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Making good ground


Piling methods that save on landfill and muck disposal costs could be the next big development for foundation specialists.

Made ground at Kerseley Colliery near Coventry presented a quandary for main contractor Winvic this summer. Fill material inconsistent in type and depth was only partially and unpredictably consolidated after the site was levelled and back-filled in the early 1990s.

Winvic needed a reliable foundation method for a new warehouse development on the site. Roger Bullivant Ltd (RBL) came up with the answer: combining two different ground improvement techniques, lime cement stabilisation and dynamic compaction.

The unusual solution proved a highly successful means of creating a stable platform, quickly and economically.

BL has been making strides in innovative ground improvement methods of late and the next innovation may come as an extension of amelioration of bad ground to more uses. The technology, implemented with plant developed and built by RBL, offers to cut down on quantities of muck taken away from sites and improve the environmental side of foundation engineering - a subject much on chairman Roger Bullivant's mind.

'Landfill and muck disposal costs are not going to get any lower, that's for sure, ' he says.

'I'm not going to say what we plan to introduce, but it's pretty obvious where the big quantities of work are going to be and what clients will be looking for.'

Piling and groundwork of all types tends to be the noisy and messy end of construction projects, he says. 'Quick, quiet and clean may be the watchwords for the future.'

Bullivant is known for innovative plant development.

His firm began working on its own mini-piling rigs in the early 1980s and new designs have produced powerful, short masted rigs able to cope with the demands of restricted access and high load mini piles.

The rigs occupy a space only 3m long by 1.5m wide and 4m high. They also have hefty Cummins diesel engines, delivering enough torque to power 600mm segmental augers down to more than 30m in heavy clays.

According to Bullivant, a considerable share of company money won from the piling market is being ploughed back into developing more innovations. RBL is introducing 10 new piling rigs to its fleet, at a cost of £2.5M, with low emission engines, remote controls and synthetic materials to quieten the hammer blow. There are more plant developments to come, but for now, Bullivant is keeping tight lipped.

'There is always a way of doing something better or differently to improve efficiency and safety or to give an advantage over the competition, ' he says. 'We have to keep innovating. This market is one of the most competitive, but the business of building foundations and engineering in the ground has provided plentiful opportunity to innovate.

Innovative plant was key, at least in part, to Bullivant's work at Kerseley Park. The firm's NRG cone was used to apply dynamic compaction to vibro stone columns and increase the rate of consolidation on the site.

'Some form of improvement of the ground was essential to speed up the consolidation process, reduce overall settlement and control the differential settlement of the new warehouse, ' says RBL ground improvement manager Jason Redgers.

RBL proposed two stages of lime cement stabilisation with dynamic consolidation using the NRG cone. Its stone columns are unique in that they are created with a shaped mandrel, applying vertical instead of horizontal vibration. This, says Redgers, increases column density and improves bearing capacity and settlement control.

'Use of two techniques may initially appear to involve more work, but it actually worked out quicker by five weeks and cheaper to the tune of £1M overall than conventional piling.'

Made ground at Kerseley consisted mostly of clay, containing thin gravel and sand layers as well as colliery and demolition waste. According to Redgers, dynamic compaction used on its own would have improved the ground at depth in the long term, but would also have raised pore water pressures in the clay fill.

'By installing the vibro stone columns first and then applying the dynamic compaction, the top of the column 'bursts' to create a drainage pathway and a stiffened soil raft. This way the dynamic compaction improves the ground and the pore water can drain rapidly through the granular columns.'

Success at Nine Elms Use of Roger Bullivant Ltd's (RBL's) short masted mini piling rigs has enabled new foundations to be installed in a tricky location: beneath Railtrack's Nine Elms viaduct in London. The brick and iron composite structure was to be strengthened by main contractor Nuttall, with rail services in operation above while the market's wholesalers make use of the arches and available space below.

The viaduct consists of a series of masonry arches and an iron frame structure built later as a widening extension. Structural assessments for Railtrack indicated the iron structure needed strengthening with steel portal frames supported on new piled foundations.

RBL's underpinning and mini-piling division took on the job, which involved construction of 216 segmental flight augered concrete piles, each 300mm in diameter. Groups of four piles have been installed at 1m spacings on a square grid. Nuttall had the task of erecting the new steel portal frames, on 54 pile caps over RBL's piles, and to the underside of the viaduct's wrought iron beams to provide the structure with additional strength.

RBL's work had to be carried out in confined spaces, with only 3.7m of headroom and piles constructed as little as 0.5m away from the masonry structure's foundations. It also had to keep the market free from disruption.

Temporary partitions were erected to provide a barrier protecting RBL's work from pathways for movement of fruit and vegetables between the masonry arches and the market's open concourse. Plant movements had to be managed carefully. Piles down to 29m demanded removal of significant quantities of spoil and delivery of similar amounts of concrete, all of which had to be moved with a small dumper.

'Small items of plant were essential for this job, ' says RBL's south east mini-piling area manager Adrian Mercer. 'There was not a lot of room to spare, but once the plant was in place we were able to reach our target of two piles each day.'

RBL's mini piling rigs were used to drive steel casings as well as auger the piles. The firm expected to be using 7m of casing for each pile.

Borehole data had shown sand and gravels down to this depth, but a series of underground streams were encountered. Depth to clay and required length of casing varied from 7m to 14m.

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