A contaminated East London site required a pile that would not carry pollution to aquifers.
London's Beckton gasworks did a remarkable job doubling as war torn Vietnam for Stanley Kubrick's film Full Metal Jacket. Nearly 20 years later, the site is being redeveloped as a warehouse and offices, and geotechnical engineers are charged with ensuring that contaminants in the ground do not reach the aquifier down the side of the piles. And while they will not encounter the deadly defoliant known as Agent Orange - used to flush out the Viet Kong in the forests of Vietnam - piling work must ensure any residual pollutants are kept at bay.
A 200m by 165m portion at the south end of the site is now being redeveloped by retail industry supplier Kesslers International, which is relocating its warehouse and offices to the area close to the Royal Albert Dock.
'Kesslers is moving away from its Stratford site because the Olympics will be held there, ' says Keller regional manager, Derek Taylor.
'There's a coincidence here because Keller worked with the same main contractor as on this site - Loach Construction - to build Kesslers' previous warehouse 16 years ago.
When I joined the company I did the trial pits for the Stratford site and 17 years later I'm doing their new site.'
An important consideration for the pile designers was isolating pollutants.
Although the ground has been remediated to a depth of 5m, preventing any movement of residual contamination further into the stratum is vital. 'One of the reasons vibro concrete columns [VCCs] are popular on contaminated sites is that they offer a good bond between pile and surrounding ground, meaning there is less likelihood of contamination passage down the sides of the pile into any aquifer, ' says Taylor. 'VCCs also have a place on these sites as no-one wants to clear away spoil.'
Ground conditions at Beckton consist of 4m of remediated crushed concrete ll lying over soft to rm clays which are followed by 1m to 2m of peat before reaching the founding river terrace gravels layer 9m to 11m down. Keller installed 1300 piles 12m to 14m deep in order to cater for loads of up to 800kN.
Taylor says: 'Interestingly, of the 1300 columns, about 1100 are to support a two way suspended steel bre reinforced slab.'
To cater for this, the rig crews provided Loach with enlarged 900mm pile heads. Below the head these piles shrink to 430mm diameter before again enlarging to 600mm at the base. 'We are constructing the pile heads to a very ne tolerance - +0 to -25mm - meaning the next process is laying the floor slab which is a big advantage, ' Taylor explains. The pile heads must be no higher than flush with the bottom of the floor slab as it is easier to top up a pile than to cut one down.
Two rig crews that were operating 32t [base unit] Vibrocats installed between 60 and 70 columns each day. To get the columns through the dense remediated layer the site workers needed to pre-bore with an auger to make a shallow hole before pushing the vibrator down.
'Once at full depth we turn the concrete pump on and lift the vibrator about 1m to ensure there is enough concrete at depth. We then lower the head into the stiff wet mix concrete which has a 60mm slump.
Re-immersing the head means the pressure of the vibrator added to the pressure of the concrete will enlarge the base, ' says Taylor. 'A less stiff mix would not form the base and just go all over the place and up the shaft.' At the top of the poured column, Keller installed a cage of loose bars to form the enlarged head.
A lorry mounted concrete pump was needed to provide the stiff mix to enlarge the bases. A smaller, trailer mounted pump might have struggled to cope.
Keller began work on site for the £540,000 contract in mid-August and was due to complete at the end of last month.
Big warehouse, big piling scheme On another London project, this time south of the River Thames, Keller has been busy piling for a large Sainsbury's warehouse.
The site has a clear view of the Queen Elizabeth II Bridge which spans the river at an impressive 65m above water level, allowing the tallest vessels to pass beneath.
At 450m long it is also Europe's largest cable-stayed bridge.
Keller has been busy using a mixture of techniques to prepare the diverse ground to take the supermarket's distribution centre.
'We are using about 6000 cast insitu piles where there is peat at the north end of the site. But at the other end we have used both vibro compaction and dynamic compaction as there is no peat here, ' says Keller regional manager, Derek Taylor. Ground improvement at the south end of the site finished a few months before GE's September site visit.
The piles, like those at the Beckton proect (see above), have enlarged 900mm diameter heads while the shaft is 340mm. But unlike those at the East London site they do not have enlarged bases. Five rig crews put in the driven cast in situ piles with a steel tube and a sacricial shoe, withdrawing the tube afterwards. Founding depths range from 20m where piling is close to the river, to 10m further away from the Thames where the river terrace gravels are higher in the strata.
'The engineers originally wanted to pile throughout the site but we came in and did some trial pits before deciding to use ground treatment where there wasn't peat and columns where there was, ' says Keller. 'This saved about £2.5M from the all-piled £6M scheme and so Keller won the £3.5M job from main contractor Fitzpatrick.'
The pile head tolerance on the Dartford job was 0mm to 50mm below ground level, so as for the Beckton project, the ground slab will go directly on top of the enlarged pile heads. Keller expected to complete work by the end of September.