Consultation with Rail Infrastructure, Sydney Water and CLM was necessary to develop a method statement that satisfied the stringent requirements to bore under the rail. The railways required a depth of 5 m cover to ensure no movement to the line during the operation, and due to this depth and the exit angle, the bore was designed on a bore planning software package to ensure the profile was acceptable to the railways and the bend radius of the large diameter pipe prior to sign off.
To assist in working to the tolerance a surveyor was engaged to peg out the route and verify the profile. Due to the rail line being in a 4 m deep cutting, the bore being 5 m under the rail and the exit point being only 20 m from the edge of the rail cutting, the exit angle and grade were critical. To complicate the bore further it had a 10 degree bend to negotiate as the exit angle was being reached. Again, the bore planner and experience of the operators were essential to ensure that the tolerances were met and the exit target was reached.
To expedite the project, the pilot bore was originally attempted with a 12 tonne clay machine as the profile of the ground and the geotech report indicated medium strength shale. However the pilot bore fell short of the required depth by 1.5 m when the ground became too hard to drill. UEA’s 25 tonne Vermeer rock drill was then used to complete the pilot bore using a Wenzel mudmotor. Rail track access was needed to locate the bore across the line and Rail Infrastructure provided safety officers to oversee the process. The bore reached the target and upsizing of the hole commenced, starting with forward reaming to 250 mm, then progressive back-reaming till the required diameter of 700 mm was reached. Due to the hard ground conditions this had to be done in 100 mm increments with conventional barrel reamers.
The project was designed so that the pilot bore and back-reaming were all done from the low end of the bore, however due to the exit area being purely residential, there was no room to pre-weld and string the pipe. The pipe was therefore pre-welded in four separate lengths on the entry side of the rail line while the drilling continued.
During the project, a new slip road was being constructed to gain access from the new development to Homebush Bay Drive, further impeding the drill site and access for welding the pipe. Negotiations with the developer allowed for road crews to work in with the drilling and welding operations to try and minimise the disruption for both parties. With the hole complete, the drill rig was demobilised from the entry end and remobilised at the exit end, ready to pull pipe. The pipe was pulled in stages with three separate PE welding operations during the pullback.
All up, construction on this project took 5 weeks to complete, with the line pressure tested, grouted, chlorinated and commissioned in February 2005.
Project Challenges
“¢ Boring under a main rail line
“¢ Tight tolerances to bore due to depth required by rail authority, ground profile and restricted area for exit (3 m wide lane with a swimming pool on one boundary)
“¢ Ongoing construction of a slip road to Homebush Bay Drive which impeded construction
“¢ Harder than expected ground conditions
“¢ Large diameter pipe with restricted area for welding and pipe stringing.