Why read this
Explains environmentally sensitive demolition
Inner city architecture
St Paul's Cathedral
Over the past few months, views of St Paul's Cathedral - unseen for 30 years - have gradually been revealed as the demolition of the unloved Paternoster Square proceeds.
The Paternoster 'masterplan' redevelopment scheme is progressing each day, leaving fewer reminders of the 'draughty and harsh' 1960's development that previously stood on the 2.16 hectare site.
No doubt few people, including owner and developer Paternoster Associates, will be upset to see it go. Widely criticised by architects and the public alike, the development obstructed pedestrian circulation and diminished the presence of the 17th century cathedral.
Four of the previous ten storey high rises - Courtenay, Walden, Grindall and Laud House - have all but disappeared. Only the raised central square, basements and parts of Laud and Grindall remain to support nearby office blocks and the Paternoster Row walkway below.
The Row is currently the main thoroughfare to the cathedral's chapter house, and is being left intact until other access arrangements can be made.
'This is typical of the efforts made by the development team to accommodate third party requirements', explains John Towers of Waterman Partnership, consultant structural engineer on the project.
'Another example is the basement NCP car park under the central square that couldn't be closed until alternative parking space had been agreed. These were causing delays so we continued work around it,' he adds.
The current situation signals completion of the first phase of demolition, that began last May. 'Soft-strip' works - or the removal of non-structural elements - are taking place in the remaining basements before the start of phases two and three.
Phase two will complete demolition to foundation level of the west side of the site, including the last remaining building Bancroft House, and the basement car park. Phase three concerns the eastern side.
But for now most passers by remain unaware of the scope of changes at Paternoster Square. The decorative facades shielding the site from view are designed to manage disturbance in the busy area around St Paul's.
Reduction of noise, dust and vibration associated with demolition were set out in the environmental plan, says Towers.
'A ball and chain approach was unacceptable in an area such as this.' he adds. 'So instead, both manual and radio controlled excavators fitted with hydraulic pulverisers and breakers, were used - a much quieter, controlled option.'
The seven-tonne excavators were lifted via tower crane up to the buildings and lowered though access holes punched through their roofs. Once in place on the top floor, contractor Griffiths McGee began systematic demolition, or 'deconstruction' of the buildings with the excavators - floor by floor. The lower floors were supported by temporary props to prevent possible collapse.
Concrete and steel rubble, much of it reclaimable, was then loaded into skips and transferred down to ground level, again by tower crane. The skips were loaded on to lorries and removed from site.
Minimising ground movement was particularly important because Paternoster is so close to the Cathedral.
St Paul's is particularly vulnerable to subsidence as it lies on a shallow foundation built over firm, silty 'potter's clay'. Since 1935, the cathedral has been protected by a Preservation Act that prevents water abstraction and excavations below 9.14m OD near the cathedral.
Unfortunately for the developer, the square lies within this area. Waterman and Griffiths McGee have since been constantly monitoring noise, earth movement and groundwater levels throughout the site, particularly near the cathedral's crypt.
London Underground also raised concerns that the redevelopment may subject the subterranean tunnels of the Central Line, running to the north of the site, to ground movement. This has resulted in continuing strain monitoring and real-time finite element analysis.
Following the completion of the demolition contract in August, some excavation will be required to return Paternoster to a blank canvas.
'There are pockets of earth, previously untouched, that will need to be excavated to allow future construction. We've not had many unexpected problems so far but, of course, risks increase as we go underground,' says Towers. 'Archaeologists from the Museum of London are on hand should anything interesting be unearthed.'
Waterman Partnership began detailed design for the redevelopment in December with tenders due out next month. It hopes that the previous 1.2m thick reinforced concrete raft foundations can be left in place beneath the new development depending on their condition. The existing foundations lie at the maximum 9.14m OD depth prescribed by the St Paul's Act, serving as a guide for future redevelopment.
Redevelopment of Paternoster Square was first proposed in 1985, but property market changes, architectural disagreements - most famously Prince Charles's complaints in 1987 - and financial issues caused endless delays to a stream of proposed schemes.
The puzzle of Paternoster Square has finally been solved however following the planning acceptance of the Paternoster 'Masterplan' redevelopment scheme. This was produced by architects Whitfield & Partners in 1996 for development owner Paternoster Associates.
'The Masterplan is unlike any of the previous schemes,' explains John Towers of Waterman Partnership, consultant structural engineer on the project, 'because it creates several different schemes in one, rather than depending on a single unchangeable substructure'.
Six new buildings are planned, which can each be built independently. This provides far more flexibility, both in the construction and commercial sense - perfect, the planners agreed, for the City's requirements. The new buildings will produce 82,000 m2 floorspace over the existing area, with 90% going to office accommodation, and the rest to retail and leisure. Construction costs are estimated at £200M.
There will also be a central open public area, complete with column for an architectural focal point, intended to 'create a distinction between the public and commercial realms'. The public area, designed to be at ground level rather than the previous raised platform, will improve pedestrian circulation between the surrounding streets, Cheapside, Ludgate Hill and Newgate Street. The plan also includes a two-storey underground car park beneath the public area, creating 73 parking spaces. The car park will be reached by a circular access road known as the 'gyratory'.
On the spot
Name: Barry Dobbins
Qualifications: BSc. CEng MIStructE, AMICE
Company: Waterman Partnership
Current job: Associate director of Paternoster Masterplan, working with John Towers
Best thing about the job: Setting concepts and seeing them come to fruition, especially when working in such close proximity to a neighbour like St Paul's Cathedral
... and the worst: Endless meetings about peripheral issues, rather than engineering solutions
The most useful lesson you have learnt as an engineer?
Always to think laterally, keep up to date with new products and technology and, most importantly, to be approachable and open to suggestion
Advice to young engineers just starting out? Read textbooks, and yes listen to what older engineers have to say. Common sense is also a great virtue
Anything else? I'm getting into adventure sports such as climbing, sailing and skiing. I guess it's a way of letting off steam from the high pressure environment in which engineers now work