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Pilot tube microtunnelling: a guide

Pilot tube microtunnelling is an on grade pilot hole system capable of installing 100 mm pilot tubes in displaceable ground up to a distance of 140 m in length. Utilising a pilot guided boring machine system (GBM), the technique is predominantly used to install on-grade casing pipes and can be used either on its own or in conjunction with a conventional auger boring system.

Highly accurate, fast and environmentally friendly, the GBM system is suitable for a range of infrastructure projects, especially the installation of on-grade sewer or stormwater lines in displaceable ground types.

Research, development and deployment

The first examples of the GBM system were developed by trenchless equipment manufacturer Akkerman in the United States (US). Following extensive use in the US, the system made its way into Australia.

UEA Trenchless, a division of the UEA Group, employs an Akkerman GBM 240A in combination with a number of different conventional auger boring machines and crane truck for the bulk of its GBM projects. The choice of borer is determined by a number of factors, including ground type, size and bore distance.

Comparing the trenchless techniques

It is tempting to compare the GBM system to other trenchless techniques such as horizontal directional drilling (HDD) or microtunnelling. However, these trenchless techniques differ in several fundamental ways.

First, a GBM is only used for straight online grade critical bores – an operation type that HDD cannot always undertake.

Second, the GBM system is both fast and accurate. A 140 m pilot bore can be completed within one day and to great accuracy levels – up to +/- 25 mm at a length of up to 140 m, which is not an accuracy HDD can guarantee.

Third, because of its displacement of soil as it is thrust forward, the GBM system must be used in displaceable soil and is most suitable for ground conditions up to 10 Mpa. Lastly, no drilling slurry is created with a GBM in displaceable ground.

Environmental benefits

A mixture of bentonite and other polymers, drill slurry is a potentially costly by product of both HDD and microtunnelling machines that must be removed and disposed of at a licensed disposal area or recycled via other means.

When using drill slurry, a construction environmental management plan (CEMP) must be in place to outline how drill slurry needs to be removed from site and how to deal with a frac-out or spillage, should the worst occur. The GBM system, however, avoids this requirement.

With the augmented environmental benefits of the GBM system, UEA Trenchless has established a robust set of environmental objectives for every pilot tube microtunnelling project, including the protection of physical features, surface stability and to control erosion; the protection of water quality; the minimisation of noise and air pollution; appropriate waste management practices; and minimal disturbances (including, where possible, sites of cultural heritage value).

Overview of a GBM pilot tube installation

The GBM pilot tube is 100 mm in diameter and works by displacing and packing soft ground. The rods remain in situ to support this ground until they are replaced by the steel enveloper pipe which pushes them out into the receipt pit. The enveloper pipe is welded into a continuous length, which provides a structural element to support the bore at all times.

The lead pipe has a cutting shoe welded on and the cutting swivel head on the lead auger is situated on the outside edge of the lead pipe.

The head is retractable and tooling can be changed to suit conditions, such as for harder than expected ground conditions.

For softer than expected ground conditions, or running sand, the head can be retracted back into the sleeve to help pack the ground hard against the face as the sleeve advances.

This controls the amount of material removed and the operator matches his advance rate to the excavation rate and prevents over excavation and sand running freely down the auger.
If an area of very soft ground is unexpectedly encountered, the pipe and augers can be thrust quickly forward to prevent over excavation. In areas of high water content and running sands, clays can be reversed along the augers packing up the face and allowing work to be suspended without fear of loss of ground and flooding of the pit.

For bulk water ingress from groundwater or surface bodies, an alternative method of hammering the case through these sections can be employed – or this risk can be “÷engineered out’ by providing more cover via lowing the bore level.

Precise installation

The site of a pilot tube installation measures approximately 10 m long and 20 m wide in the direction of the bore. The launch pit measures 10.5 m x 4 m and is offset 1.8 m left of centre in the direction of the bore and 0.76 m below the centre of the pipe. The receipt pit, meanwhile, measures 2 m x 2 m and 0.5 m below the centre of the pipe.

The pilot tubes are inserted through the ground from the launch pit to the receipt pit. On the leading end of the pilot tube is a steering head with an angled tip. In the steering head is a battery powered LED-illuminated target that is visible to the operator by means of a theodolite, camera and monitor. The theodolite is set on line and grade and is positioned to view the target through the bore of the pilot tube with cross hairs of the theodolite visible on the monitor along with the illuminated target.

The operator rotates the steering head as needed to steer and maintain the desired line and grade.

Angular deflection and avoidance of in-ground structures

The GBM system ensures straight and accurate pipe placement within millimetre-sized tolerances. The head can be pulled back and steered if it is forced off line.

If an unacceptable deflection is experienced due to latent ground conditions, the pilot tubes can be retracted and a new drive started beside the original. This saves the costly removal of augers with enveloper pipe and backfilling of the bore due to a deflection or blockage.

Where known or suspected structures may clash with the design alignment, a test pilot can be made to verify clearance. The pilot can be offset to outside the proposed pipe alignment on one or either side, top or bottom to prove clearance to the structures rather than excavating a full sized bore that would have to be abandoned.

Advancing augers and steel enveloper along pilot path

Once the pilot is complete, the end of the pilot tube is fitted with bearing swivel joint to connect to a splitter head which is fitted to the lead steel enveloper case. The splitter head is sized to overcut the outside diameter of the steel enveloper pipe surrounding the steel pipe to avoid jamming the casing later as it is inserted. Different types of cutting head tooling are available to work in various ground conditions.

Once each auger and section of tube is buried, the machine is disconnected from the auger and walked back along the rail. The next auger and tube is then lowered into place and welded and the process continued until enough augers and tubing are installed to break through into the target pit.

With the addition of each section of auger and tube in the drill pit, a section of pilot tube is removed from the target pit. This process ensures the enveloper pipe is installed on-grade and the cut tunnel is cautiously supported.

Installation and grouting of product pipe

Encased with timber or metal spacers to centralise it on insertion into the enveloper pipe, the product pipe is pushed into the enveloper by the thrust borer or excavator from the launch pit until fully inserted.

For the grouting of the product pipe, a 100 mm flexible feeder tube is inserted at 2 o’clock and a 100 mm flexible breather at 12 o’clock on the upstream end of the annulus between product and enveloper pipe.

The annular space is blocked at each end, with filling foam or mortar, sand bagging and backfill if required. Grout is then gravity fed into the annular space until visible at the breather. Grout quantities are checked against the theoretic annular volume.

Past, present and future

To date, UEA Trenchless has completed a series of significant GBM projects such as a 95 m sewer bore in sandy clay ground conditions in Maitland, a 120 m LPG line bore beneath Charlottes Road in Sydney’s Port Botany, and a 60 m sewer bore in shale ground conditions at Leppington.

Each of these respective projects were completed well within the +/- 25 mm tolerance, with the Maitland project coming within 8 mm of the designed grade, the Port Botany line bore coming within 12 mm and the Leppington sewer bore finished within 10 mm.

One of UEA Trenchless’ most high profile GBM projects was in support of Queensland’s GoldLinQ Rapid Transit Light Rail System on the Gold Coast. UEA installed trenchless sections of steel enveloper pipe to encase DICL water main crossings under the Gold Coast Highway in Broadbeach.

UEA used a GBM to drill a laser guided pilot hole achieving on target accuracy of less than 10 mm in unstable fine sand – conditions that had seen other contractors undermine the main highway through the Gold Coast precinct.

The installation auger followed the pilot bore, pushing out the 100 mm pilot tube and replacing it with the 550 mm steel enveloper pipe. Once the steel enveloper pipe was installed the DICL pipe was inserted using an auger boring machine to jack the pipe through, after which the annulus was filled with an approved grout mix.

Throughout the operation, UEA Trenchless did not experience any undermining or over excavation subsidence on the drives using the pilot tube microtunnelling method. This result gave Baulderstone, the project’s principal contractor, confidence that strict tolerances of grade and safe clearance from existing infrastructure could be constantly met.

Looking ahead, UEA Trenchless is currently working on the South West Growth Centre – a Sydney Water Sewer Scheme. The company has also recently been awarded a contract for four bores at another Sydney Water Sewer upgrade project in Pagewood.

For the Pagewood project, the ground conditions are sandy, which, as demonstrated by the GoldLinQ project, is highly suited to the GBM.

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