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SUPER SONIC

GEOENVIRONMENTAL ENGINEERING

A modied rotary drilling rig tted with a sonic drill head helped save time during the investigation and remediation of a domestic oil spill on a site in West Sussex. Alexandra Wynne talks to the men who came up with the idea.

Insurers called in Ecologia Environmental Solutions to investigate a reported 3000l kerosene leak on private property in West Sussex last summer. There was considerable concern that the contaminant could pollute a nearby spring supplying water to local homes.

'Removing the bulk of the source of the contaminant as quickly as possible was the most pressing issue we faced, ' says Ecologia managing director, Giacomo Maini. 'As soon as the rain starts, the contamination would spread.'

Initial investigations revealed a leak from the pipe carrying the kerosene from a 5000l tank where it sits above ground around 100m away from the house it supplies. Ecologia employed an excavator to quickly remove more than 20,000t of soil 3m to 4m deep from around the visible area of contamination.

Engineers decided that the company's recent purchase, a modied Hydra Joy 3 drilling rig, was the only equipment they owned that was suitable to reach the 14m to 18m depths needed for the next phase of the site investigation.

But rig crews were surprised to d that an anticipated shallow layer of sandstone went further down than expected, once drilling started, for the nine 101mm diameter boreholes on the about 50m by 100m site. After drilling ve SI boreholes, they opted for a different approach and sent for some sonic drill rods to t to the rig.

'It was a good opportunity to test the sonic drill down to a deep level of sandstone, and although we expected it to reach the requisite depth, we still thought it would be a dif cult job, ' says Ecologia drilling operations manager Ashley Nicholls. 'But it worked about twice as fast as the rotary drill. It surprised us.'

Nicholls says that although the rotary drill could cope with the ground conditions, each borehole was taking around a day to drill. Up to 1.5m of made ground sits over yellowish sands that are occasionally banded with gravels, and poorly cemented sandstone, turning into sandy clay over the groundwater at 14m to 18m below surface.

Dutch company Eijkelkamp and Italian manufacturer Hydra worked together to create the hybrid rotary and sonic drill rig to Ecologia speci ations. Hydra produced a modi ed version of its Joy 3 rig to include a bespoke cassette that would accommodate a specially commissioned S2x7/5 sonic drill head from Eijkelkamp. Nicholls says the modi ation was 'like attaching a terrier to a pit bull'.

Ecologia was reluctant to ush during rotary drilling in case it disturbed the contamination. But this made it dif cult to keep the drill rod cool. The company was also nervous that if the drill rod overheated it could burn off the leaked kerosene and skew the analysis of the extent of the contamination.

Drill operators were forced to advance the drill slowly and were able to hold off using ush until they reached 10m where the ground became mer. At this point it became necessary to cool the rod by lubricating it with a low pressure ow of water.

Using the sonic drill removed these concerns. High frequency vibrations are concentrated down the drill rod, causing between 1mm to 5mm of the surrounding soil to become liquid, reducing friction and allowing the drill rods to easily penetrate. The drill works well in weak clay and soils that are sandy or are rich in gravel so no ush is needed.

Another benenfit of the sonic drill is that it has a 2m long sampler compared to the 1m long sampler on the rotary drill. This means that it takes soil samples less frequently and is therefore quicker on the job.

The rotary sampler is attached to the front of the drill rod and its tungsten carbide cutting shoe enables the steel case to advance down and collect a soil sample in the plastic cylinder held inside, similar to the way an apple corer works, says Nicholls.

SonicSampDrill's AquaLock sampler vibrates down through the ground disturbing only a few millimetres of soil around it. A piston is held inside the cutter head by a trapped water column. When the sampler reaches the required depth a valve is opened, releasing the water into the drill rods above and drawing back the piston to take the sample, much like pulling on the plunger of a syringe.

Because water doesn't compress in the way that air does, the AquaLock can maintain a vacuum to produce an accurate sample. The water creates the pressure that stops the piston taking the sample as soon as it starts to penetrate the surface.

Each of the four 14m to 18m deep boreholes made by the sonic drill took around four hours - including the time taken to collect samples. This was almost half the time it took Ecologia to drill each of the first five boreholes with the rotary drill.

Nicholls says that another benefit is not having to mobilise two separate rigs for variable ground conditions, thus saving money. He claims that Ecologia has achieved a first with its new quick change rig. A mere tug of a lever and a change of rods can transform the 5.5t machine from a standard rotary drill to a sonic drill within five minutes.

Remediation work started in early December following the installation of monitoring wells into each of the 101mm boreholes.

The company then inserted 63mm plastic pipes carved with horizontal slots, 14m to 18m deep so that they reach groundwater level. Gravel is placed in the annulus between the borehole and the outside of the pipe and bentonite used to seal the pipe off from anything coming in from the top.

Maini claims there is a general mistaken belief that sonic drilling is limited because it works with narrow diameter drills. But he says: 'If you double the diameter of the monitoring well in remediation, it doesn't mean you automatically double the efciency.'

In the worst of the monitoring wells, 300mm of kerosene (free product) was floating on top of the ground water. The company's ECOVAC and oil water separator system has recovered 450l of kerosene from the contaminated land to date.

Maini explains that internal vacuum pipes skim off the free product from each of the four wells on a cycle, operating on one at a time. Liquid and vapour is drawn off from inside the recovery wells and treated by a dual phase extraction. A carbon filter traps any hydrocarbon vapours and releases the clean air into the atmosphere while an oil water separator extracts the contaminant before discharging clean water into the ground.

Contaminated spoil from the initial 20,000t excavation along with barrels containing the 450l of recovered product have been removed to a hazardous waste land ll site. But no additional spoil has been generated by using the hybrid drill.

Ecologia is liaising with the Environment Agency and the local authorities to establish remediation targets on the project that has been worth £60,000 to the company so far. Maini anticipates that further work will establish the perimeters of the contamination plume. In addition, it is likely that the remaining four monitoring wells will be attached to the ECOVAC and oil water separator for a more widespread remediation of the kerosene.

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