SCOPE OF WORKS
- 66 metre bore at 1.77% grade
- Install a 600mm steel casing for the client to install the carrier pipe inside
Geotechnical information indicated the ground conditions were sandy clay – light grey with red-brown mottling, of high plasticity, and with medium to coarse grained sand (Alluvium). These ground conditions are conducive to guided boring, which requires material to be displaceable.
UEA set up the GBM within the excavated launch pit and completed the pilot bore. 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.
UEA decided to thrust the pilot tubes to completion under constant rotation, as there were only six metres of the pilot bore remaining and the previous 60 metres were installed within the allowable tolerances of +/- 25mm.
THRUST BORE AND STEEL CASE INSTALLATION
To undertake the thrust bore and steel case installation within these ground conditions, UEA chose 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.
CHALLENGES & COMPLETION
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 augers and managed to complete the bore with an average progression rate of 14 metres per day.
UEA ensured the client was informed about changes in ground conditions, and any 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.