Contractor case study: Pilot auger boring success
When specialist contractor Conwell Contracts was brought in to assist on a drainage scheme in Carlow, Ireland, it proved a memorable challenge.
As part of the ongoing €19.5m Carlow Town Surface Water Drainage Scheme, Flood Relief Scheme Phase A and Water Conservation Project Phase 2, which started in May 2010, a section of 950mm diameter sewer pipeline was required to be installed over a 40m length in a very difficult access location.
Positioned in a narrow side street in Graiguecullen in Carlow, the location was deemed to be inaccessible using the open cut techniques previously utilised on significant tracts of the project. Given the limited space and a high density of existing other services that would have had to have been negotiated using the open cut option, it was decided the pipeline could only be installed using a trenchless technique.
Main project contractor Wills Bros brought in specialist contractor Conwell Contracts early in the planning phase for the 40m long section to offer advice on potential alternatives. It was decided the best option for the Graiguecullen works was to utilise the pilot auger bore technique for the installation.
Pilot auger boring is an established technique but is usually applied on installations of up to 600mm diameter. The 950mm diameter requirement for the Graiguecullen installation meant the project would bring with it significant challenges even for experienced contractors.
Conwell employed its Perforator PBA155 model pilot auger boring system equipped with the Big Bore System (BBS) to complete the installation. Supplied by Sheffield-based Perforator, the company also provided an establishment crew as this would be the first time that Conwell had undertaken a project of this diameter with the BBS.
As with all pilot auger operations the installation required construction of both a start shaft to accommodate the boring system and a reception shaft at the end of the bore route. Whilst the reception shaft was easy to access − it was located at a junction of several of the surrounding narrow streets − the start/launch shaft was a different proposition. Both launch and reception shafts were to be sunk as sheet pile coffer dam constructions.
Located between a residence and a block of flats in a narrow street, the start shaft was difficult to excavate because the manoeuvring room for the excavator used was limited and the machine could not complete a full 360° swing.
After initially sheet piling the required area and opening the shaft to an initial depth it became impractical to use a large excavator to complete the shaft sinking work. A mini excavator was used to complete the dig with the larger excavator simply removing spoil from the centre of the excavation.
The shafts were sunk to a depth of about 4.3m, with the start shaft having a concrete base and a rear thrust wall included as part of the construction. At the front of the shaft where the bore would start, a concrete wall was installed with a circular access for the boring machine to pass through.
Once the shafts were prepared the boring system’s jacking unit was positioned on the floor of the launch shaft and anchored into position on the approximate line and level of the required bore. Establishing alignment prior to commencing the boring operation minimises the requirement to apply major steering adjustments to the boring head as the pilot bore takes place, such that steering adjustments that do take place are simply to keep the pilot head on or as close to the desired alignment as possible.
The completion of the pilot bore went smoothly with the pilot head arriving on target at the reception shaft. Once completed the pilot bore acts as the guide for the upsizing of the bore to the final desired diameter for the installation of the final product pipe.
To achieve the bore upsize the auger head and hollow stem auger string were placed inside steel pipe which was advanced along the bore path a section at a time with each subsequent steel pipe length being welded to it preceding length once it was buried in the ground by advance of the jacking frame. This process was repeated until the auger head arrived at the reception shaft.
Once this second phase of the installation was completed the auger string was removed from the bore at the launch shaft using the jacking frame to retrieve the auger sections.
To complete the installation each 2m long product pipe was placed in the jacking frame and pushed into the bore. As the product pipe advanced the steel casing pipe installed during Phase 2 of the pilot auger process was extruded into the reception shaft where it was cut and removed.
The next length of product pipe was then placed in the jacking frame pushed into the bore. This process was continued until the lead product pipe arrived at the reception shaft. The installation was then complete.
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