by 2m, and weighing about 1.5t, are sawn and cut out while supported on the underlying props. Holes are then core drilled through the cut section of the slab to attach slings for lifting out by the tower crane. Any exposed asbestos is carefully covered and sealed prior to placing the section of floor directly into an ISO skip container at ground level. Just a single and congested site access point in Guildford Street serves the entire operation.

Demolition of the remaining structure is achieved using 5t mini excavators equipped with hydraulic shear attachments these are also moved from floor to floor using the tower crane and used to quietly dismantle the interior and exterior walls. Constant attention to spraying the structure with water has succeeded in virtually eliminating the nuisance of dust. The dampened rubble is placed in skips on the floor and when full, lifted by the tower crane and tipped into containers at ground level.

Wall removal

The floor sawing and wall removal are carried out simultaneously and the whole top-down sequence is repeated floor-by-floor to ground level. Dismantling of the scaffolding also follows on behind the demolition of each floor.

At lower levels, two large Komatsu excavators equipped with hydraulic attachments are used to dismantle the lower floors and walls within their reach, and load away the resultant debris for off site recycling.

"We've asked for specific methods of deconstruction, such as pulverising, munching, core drilling and saw cutting," says Gardiner & Theobald Management Services site project manager Lee Houghton. "We said to the hospital that there will always be some noise during the demolition, but have chosen what we believe to be the quietest methods.

"There are noise restrictions of 75dBA at 1m from the existing buildings and we have noise monitors to check this. There has been the odd occasion when we've exceeded this level, but that's only been for an instant rather than a continuous problem. So far, we have had very few complaints about noise."

Dismantling process

The final structure to be dismantled is The Cardiac Link Bridge and for this task John F Hunt will bring in one of its Brokk Robotic excavators equipped with a concrete cracker. The Brokk will be used to first 'skeletonise' the structure back to its basic components, and thereafter the structural members will be cut and lifted clear using the site tower crane.

During the entire dismantling process, pedestrian access under and adjacent to the Cardiac Unit building must be maintained. To cater for this, the company has erected a double layered, protective scaffold tunnel to facilitate the necessary pedestrian access required by hospital personnel and staff.

Once John F Hunt has completed its work, HBG will then follow on with its design-and-build contract and make a start on the replacement 10-storey Morgan Stanley Clinical Building, which is scheduled for completion at the end of 2011 and occupancy early in 2012.

PROJECT FACTFILE: Great Ormond Street Hospital For Children NHS Trust pre-construction services

• Name: phase 2A new clinical building
• Contractor: BAM Construction
• Demolition contractor: John F Hunt Demolition
• Subcontractors: still in negotiation
• Value: construction costs approx £80m
• Construction start date: Jan 09
• End date: Jan 2012