Project One Lothbury Project
Cost £34 million
Client One Lothbury Ltd
Main contractorGalliford Try
Slipforming subcontractorSlipform International
Formwork and reinforced concrete subcontractorReddington
Two into one doesn't go and you can't fit a quart into a pint pot. Simple, unarguable, statements of fact.
But in the City of London Galliford Try is in the middle of a project that threatens to debunk these ideas.
Sandwiched between Lothbury and Old Jewry in the shadow of St Paul's Cathedral the One Lothbury development is seeking to squeeze and pummel extra ceiling heights and more floor space from an existing 1940s office building in a bid to bring it up to modern standards.
Galliford Try senior project manager Barry Kingscote is the man charged with defying logic and shoehorning extra space into the building without altering its facade. "Actually we are altering its facade," admits Mr Kingscote, "but only slightly. We are stretching it a little over the top few storeys and have lowered the ground floor slab so we can get the 2.7 m ceiling heights the client needs."
The original building, part of the Bank of England's property portfolio, was built in two phases during the 1940s and 1950s and at one point it seemed destined to become a victim of the demolition wrecking ball.
Thankfully there was a move to save its Portland Stone clad facade and engineers at Oxford-based consultant AKS Ward designed a steel frame that would help retain the facade during the contract.
Five steel towers, stitched together using steel latticed beams and built using 305 x 305 mm universal columns, support the facade while demolition contractor McGee clears the original steel frame and concrete floor slabbed building away ready for Galliford Try to re-erect its newly proportioned interior.
But the stretching of the original design means that the top two floors of the building's facade have been dismantled and stored off-site while the bulk of the work is carried out, explains Mr Kingscote.
"The top floors of the facade have been dismantled down to the top of the seventh floor. These pieces of stone facade will be restored and replaced toward the end of the contract. It gives us the room we need to rejig the building," he says.
Stone masons from specialist company PAYE took down the facade as part of its contract with demolition contractor McGee but south London-based stonework specialist Szerelmey has been tasked with rejuvenating the original Portland stone of the neo-classical Greek revivalist primary facades.
Fast demolition job
Their dismantling involved the erection of a cantilevered scaffolding system across the upper storeys of the building while the stone facade was taken down piecemeal. Nothing of the institutionalised banking hall interior was retained enabling McGee to do a fast demolition job behind the frontage.
The five steel towers are bolted into the basement slab and the steelwork fed through holes punched in the floor slabs.
They are then tied into the existing steel frame of the building to help retain its frontage and linked with the latticed steel girders to help provide more rigidity to the temporary works.
This allowed McGee's demolition team to take down each reinforced concrete floor slab without disturbing the facade which is closely monitored for movement throughout the timescale of the scheme.
"The towers are designed to ensure the facade does not exceed the maximum level of movement - something like 10-15 mm," says Mr Kingscote. "All our readings so far have shown a movement of less than 10 mm."
That level of movement or sway in the facade will decrease as more and more of the new steel frame for the structure is fixed onto the existing steel helping tie it back in.
A secant wall has been installed around the perimeter of the new basement but the close proximity of other offices around the site, as well as the footings for the existing building, meant painstaking progress to install these piles. Stitch drilling methods were used to accurately locate the toe of the existing structure and a 1.5 m deep trench was dug alongside it.
This was then filled with bentonite clay before polystyrene piling guides were placed at 900 mm centres for the 600 mm diameter continuous flight auger piles.
These were driven through holes punched in the reinforced concrete floor slabs with the piling rig sitting two storeys above the ground slab level. But any fears that the ground conditions may be particularly problematic proved unfounded despite the ground water table sitting some 600 mm above existing basement level.
"We thought there would be major problems with the ground," says Mr Kingscote.
Choosing slip form
The Walbrook - a tributary to the River Thames - flows through this part of London and there were concerns it may pass through the site.
But other than the discovery of alluvial deposits toward the east of the site ground -constions have not caused headaches. One area that did concern the project team was the two cores for the building.
A host of options were considered including precast concrete, steel, jump form and slip form techniques before finally deciding on the slip form method.
The long lead in that Galliford Try had on the project allowed the delivery team to look into every aspect of the build before making the final decision on what system to use.
This helped trim final project costs as well as the overall contract length but it wasn't always the cheapest -solution that got the nod. "The slip form system was not the cheapest option we had but the extra we spent on using it we more than clawed back thanks to its efficiency," Mr Kingscote explains.
The first core is far from an ideal lay out. It is as Mr Kingscote puts it a "little bit bitty".
Effectively there were three unattached components in the first core forcing the use of a forest of temporary props to keep it in line.
Thanks to its complexity the first core took a while longer to erect than the second, simpler core. Five weeks to the second core's three. But the pace of forming is all down to accurate initial set up work _ and not trying to go all out for speed, according to Mr Kingscote. "Slip forming is all about the initial setting up and getting comfortable with the system. Overstep yourself and the problems start to pile up," he says.
With the two cores now completed the focus is now on steelwork contractor SCWS to get the frame erected so that the connections to the existing facade can be made.
Some 90 per cent of the welds to the existing steel frame have already been carried out. Mr Kingscote is placing an angle welded to the frame as a launching plate.
This allows the new beam to be placed on top then packed up to its exact position as required. Its another time saving option that will help the project team move another step closer to its August 2009 completion target - proving that sometimes a pint pot can accommodate a quart.
Moving waste proves problematic
Any construction project in the centre of any major city has its own logistical issues.
Even the simple delivery of materials to site can cause major problems for unwary contractors but when there are 3,000 cu m of earth to be shifted from a site in the centre of the City of London those problems are magnified.
"Getting the muck away was difficult," says Mr Kingscote, "It all had to go over the wall into Lothbury. It took about an hour to turn around one lorry."
Further constraints also meant that noisy work could only continue on a two hour on, two hour off basis, adding to the site team's headache.
Long lead in helps buy out risk
One aspect that makes the Lothbury project differ from other construction schemes is the lead in time Galliford Try had before its official start date.
Initially client One Lothbury Ltd let out the demolition package and building work as two different contracts but while McGee was on site, the client asked Galliford Try to take on McGee as its demolition contractor.
Galliford took the job on in February 2007 but were unable to start construction on site until December - leaving the team with an unusually long lead in time.
The 10 months was used to help focus on certain areas such as the method of constructing the cores and by introducing subcontractors to the project earlier than they would normally expect. The move has paid off handsomely so far with the long lead in enabling Galliford to buy in material at lower prices.
"All of the key subcontracts were procured as early as possible," says Mr Kingscote. "By getting key suppliers and contractors on board early we were able to close out any risk of material price hikes, it also allowed us to look at different systems which could speed up the project," he adds.
The shortest or 'worst' lead in was the 24 weeks afforded steelwork subcontractor SCWS but this has to be compared with the 16-week notice it would normally expect on projects of this type.
"The packages were put out to a shortlist of suppliers and contractors drafted from our supply chain," says Mr Kingscote.
"There were only two firms on the shortlists for each of the packages - they knew they had a 50 per cent chance of winning the work so they were more focused on it.
"We wanted value from those specialists and we have got it," he says.