From the magazine, HDD

Pushing the boundaries of thrust boring

Traditionally thrust boring has been seen as a methodology used for bores up to approximately 60-70 m in length, with limited accuracy due to being non-steered. Codmah has six thrust boring rigs, all of which use steering heads and are capable of achieving shallow grade bores. Additionally, with the development of other methodologies, the rigs are capable of installing pipes of far greater length and diameter than traditionally undertaken with this installation method.

AGL Energy Gas Gathering Project, New South Wales

The tender documents for the Gas Gathering Project called for microtunnelling methodology to be utilised, however, Codmah proposed the use of steerable thrust boring rigs and was awarded the project.

Getting on-grade

For the first bore, AGL required a 600 mm steel casing pipe to be installed under the Hume Highway at Menangle. Due to access problems because of private land surrounding the highway, the bore had to be extended to a length of 156 m. The casing also had to be installed on a grade of 1 per cent.

Normally a 10 m launch pit is excavated for the boring rig and 6 m lengths of casing are installed, but due to the length of the bore it was decided to excavate an 18 m pit and push in 12 m lengths of pipe to restrict the time spent welding and installing pipe. The bore was completed on-grade and to the required location and to the satisfaction of the client and the private land owners. The carrier pipe was later installed and the annulus grouted.

For the second bore, 80 m of 900 mm diameter steel casing was to be installed under a rail corridor and surrounding private land on a grade of 1 per cent. As the rail line was not electrified, a concrete casing was not required. Codmah used a steerable thrust boring rig and encountered cobbles of approximately 400-500 mm in diameter during boring. These cobbles were successfully removed and the bore was completed to the desired grade and location. The carrier pipe was later installed and the annulus grouted.

Suir Excavations Balmoral Road Sewer Carrier Project

Due to an environmentally protected area, Suir Excavations required a 200 m section of the project to be installed by trenchless methodology. Initially microtunnelling was considered for installation, but the price was found to be prohibitive. Codmah was questioned about the possibility of thrust boring, but after review decided that traditional thrust boring was not viable due to the length, grade and accuracy required.

HDD and thrust boring combine

The Codmah project team proposed a new methodology where a pilot bore would be drilled with a HDD rig, then the rods would be used to guide the thrust boring rig as it installed the casing. The proposal was accepted by Suir Excavations and Codmah used a Vermeer D80X100 HDD rig to undertake the pilot bore on a grade of 2-3 per cent. The bore also had to intersect three manhole locations. Once the pilot was completed the thrust boring rig was attached to the drill rods, which guided the casing installation. The bore was completed to the desired grade and arrived at the manhole locations.

CLM Caddens Release Project

CLM was awarded the Caddens Release Sewerage Project by JK Williams and identified that a sensitive riparian corridor section required installation by trenchless methodology. This required installation of 108 m of 500 mm steel casing on a shallow grade of 0.8 per cent between manholes under the sensitive area.

Codmah used a steerable thrust boring rig to undertake the works. During installation the ground conditions were extremely changeable, regularly going from clay, to rock, to river sand and back to clay. The crew overcame all these conditions and delivered a successful bore on grade and location.

Perfect pipes at the Old Carlton Brewery

Perfect Pipes undertook the sewerage connection for a new building on the Old Carlton Brewery. The only available connection point to the existing sewer was found to be into a 100-year-old brick oviform pit.

Sydney Water would not allow excavation at the oviform, so trenchless installation was required to get the new pipe from the basement of the new building to the connection location. To achieve the required grade it was necessary to begin the installation through the building wall 2 m above the ground. Codmah proposed to use a thrust boring rig perched on a modified platform with a bin below to capture the returning spoil. The proposal was accepted and the crew successfully completed the delicate installation using great skill and experience.

Thrust boring has advantages over other trenchless technologies which include, being able to contend with changeable ground conditions during boring and the use of retractable heads which, in the event of being unable to complete a bore, means the head and gear can be retrieved at any time, unlike a other trenchless rigs which have to be either excavated to retrieve or else left in the ground. Codmah plans to continue to expand the technology and find alternative solutions to projects outside the norm.

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