The project, undertaken by UEA, involved the HDD construction of a 1.2 km, 355 mm PE rising sewer in an environmentally sensitive area within the Parliamentary triangle in Canberra.
The main risk with the HDD component of the project was the grade of the main. The bore path was designed to accommodate existing services and keep to a depth that would avoid known rock outcrops.
Further, as the project was only close to Lake Burley Griffin, ground water was known to be a potential issue. The rising main therefore needed to be constructed at -1.5 per cent grades for the first 450 m to the scour valve and then +0.9 per cent for the remaining 750 m.
To compound the difficulty of the project the bore was designed on a curve of 30Ëš over 150 m during the first 450 m to the scour valve, then a further curve of approx 30Ëš after the scour valve, adding a challenge as the grade at this point was down to 0.9 – 1.1 per cent. To meet these specifications the client limited the project tolerances to +/- 150 mm in the horizontal and +/- 30 mm in the vertical.
The survey also plotted the point at which the main grade went from – to +, this was to be the location for the intended scour valve.
Bore construction
To undertake this project UEA used its 25 tonne Vermeer D50 x 100 HDD. This drill has a relatively small footprint for its power and its ease of operation made it ideal for the project.
The bore could not be completed in a single shot due to design restraints, so UEA broke the project into four smaller achievable bores ranging from 220 to 400 m. The site at which the bores could be positioned was restricted by the location of trees, roads and pedestrian access.
The first bore was just over 300 m at -1.2 per cent grade with a gradual curve of 30Ëš in the horizontal. The target was reached and the drill head depth surveyed in the exit pit prior to any further up-sizing taking place.
As the ground at chainage 300.00 was so soft due to the water table, to overcome a possible drop in invert of the pipe, the drill crew and UEA’s engineering team developed a reamer that would support itself and not cut into the invert as the reaming process took place.
This became typical for most of the bore where minimal pre-reaming was carried out and the product pipe was installed with few passes or conditioning of the bore hole.
The second bore was to go through the deepest point of the project and included the scour valve. From a timing perspective this section was the most critical, as the project only had a 12 week construction period, and there was a chance that the contractor would need most of that time to build and test the scour valve.
The bore had to bottom out at exactly the right place and on line, as the scour valve area was constrained by the head wall of the open storm water channel, a road and a tree overhanging the site.
Once past the scour section the bore then had to negotiate a similar curve to the first bore but on a slightly flatter grade.
This bore was achieved in less than half the time of the first bore due to refined work methods and familiarity with the ground conditions on the first bore. Once the scour section of the bore was complete the remaining work was installed with few issues.
Commissioning the main
Once the electrofusion and other joints were completed and the scour valve commissioned, the main went through final testing.
At this point it became apparent that there was an issue with the transitional connections between the PE main and the scour assembly. A redesign and prefabrication of the connection to the valve chamber overcame the problem and the main was passed.
Initially the rising main was to be connected straight into a larger carrier main, but due to a bank of Telstra conduits being constructed on top of the main, a further redesign was necessary before the final commissioning could take place. This involved the repositioning of a manhole and the fabrication of a stainless steel oblique connector.
The success of this project resulted from the “˜can-do’ approach of all parties involved. The design was well thought out, the job was well planned and the entire team worked well together to achieve a successful result from what was initially a very risky and technically difficult project.