In 2019, Downer Pipetech was engaged by Sydney Water to install 870 m of 226 mm uPVC sewer main for the future developments in the Spring Farm residential area of New South Wales.
With the project requiring the completion of 10 under bore sections, Downer engaged auger boring specialist Bortec Laser Bore from Narellan NSW, which uses machines from expert German supplier Bohrtec.
During consultations with experts from Germany, it was noted that geotechnical reports indicated displaceable soft sand, non-displaceable stiff clays and soft sandstone would be encountered.
These ground conditions suited either the Bohrtec pilot tube system for the displaceable soil, or the Bohrtec 406 mm FrontSteer system for the non-displaceable ground.
Although Bortec Laser Bore possessed both a Bohrtec BM 400 LSC and a Bohrtec 400 LS, it was decided the LSC machine would be better suited to the launch shafts due to its more compact design.
While the project proponents had top-quality Bohrtec machinery at their disposal, the works were not without their challenges and a range of equipment was required.
The required bore lengths ranged from 69 to 111 m and encompassed a total of approximately 830 m of boring across the project, while the launch shafts ranged from 6 to 13 m in depth.
Due to changing ground conditions, the pilot tube method was used in the softer displaceable soil, and the FrontSteer method was used in the harder non-displaceable clay and soft sandstone areas.
This method has both vertical and horizontal steering capabilities with an accuracy of up to ± 25 mm for drive of a length up to 100 m.
The connection point for the sewer was into an existing sewer access chamber 13 m deep in the middle of a roadway.
Bortec and Downer discussed the possibility of directly boring into the existing manhole and eliminating the excavation of the roadway to a connection, and Downer agreed to set up a platform in the existing access chamber in the roadway and internally core out a 600 mm core hole where the bore was to come out.
Bortec was able to use the pilot system to move through the soft sand and, with the use of the optical guidance system, could come out directly in the centre of this core hole.
Afterwards, the hole could be upsized and bored in the 406 mm steel pipes.
Manholes up to 13 m deep were constructed with tight access, with flat grades ranging from 0.45 per cent to 0.65 per cent with minimal tolerances between the bores.
A DN 406 mild steel casing was installed, with the DN 225 uPVC carrier pipe then inserted inside.Centralisers were also installed at 3 m minimum intervals, while the annulus between the uPVC and steel casing pipe was grouted.
Vacuum testing and CCTV of the installed sewer were conducted and all spoil and slurry from the launch pit was removed.
The project was completed successfully and within budget, with no recorded safety incidents, while the connection made to the existing access chamber was made without the need to excavate the surrounding roadway.
Thanks to the technical assistance from Bohrtec, the optical guided system was successfully employed when using either the pilot tube or FrontSteer systems in order to maintain grade and alignment as per design.
For more information visit the Bohrtec website.