|BALNARING BEACH - TRENCHLESS TECHNOLOGY
As part of South East Waters Backlog Sewerage Program the Balnarring Beach Sewerage Project is being 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.
Back to the drawing board! 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 metres 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 late1998 a directionally drilling company, Blaxland Pacific Pty Ltd. had successfully undertaken a tricky directional drilling job for us, installing a 77 metre 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 USA, which is accurate to 0.1% 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 $7,000 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 metres 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 metres "Murphys 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. It was a strange sensation standing on the sand listening to the drill head crunching across the reef four metres below. 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 metres from the start.
Mark Scheide from the Adelaide company, Independent Drilling Fluid Services, was on site for the first two days to provide technical advice. The drilling fluid used for the pilot bore was primarily Bentonite (refined clay) with a polymer additive mixed with water. 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 metre lengths of 180 mm diameter polyethylene pipe were being fusion welded together into a 156 metre length strung out along the beach ready for the next stage of the work.
Enlarging the pilot bore and drawing the polyethylene pipe through is 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 is coupled behind using a universal joint, so that pipe doesnt turn with the rotating reamer. The reaming was 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 fails, or the drilling machine is no longer able to provide enough thrust. The drilling fluid was modified with powdered wheat husk being added to the blend. During the day there were phone calls between the site and Adelaide, checking progress and modifying the blend. After the intervention of "Mr Murphy" a few more times with the newly reconditioned slurry mixing pump failing and debris jamming valves in the drilling machine slurry pump, the auger eventually appeared in the bottom of the upstream manhole excavation 12 hours a fter starting.
The 30,000 litres of slurry removed during the project was disposed at the local Sewage Treatment Plant.
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 500mm, which was totally unacceptable. Excavation at a distance of 14 metres 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 job was not without its humour. At one stage the Foreshore Ranger advised that he had received a complaint about camping on the beach. The fusion welding equipment being used on the beach was enclosed in a small tent to ensure that the welded joints were not contaminated. We are not sure how the complainant explained the 140 metres of black polyethylene pipe emerging from the tent. Some time later an elderly couple strolling along the beach, mystified by the sight of the huge black pipe silently, slowly, disappearing into the beach, were relieved to come upon the drilling machine 200 metres along the beach as the explanation.
The finished pipeline meets all objectives, has doubled the distance of previous directionally drilling undertaken for SEWL and in addressing the challenges of grade accuracy, site geology and direction changes at the start and finish of the line, has shifted the benchmark towards the top of the difficulty rating.
Article supplied byRoger Bartlett - Project Management EngineerSouth East Water Limited
Last Updated on 23 April 2000