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Switching sewer samples

A new multi-purpose site investigation rig is being used on a proposed sewer project in Hereford and Worcester.

Keeping site investigation competitive is not a matter of relying purely on the price of work, but on finding better ways of doing it. There has, after all, been little development in routine cable percussion drilling techniques for two centuries.

The industry, according to this line of thought, has brought its current predicament upon itself by perpetuating the use of old techniques and by failing to embrace developments in drilling technology that give a technically superior and better value product.

Gloucester-based Geotechnical Engineering has responded to this, and has just completed site investigations using its new multipurpose rig on a proposed sewer project in Hereford & Worcester.

Conventional cable percussion boring and rotary drilling would have meant huge disruption to residents and commerce in the quiet town of Pershore, nestling in the Vale of Evesham. Both techniques take time to mobilise and set up, are noisy and produce considerable mess, which in the town's narrow streets would have been to put it mildly, undesirable. But this was the only viable option for the contract, until Geotechnical Engineering suggested using its rigs that combine dynamic and rotary sampling to obtain a continuous profile of the ground.

The rigs are compact enough to get through a standard doorway with their rubber tracks retracted, and can be brought to site with all their equipment on a trailer, greatly reducing mobilisation time, and of course, cost.

'The rigs are ideal for this type of work because of their versatility and size,' says Geotechnical Engineering managing director Andrew Milne.

The Pershore resewerage scheme is being carried out by Severn Trent Water for Wychavon Dis- trict Council. Geotechnical Engineering is working for Severn Trent consultant com- pany Haswell Consulting Engineers.

The new sewer pipeline will replace the existing system that is unable to cope with current demand and has caused poor drainage. It will link in with the pump- ing station on the edge of town.

The River Avon flows to the east of the town and as a result the geo- logy is alluvial deposits of sand and gravel overlaying Lias Clay which is weathered towards the top. All the boreholes are 15m deep and penetrate the unweathered clay.

Instead of using a cable percussion rig to bore through the alluvial deposits to refusal and then putting a rotary rig on to obtain core from the Lias Clay, the multipurpose rig can do both, Milne says. Dynamic sampling cuts through the coarse material with rotary coring following on in the clay. The new system allows boreholes to be completed at a rate of one per day.

The system was first developed with UK rig manufacturer Technodrill, more usually associated with mini piling machines, which built the first prototype rig. Subsequent rigs and further development has been carried out with Italian manufacturer Commacchio.

The rig switches between dynamic sampling and rotary drilling by the use of a sliding frame on the mast that carries both an hydraulic jackhammer and a rotary head. The frame can be slid hydraulically into three positions by the driller allowing rapid changeover of systems and also allowing the winch to be

used.

Dynamic sampling uses a high powered hydraulic jackhammer, similar to a stripped-down version of a JCB-type pick, to push large diameter steel tubes at any angle into the ground producing continuous high quality samples from 60mm to 112mm diameter in clear plastic liner.

Conventional insitu testing can be carried out in the borehole and because of the large diameter, U100s can be also be taken, although Milne argues that high quality 70mm diameter samples are just as useful, with the added advantage that they can be used with smaller diameter equipment. All casing, sample tubes and core barrels are compatible.

When the ground becomes too hard for dynamic sampling, the driller simply moves the rotary head over the hole and rotary coring can start. This is a conventional system and coring can be carried out in runs of up to 3m.

The rig-mounted pump is, as with practically everything on the machine, compact and powerful and can handle a variety of flush mediums, including water, polymer muds and even bentonite cement grout. Rotary percussive work can also be carried out if a compressor is used.

And because either system can be selected at any time, it is possible to drill through obstructions and then switch back to dynamic sampling if necessary. This is particularly useful when sampling interbedded soil and rock. 'You get a continuous sample across the critical rock/soil boundary, which can be lost using conventional methods,' says Milne.

He adds that the continuous nature of the sampling removes selection errors by drillers, and the larger diameters combined with the use of core liner allow more representative sampling, with less disturbance and little loss of fines.

Milne says the rig is especially suited to environmental contracts, because samples are less disturbed and are sealed in the core liner immediately the core comes out of the ground, especially important in this sort of work when contact with the air can affect pollutant levels. 'Sub samples can be taken for on site testing and the core liner resealed for further sampling at a later date.'

And because the samples are sealed, site operatives have no contact with contaminatation, an advantage over conventional systems with regard to health and safety issues.

The company fleet currently includes five of these rigs, along with conventional drilling equipment. Two of the rigs have cut down masts that allow them to work on restricted access jobs with as little as 2m headroom.

'These newer versions also have rotating masts that will allow them to drill in 360degrees, even vertically upwards, for bridge work for example,' says Milne.

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