As part of the biggest upgrade to Sydney’s busiest railway station in decades, Directhitt Trenchless were contracted to supply and install four DN 630 HDPE liners and PVC electrical conduits to provide 33KV power to the new underground Metro station.
The boreholes constructed by Directhitt, connected the 27 m deep metro station box level B2 to an existing ghost tunnel, which was built in the 1920s as part of the Eastern Suburbs Railway line for Bondi and the Illawarra. However, it was never completed. Platforms 26 and 27 have sat unused since and will now house 17 new communications and power rooms to support the new Sydney Metro at Central Station.
The 90 m long microtunnells were constructed using an Akkerman Guided Boring Rig, which used a Pilot Tube and Laser Guidance System to accurately drill the pilot hole to line and grade in Class II Hawkesbury Sandstone.
The confined site prevented the use of a typical HDD rig set up, so Directhitt chose to use a Guided Boring rig with a compact 3 m jacking frame.
The Akkerman dual-wall pilot tube design allows the pilot tubes to be installed with pinpoint accuracy while reducing pressures necessary for the long installations.
The annular space between the inner and outer tube was used to pump drilling fluid lubricants to the Tri-Hawk drill bit.
The inner tube is treated for corrosion resistance and sealed for a clear site path to the target located directly behind the steering head and 165 mm drill bit. Once the pilot hole was completed the boreholes were then enlarged to 710 mm before the HDPE liners were fusion butt welded and installed into the boreholes, using the jacking frame. Given the confined site, the DN 630 HDPE liners were welded and pushed into the boreholes one section at a time.
Directhitt then installed the DN 280 PVC Electrical ducts into the HDPE liners and grouted the PVC duct annulas with a low thermal resistivity cementitious grout before final testing and commissioning of the conduits was completed.
The project presented many challenges for Directhitt to overcome, with COVID-19 shutdowns, border closures and managing crews in working bubbles to maintain a COVID-free workplace. Together, with the site access restrictions presenting constant challenges for the crew, Directhitt stepped up to the challenge and completed the project successfully.
The addition of the pilot tube system to Directhitt’s trenchless capabilities allows for the provision of varied and hybrid trenchless solutions utilising the best of grade control and accuracy from the Akkerman system with trusted hole opening technology and drilling fluid management from the HDD industry.
For more information visit Directhitt Trenchless’ website.