UEA set up the guided boring machine (GBM) within the excavated launch pit and completed the pilot bore by the next day. Ground conditions were well suited to the GBM (displaceable), with thrust and rotation pressures remaining constant up to 60 metres out. However, a problem arose with six metres 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 or an 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 six metres of the pilot bore remaining and the previous 60 metres were 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. This technique, combined with the excavation rate of the augers matching the advance rate of the steel pipes, eliminated over-excavation at the head and ensured that the bore remained supported.
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 60 metre point where the GBM encountered tight ground conditions. This provided UEA with a contingency plan in the event of auger failure – the augers could be retracted, the auger borer removed from the rails, and the remaining steel case pipe rammed into place using a pneumatic hammer.
In an ideal site set-up UEA prefers to install six metre cases at a time to maximise the daily progression rate, ranging from 18 to 22 metres in these types of ground conditions, however a six metre case installation required a 10.5 metre long launch pit. Unfortunately this site could not facilitate such a large pit and as a result the maximum case size that could be installed was three metres, which doubled the welds required and slowed the progression rate.
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, at the point of near-failure. UEA was able 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 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 60 metres which, based on the rotation and thrust pressures encountered, were 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 informed about changes in ground conditions and their impact on the bore’s design. The team’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.