How do you sink piles through an old rubbish dump full of potentially hazardous asbestos waste where no spoil can be brought up to the surface and where noise or vibration are also discouraged?
The answer, says foundations specialist Bachy Soletanche, is to develop a new rotary piling technique. This centres on displacing the soil instead of removing it, and at the same time inserting a hollow pile casing.
Seeing its premiere on the Channel Tunnel Rail Link's Medway bridge site in Kent, this novel form of rotary displacement piling is being used to install four dozen, large diameter piles through the 20m deep tip with the help of a cone shaped screw to force aside the asbestos-laden ground. The hollow casing, lowered immediately behind the tapered cone, allows a conventional concrete pile to be formed within it and bored a further 16m into the chalk beneath.
With all piling due to be complete this month, the closely guarded technique is being heralded a major success. Noise, vibration and ground disturbance have all been negligible.
And, even more crucially, so has any trace on the surface of the dump's sensitive contents.
'Air and ground monitoring samples contain only negligible concentrations of asbestos dust, ' says Bachy Soletanche construction manager Robin Abraham. 'We have only ever recorded a fraction of our permitted safety levels.'
Factory Farm is an inconspicuous tract of land on the eastern banks of the River Medway in Rochester, used for years as a licensed tip for a wide range of domestic and industrial products. It might have remained inconspicuous had not alignment for the CTRL's 25 span Medway crossing clipped the side of the now capped and sealed dump that contained most of the asbestos.
'Records are incomplete, but we suspect there is asbestos in all forms - lagging, boards and sheeting - both loose and in plastic bags, ' Abraham explains.
'Site investigation was initially limited as we were not allowed any coring or direct sampling.'
The £30M Medway Bridge, designed and project managed for client Union Railways by Rail Link Engineering, will be supported on 24 solid concrete piers, each demanding an average of six, 1,200mm diameter piles bored up to 30m into chalk bedrock.
Bachy Soletanche, through its £2.5M subcontract for main bridge contractor Eurolink, last year sunk the bulk of the structure's 460 foundation piles.
Only at Factory Farm, the site for four of the bridge's supports, is the neat 1.3km long line of white piers interrupted by remaining piling rigs. Completion was deliberately programmed separately to allow for approval of different methods.
Engineers were thankful of the additional design time, for the list of constraints accompanying this narrow, 300m long area is onerous. The presence of asbestos triggered an immediate awareness from RLE that Factory Farm be treated as a contaminated area.
Houses stand just 170m away and, not only do the environmental constraints include tight noise and vibration limits, but the client was determined not to concern the residents.
Twelve, 900mm diameter piles are needed for each foundation and original ideas centred around boring concrete piles through a driven casing. But, while these kept any contamination buried, they still triggered noise and vibration.
Discussions between Bachy Soletanche and RLE led to a environmentally friendly solution.
The resultant displacement piling technique, with its follow on casing, is the outcome of over a year's development and is claimed by the foundation contractor to be a world first.
'Not only does it satisfy all the environmental constraints but, in these uncertain ground conditions, it reduces risk both to us and the overall project, ' says operations manager Chris Merridew.
Key to the system is a 2.6m long solid steel, tapering cone with its lower half formed as a helical screw. Using a modified high torque rig, the 1t cone is fixed to the end of the kelly bar and lowered down to the leading edge of a standard pile casing.
As the cone is screwed relatively slowly into the ground, the casing - supported separately by the rig's oscillator ring - follows just millimetres behind. Cone and casing are independent, but are sunk at the same speed.
While the cone displaces the soil sideways, the resultant bore is immediately filled by the casing. Up to three, 9m long tubes are needed to bore through the 20m deep rubbish dump, and temporary removal of the kelly bar and cone - to allow additional casings to be welded on top - signals the only time when potentially contaminated soil, stuck to the cone, could be brought to the surface, The cone is immediately inserted into a water-filled capsule alongside where high pressure jets remove any spoil and wastewater flows into a closed disposal tank. The cone is then returned to the bore to ultimately found and seal this upper, cased section of pile some 2m into chalk bedrock underlying the tip area.
Once the average 22m hollow casing has been sunk right through the dump for all 12 piles needed for a pier, numerous surface soil samples are taken on and around the foundation to confirm no contamination and allow fencing to be removed before second stage piling starts.
The piling crews then bring in a KCA 100 rig to bore conventional, 36m deep concrete piles through the hollow casing and deep into the chalk beneath.
Robin Abraham, Bachy Soletanche Tel: (01483) 427311 www.soletanche-bachy.com
The row of site huts bordering the fenced off Factory Farm compound look like any other, until notices referring to 'contamination area' and 'leave all clothing here' reveal that one of them doubles as a decontamination lock controlling all access to the piling area. Both Bachy Soletanche, and main contractor Eurolink, had to register as licensed tip operators bringing on board a raft of safety and environmental demands in consultation with both the Health & Safety Executive and the Environment Agency The discomfort for the 12 strong piling workforce always having to wear protective clothing and full face masks, encouraged Bachy to reduce the standard 10 hour shifts to just four hours. But for the team, timing can be more personal, as just a visit to the loo demands planning 20 minutes in advance to negotiate the decontamination hut.
All waste from the compound is bagged, and leaves in sealed skips. A dozen air monitors around the site perimeter, plus more on the workforce, constantly record contamination levels.
And, after first stage piling, when the compound transfers back from a restricted to a clean site, numerous soil samples are taken.
Such precautions seem to have proved their worth. The maximum recorded asbestos readings around the site perimeter of 0.004 fibres/ml of air are less than half the permitted safe limit.
Turning up the torque
Bachy Soletanche's displacement piling technique relies on imparting high torque directly down to the drill bit - a large conical screw. This requires the company's highest powered rig - a Casagrande C90, providing an impressive 55tm torque through an HT55 rotary head - to be modified so that a strengthened kelly bar can be passed through the rig's rotary drive and be powered directly by a sleeve in the centre of the casing adapter.
The rig's full torque, normally used to install casings, can then be redirected to the kelly bar and down to the cone. The adapter also holds and rotates the casing at the same speed as the kelly bar. A fine water mist is sprayed into the casing to suppress dust.
The diameter of the cone is only fractionally less than that of the casing through which it protrudes. The two units are not directly connected, and the cone can move 700mm vertically in relation to the follow-on casing.
But the aim is that both advance together so no soil can enter the hollow tube.
On the spot
Name: Robin Abraham
Job: construction manager, Bachy Soletanche
Qualifications: BSc MICE CEng
Best thing about this job: Handing over difficult piling works in the tip and river areas knowing a good job had been done.
And the worst: Facing the M25 to get here.
Best job ever: A remote dam site in Ghana where you couldn't call for outside resources.
Most embarrassing moment: Several hours in jail at Algiers airport trying to prove I was not Jewish.