In the name of sustainability, foundations from a demolished building are being reused; Damon Schünmann visits an old-for-new scheme in the heart of London.
'It's all about the 'S' word, ' says Stent senior engineer Max Gwynn, referring to sustainability and the subcontractor's reuse of old concrete piles, a basement slab and retaining walls at a site adjacent to the famous Bow Bells church in the City of London.
The church of St Mary-le-Bow is Norman in origin, but has had something of an up and down existence. It was destroyed in the Great Fire of 1666 after which it rose again over the following two decades from a design by Sir Christopher Wren.
Damaged by bombing in 1941, it was rebuilt once more - for the most part to Wren's design.
Although many non-Londoners refer to those raised in the capital as cockneys, tradition dictates that only those born within earshot of the church bells should bear the name.
Cockneys may be known to quibble about who should and should not come under this umbrella - and the Eastenders television soap opera portrays them as rarely being short of something to argue about. But few could nd cause for complaint about the sustainability effort for the forthcoming Bow Bells House ofce and restaurant development soon to rise from the rubble of the former ofces that recently stood on the plot, just a few strides from St. Paul's Cathedral.
The new seven storey structure will comprise a reinforced concrete core with suspended steel frame and glass cladding.
'The Corporation of London - who we ok working hours with - said if it's ok with the reverend [George R Bush] it's ok with us, as he's an important man in the area, ' says Gwynn. 'It's also the last church Wren designed, so the key here is not to crash a crane or a piling rig into it.'
Stent installed 95 piles under a £1.03M contract for developer Bovis Lend Lease and the price allowed for new piles encountering old tripod-installed piles from the 1950s.
Ground conditions are 8m of river terrace gravels over London Clay within which all piles found. 'The clay is one of the deepest bands in London at up to 34m thick so the SI went down to 45m to verify that, ' says Gwynn.
'We didn't want to go into the water bearing and unstable Woolwich and Reading beds below.'
Gwynn says: 'Sustainability is a big issue at the moment and our overall client Mitsubishi UK was keen to use as much of the old building as possible, such as the basement slab, the retaining wall and some of the original piles. But these old piles are renowned for being banana shaped particularly where they hit claystone as they went down.'
Key issues for reuse - in this case with the aim of Mitsubishi providing a warranty - are what records exist, how accurate they are, how many piles should be tested and with what methods.
Fortunately, the existing tripod pile records were unusually good, and included loading data, diameter and length. So consultant Buro Happold asked Stent to integrity and load test them for possible reuse. This was in line with RUFUS guidance (reuse of foundations on urban sites), a research project being developed by several European countries.
A 26m deep test pile with a 1050mm diameter withstood a maximum test load of 7.4MN requiring four reaction anchors. This allowed the design philosophy to be agreed before the start of main works.
'We will be able to reuse about 35 of these old 24 inch [610mm] 12m to 15m piles which had a 100t safe working load, ' says Gwynn.
'Buro Happold asked us to take one to a factor of safety of two. We used a static load test and up to 100t it performed well with about 2mm settlement, but after that it increased quite rapidly. This meant the safe working load had to be reduced to about 80t to get this factor of safety.'
Although Stent designed the new foundation layout around what already existed so as to avoid clashes where possible, 13 piles conflicted with the old foundations. But a huge Bauer BG 36 piling rig made short work of these obstructions and powered through those existing foundations not earmarked for reuse.
Weighing in at 130t and producing 37t torque, Gwynn likes to compare it to a typical Porsche that might manage one hundredth of this, perhaps 367 kN/m. And the extra cost of this powerful rig is offset by the fact that Stent does not need to revisit areas of the site after encountering obstructions.
The new foundations include 80 large diameter traditional dry rotary bored piles ranging from 900mm to 1800mm, with the largest designed for maximum loads of 10,250 kN. Stent built these using temporary steel casings to about 9m below platform level to avoid river terrace gravels.
London Underground's Central Line passes within 5m of the site Client Mitsubishi UK was keen to use as much of the old building as possible, such as the basement slab, the retaining wall and some of the original piles.
Metronet needed to be convinced that Stent's piles would go in straight and not affect the line. The piles nearest to the Central Line - located at the development's northern edge and facing onto Cheapside - were originally designed to be permanently cased to 15m depth ensuring there would be no signifi cant load transfer to the tunnel crown. However, consultant Buro Happold's settlement analysis showed a very nominal load transfer of about 2kPa to 3kPa, similar to that imposed by the previous building, and so the final pile design did away with casings.
But casings were needed elsewhere. Because developer Stanhope wanted to maximise the building's footprint, rig crews added precautionary measures to minimise movement of gravels around the foundations of an adjacent building. Work right up against the building's basement - at the east edge of the Bow Bells House site - saw piles installed that were cased for the top 6m.
While work continued, a Keltbray-installed 45 degree steel brace propping system supported the old retaining wall. The bottom end of each brace anchored into the original basement slab.
'This did make it tricky to get the Bauer rig, which is a beast, into position, ' notes Gwynn.
As the site is of historical importance, the Museum of London Archaeology Service (MOLAS) sent a team to investigate the ground for Roman and medieval finds. Since Stanhope is using the old building's basement slab, each pile was installed through this via 2m 2 sawn access holes and the dirt brushing sleuths targeted each of these.
'Most of the finds were uninteresting bits of pottery, old medieval building foundations and burial pits, but it would have been a nightmare for the developer if they had found anything signifi cant, ' says Gwynn. 'The basement depth was about the level of the original road into London where traders probably lived off traffic going into the city. However they did find a medieval well in the central core area of the building which was recorded, but it didn't hold up work.'
Not all the piling work was carried out with the big Bauer rig. A Klemm 709 was used to sink piles for a temporary works structure designed to support a water tank. Stent installed 11 piles of 600mm diameter, a width that was a practical maximum in terms of avoiding delays. This was because the next size up, 750mm, would have been the trigger for a further mandatory visit from the MOLAS team.
Buro Happold had originally wanted 450mm diameter piles in this location, but as Stent did not have an appropriate string available, it provided 600mm piles to 12m depth, instead of the originally intended 16m.
The subcontractor worked under a framework agreement with Stanhope that provided the opportunity for early contractor involvement. This allowed Stent to infl uence the design philosophy, for instance liaising with the district surveyor to get the design's factor of safety of two validated.
Gwynn says the district surveyor was prepared to make the validation based on a working knowledge of the ground, the techniques and confi dence in the initial pile test. He adds that using this minimum factor of safety meant a project saving of about £20,000.
'Early contractor involvement is a huge advantage as we could get the most effi cient factor of safety and do only one test. It's a trust thing as Stent would have made more money out of doing a maintained load test on a working pile as well as the preliminary test pile.'
Stent began work in late July and was due to complete last month.