May Gurney's contract to drive more than 1,000 piles at a town centre site in Bishop's Stortford is based on a piling method that keeps the soil in the ground. But risk management is in place should it need to be excavated. reports
NO AMOUNT of site investigation can make ground conditions certain when it comes to pricing a job. So for a piling contractor lump sum contracts are a risky business.
But May Gurney has confidently taken this risk for the client by accepting a £600,000 lump sum contract to install more than 1,000 piles for a major mixed use development at Bishop's Stortford town centre.
The margin of the contractor's profit ? or loss ? is dependent on how much of the job can be done by Displacement Auger Piling, under which the soil is compacted sideways, adding vertical strength to the pile. As no spoil is brought to the surface there is no need for expensive and time consuming spoil removal to landfill. Following its site investigation and preliminary test piles, May Gu rney took the calculated r isk of doing most of the piling using the cost effective DAP method.
The risk for May Gurney is that the ground needs to be of a granular consistency with enough give to be displaced sideways if DAP is to be used throughout the site. Otherwise the company will have to resor t to Cont inuous Flight Auger ? under which spoil is brought to the surface ? which could eat into the profits. As May Gurney takes on contracts worth up to £12 million it was decided the element of risk involved for a £600,000 fixed price contract was worth taking.
'In order to give the client cost cer tainty we have taken on board the risk so that the client doesn't have to worry about soil arisings, ' says Steve Longdon, May Gurney general manager, geotechnical and environmental.
'We hope to be able to use DAP for 90 per cent of the site but it is too early to say whether we'll be able to as the ground does vary. We have only started working in one area of the site at the moment so our assessment could be wrong. A lot of the work has been assessing how much risk we thought we could mitigate.' Risk assessment has included drilling three test piles at May Gurney's own expense. Two more test piles are due to be d r iven in other, yet to be worked on, areas of the site. These will be checked against the results shown up by the 150 mm bore holes d r iven at other areas.
The ground conditions start with a layer of made ground followed by a mixture of soft clay and peat and clay and silt with some ground water deposits. The founding strata is sand/gravel at approximately 9 m.
'What we have got is fill materials, made ground over alluvial deposits made up of peats and soft clays and a founding strata of glacial deposits of sandy gravel, ' says Mr Longdon, a civil engineering graduate who has already worked on many piling jobs with May Gu rney du r ing three and half years at the firm.
The percentage of DAP piling will depend on the amount of clay encountered. 'The clays are a lot more diff icult to deal with, ' he adds.
Should the tests show up anything untoward, May Gurney has contingencies in place in the shape of a CFA machine for areas where the soil is found to be too dense for DAP to work and a hybrid CPA/DAP machine, which will displace soil but also extract it where necessary.
'Taking on risk is something we are able to offer on most contracts. Clearly there are a number where there are certain factors that make it virtually impossible to do so but on the vast majority of jobs we are prepared to manage that risk for the client and we do a lot of jobs on a lump sum basis, ' says Mr Longdon.
Having taken the gamble, pile installation is continuing apace at more than 20 a day. The piles vary in depth from 12-17 m and in diameter from 350400 mm. Out off the total, 70 per cent of the piles are 350 mm that take a ver t ical load of up to 60 Kn. The 400 mm diameter piles are stationed in areas that will take horizontal loads of up to 30 Kn f rom the ground beams above as well as vertical loads of up to 60 Kn. The 400 mm piles also have more robust 7 m steel reinforcement cages of up to 6T25.
Under May Gurney's application of DAP, the auger is drilled in to the ground displacing the soil sideways.
To achieve displacement up to 30 tonne/m of torque is needed to turn the 2.5 cu m auger. The soil comes out to the lef t hand side and is replaced with concrete.
When the auger reaches the required depth concrete is pumped through the hollow stem of the auger and the steel reinforcement cage is then lowered into the wet concrete.
'The big advantage of DAP piles is that you increase load bearing capacity. We are displacing the ground sideways and in doing that we are increasing the density of the surrounding ground, ' says Mr Longdon.
'DAP piles by their nature are becoming more and more prevalent because in the right ground conditions it doesn't create the spoil, which is the reason why we are using it here. So it is more cost effective and the job is done without the vibration and noise of other methods.' The piling method is also a big financial winner for contaminated land ? although there is a very small amount of contamination on this site ? because it keeps the contamination beneath the surface, obviating the need for expensive treatment, adds Mr Longdon.
The Llamada P150 rig is usually used for CFA piling but in this instance has been f it ted with a DAP auger May Gurney has specially designed and manufactured. Further details of May Gurney's drilling head remain a trade secret.
Steve Anderson, May Gurney site supervisor, says that the piling contractor has refined the method over the cou rse of many jobs in the last decade. 'We've been using DAP for about 10 years, ' he says. 'May Gurney was the first firm in the UK to use it.' The rig is fitted with sensors at various points that enable the driver to monitor the rate of penetration, the torque rate of revolut ion and if there are any localised changes in the ground conditions from a lap top in the cab. The information helps the driver to decide whether he has driven the pile to the right depth and can begin to concrete the pile or in some cases it may show that the length of the pile needs to be changed to suit the differing ground conditions.
'We know which pile length will carry which loads so we have a minimum depth that we need to achieve, ' May Gurney project manager Mike Cowan says. 'The pile is drilled until you reach a point of practical refusal. It is drilled until you reach a point where it is its not practical to carry on feeding the auger in.'