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Trenchless lowdown

Given the high cost of reinstatement associated with conventional trenching, the case for trenchless technology is increasingly compelling. GE looks at some of the techniques.

The terms horizontal directional drilling and guided boring are becoming interchangeable, although historically there was a difference between the two.

Directional drilling was developed by the oil industry for installing pressure pipes below river crossings.The technique used large rigs to drill a steerable pilot bore below the river and, by backreaming, upsizing the hole a number of times until it was sufficiently enlarged to allow product pipe to be pulled in.Drives of more than 1.5km and up to 1.2m diameter have been carried out.

Small and medium size versions of these rigs are increasingly used to install utility pipes below roads and railways, and sometimes airports and canals, with drive lengths of 50-150m.This is probably the most active sector of the trenchless market.

Recently some contractors have attempted to install gravity systems using directional drilling, but this is considered to be at the limit of the method's accuracy.

Most directional drilling systems use bentonite, or polymer drilling fluids to support the bore during installation, but a couple of systems are dry, relying on compressed air.

Development in the UK has been slower than in some other countries.

In Australia, for instance,70% of new telecommunications installations use directional drilling, and in the US about a third of gas pipes are installed trenchlessly.

North America is by far the largest directional market.

There are estimated to be 5,000 drilling rigs operating in the US and Canada and the figure is rising by 2,000 machines each year, against an estimated world total of 8000 rigs. In Europe, Germany is the most active market followed by the Italy, UK, Ireland and France.

Recent developments include down the hole hammers that allow directional drilling in rock, using small and midi rigs. Big improvements have also been made in integrating ground probing radar and 3D mapping programs to help plan routes.

However, to most directional drilling contractors a 'site investigation'means establishing the whereabouts of utilities so as to avoid them. There is seldom any consideration of soil type and properties.

A number of unsteered displacement systems are available for short drives. House connections and other short drives are carried out using relatively inexpensive unguided systems.

Auger boring uses a rotating head to excavate the soil which is transported by auger flights operating in a casing to the drive pit. The head is recovered at an exit pit or in the trench cut for the adjacent length of pipeline.

Impact moling, in which a percussive mole is launched from a drive pit to displace the soil and form a bore, is widely used. The new conduit is normally drawn in behind the mole.

Pneumatically driven moles, in which the soil is displaced by the action of a percussive piston, have been developed in the range 30-80mm diameter, with repetitive multiple passes to achieve 250mm diameter.

Moles are commonly used by gas and water undertakings for installations of up to 40m long and for making 45-65mm diameter house connections.

Rod pushing is a technique in which a bore of about 50mm diameter is formed by displacement. A rod is advanced by a straight hydraulic push and the pilot hole may be back-reamed to the required size. The technique is used for the installation of pipes and conduits up to 150mm diameter over lengths of 30-40m.

Pipe ramming and thrust boring are similar processes where a casing, usually steel, is driven through the ground to the exit pit.

In pipe ramming, the casing is driven by a pneumatic hammer while in thrust boring it is advanced by a straight hydraulic push. For smaller diameters the casing may be closed, but in the larger sizes spoil is removed from the open-ended casing by compressed air and water.

Pipe bursting is a method for replacing and upsizing services by running an impact mole with a bursting tool down the existing pipe. This pushes the old pipe away and allows the new one to be pulled into the enlarged hole. In its infancy, the technique suffered because in many contracts pipe bursting was carried out at shallow depth, damaging pavements.

Microtunnelling used to mean remote controlled mechanised tunnelling of non-man entry size, but is increasingly used to mean remote controlled full-face mechanical tunnelling of any diameter. Apparently the term pipe jacking has bad connotations in the US.

Microtunnelling machines are the most sophisticated end of the trenchless armoury and there are few international equipment manufacturers and a relatively small number of contractors operating in any one market.

Geotechnical conditions are generally better understood on microtunnelling projects since, as in conventional tunnelling, the cutting head has to be carefully selected to deal with the expected ground conditions, with the appropriate cutting tools and crushing devices for the range of gravels, sands, silts, and clays.

Spoil may be removed from the face by an auger or slurry and it appears slurry machines are now dominating the market.

Microtunnelling machines are generally operated from a control console in the cabin at the surface. Accuracy in driving usually depends on the skill and experience of the operator.

Machines are now available to drive 100m or more in soft ground for sizes of 100mm diameter upwards, from drive shafts of less than 3m diameter.

Use of laser guidance control systems and computer monitoring, allied to steering capability, achieves a high degree of accuracy, typically within +/-25mm.

Microtunnelled sewers are often installed to a higher degree of accuracy than those laid in conventional trenches and the pipes are more evenly stressed.

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