As part of South East Water’s Backlog Sewerage Program the Balnarring Beach Sewerage Project, located in Victoria, was constructed. The design involved the construction of a sewer through one of the beachfront properties, from the beach to the first inland street where a sewerage pump station was being built.
After negotiations with a number of the beachfront property owners proved fruitless, it became apparent that pursuit of this option was likely to involve expensive and protracted legal avenues with no guarantee of success and assured bad public relations.
Review of the design indicated that it was feasible to construct a connecting sewer along the top of the beach, a distance of approximately 150 m to the nearest street with direct access to the beach. There was sufficient room between the property boundaries and the timber sea wall at the top of the beach. However, to construct this section of pipeline using conventional open cut construction methods would destroy a significant stretch of well-established foreshore vegetation and tea tree and incur the displeasure of the whole community, the Department of Natural Resources and Environment and the local council.
But there was light at the end of the pipeline – Trenchless Technology!
In late 1998 a directional drilling company, Blaxland Pacific Pty Ltd, now AHD Trenchless, had successfully undertaken a tricky directional drilling job for us, installing a 77 m sewer under a service road adjacent to Nepean Highway at Olivers Hill, Frankston. Blaxland were approached with a proposal to construct the sewer along the beach using directional drilling. Over the following months, numerous discussions took place and preliminary works were undertaken. A proposal was developed and submitted for approval. It was a gamble, given the ground conditions and the criticality of the grade of the pipeline, but the alternatives could have significantly delayed the project.
Anthony and Michael Doherty, from Blaxland Pacific, undertook detailed investigation into the blend of drilling fluid required for drilling in beach sand below sea level and arranged for the importation of an electronic drilling head sond (transmitter) from the USA, which was accurate to 0.1 per cent grade. This is sufficiently sensitive to register a change in grade equivalent to the thickness of a business card slid under one end of the pipe. This was required to monitor the drill head with sufficient accuracy for the specified sewer grade.
Care had to be taken during the drilling operation to prevent excessive heat build up in the drilling head, which could destroy the electronics in this $A7,000 item. Five vertical exploratory bores were carried out along the pipe route to verify that the underlying rock reef, which was exposed at various points along the beach at low tide, was below the pipe level.
The pilot bore drilling commenced at a distance of 30 m back from the start of the pipeline in order to get down to depth, on grade and on line. An excavation at the upstream manhole location verified that the start of the pilot bore was within the required pipeline tolerance.
Drilling continued, but within 20 m Murphy’s Law intervened when the drill head struck a rise in the reef midway between the location of the first two exploratory bores. By slowing the advance rate, the operators were able to trench through the top of the reef.
After that progress was steady and smooth. The depth, location and grade of the drilling head sond was monitored at the surface using a hand receiver (digitrac) and stepped string lines, set up between level pegs along the pipeline alignment, which were prepared in advance by our Project Inspector and our Survey Services group. The drilling head eventually passed the downstream manhole location and was curved back up to the surface, 30 m from the start.
During the drilling, the resulting drilling fluid slurry flowed back into the excavation and was continuously drawn off using a vacuum tanker ensuring no spillage to the beach. The drilling fluids used were all biodegradable thereby ensuring that any accidental spillage would not cause any lasting damage.
Concurrent with the pilot bore drilling, the 12 m lengths of 180 mm diameter PE pipe were being fusion-welded together into a 156 m length, strung out along the beach ready for the next stage of the work.
Enlarging the pilot bore and drawing the PE pipe through was the last stage of the work. This involved removing the drilling head from the end of the drilling rods and attaching a reamer slightly larger than the pipe diameter to which the pipe was coupled behind using a universal joint, so that pipe doesn’t turn with the rotating reamer.
The reaming commenced early as it was important to complete the pipe installation with minimal interruption, to reduce the risk of the pipe friction resistance building up to the extent that either the pipe fail, or the drilling machine no longer able to provide enough thrust. When levels were taken on the finished pipeline, it was found that at the downstream end the pipe was 13 mm low, a brilliant result. At the upstream end however, the pipe had lifted more than 500 mm, which was totally unacceptable.
Excavation at a distance of 14 m downstream revealed a pipe level 65 mm above designed pipe level, which was acceptable. It proved fortuitous that the design had been extended some distance past the trees so no damage resulted from the problem. On reflection, the 26,000 pound thrust applied by the drilling machine had caused the pilot bore hole to keyhole upwards in the sand where it had curved up to the drilling machine at the surface.
The finished pipeline met all objectives, doubled the distance of previous directionally drilling undertaken for South East Water and in addressing the challenges of grade accuracy, site geology and direction changes at the start and finish of the line, shifted the benchmark towards the top of the difficulty rating.
