These works formed part of a diversion and decommissioning project for one of the Balmain sewage pumping stations in metropolitan Sydney.
UEA Trenchless was contracted to install a 600mm mild steel enveloper pipe for a 66m bore with a grade of 1.77 per cent. The project was completed using the Akkerman Guided Boring Machine combined with the McLaughlin Thrust Boring Machine.
Geotechnical information indicated the ground conditions were sandy clay; light grey with red-brown mottling; of high plasticity, with medium to coarse grained sand (Alluvium). These ground conditions are conducive to guided boring, which requires material to be displaceable
Solution and Challenges
Stage 1: The Pilot Bore
UEA set up the guided boring machine (GBM) within the excavated launch pit on 9 April, 2014 and completed the pilot bore by the next day.
Ground conditions were well suited to the GBM ie. displaceable, with thrust and rotation pressures remaining constant up to 60m out. However, a problem arose with 6m remaining when the thrust pressures rose to their maximum limit preventing UEA from thrusting the pilot tubes to completion on-grade.
To determine whether the GBM had hit impenetrable ground, an object or other obstruction, UEA pulled back the pilot tubes on numerous occasions and following the withdrawal resumed with reduced thrust speed. At each attempt UEA gained a small advance on the pilot bore’s distance. This method confirmed the problem was tight ground and as the ground was no longer displaceable, UEA could not guarantee or maintain the desired grade following the current method of thrusting the pilot tubes to completion.
After review, UEA decided to thrust the pilot tubes to completion under constant rotation as there was only 6m of the pilot bore remaining and the previous 60m had been installed within the allowable tolerances of +/- 25mm. This was the only way UEA could guarantee accuracy of the pilot bore to target.
On completion, a survey of the pilot bore confirmed that it had finished over target by just 37mm high at the receipt pit location. The main reason for this was the unforeseen change in ground conditions at the 60m point.
Thrust Bore and Steel Case Installation
UEA set up the Auger Borer in the launch pit at 40mm higher to counteract the surveyed level achieved after pilot boring.
To undertake the thrust bore and steel case installation within these ground conditions, UEA choose to fit a splitter head to the lead case and set the lead auger back 50mm from the end of the case (see photograph of Splitter Head).
This, combined with the excavation rate of the augers matching the advance rate of the steel pipes, eliminates over-excavation at the head, which ensures the bore is not left unsupported at any time.
Precautions against bore failure
UEA opted to install a thicker walled case that would be structurally welded should rotation and thrust pressures fail at the 60m point where the GBM had encountered tight ground conditions.
The use of a thicker walled case, structurally welded provided UEA with a contingency should the auger bore fail.In the event of auger failure, the augers could be retracted and the auger borer removed from the rails and the remaining steel case could be pipe rammed into place using a pneumatic hammer.
In an ideal site set-up UEA prefers to install 6m cases at a time where they can maximise the daily progression rate – which ranges from 18m to 22m in these types of ground conditions, but to install a 6m case requires a 10.5m long Launch Pit.
Unfortunately this site could not facilitate such a large pit and therefore the maximum case size that could be installed was 3 metres, which doubled the welds required and slowed the progression rate.
UEA adjusted its scheduled completion and advised the client of an expected 12 to 15m per day progression rate. Due to heavy rain, ground collapse and the Easter break, a suitable window to install was determined to mutual satisfaction.
Anticipation rewarded with success
As envisaged, during the case installation UEA encountered extremely tough ground conditions at 60 metres. This lifted the auger borers’ thrust and rotation pressures to its maximum – and the point of near-failure. UEA persisted to the best of its capabilities to avoid retracting all the augers, and managed to complete the bore with an average progression rate of 14 metres per day.
The site was surveyed on 8 May and it was confirmed the bore was 40mm high within the Launch Pit and 115mm high within the Receipt Pit. The height increase was again caused by the ground conditions at 60m which, based on the rotation and thrust pressures encountered was over and above what had been reported as part of the geotechnical information at the time of tender.
However, the combination of UEA lifting the auger boring machine 40mm at the launch pit and installing a slightly oversized steel casing during operation, enabled the design grades and inverts to be successfully achieved.
UEA ensured the client was kept up to date about changes in ground conditions and their impact on the bore’s design. UEA’s ability to re-design the machine set-up allowed all parties to deal successfully with the alignment issue and ensured the product pipe was installed within the agreed tolerances.