Using Retractable Heads for Microtunnelling
Retractable heads and cutters can be bought back to the shaft in case of:
- Obstruction eg filled ground, rock anchors, services, cobbles
- Major change in ground conditions – clay to sandstone
- Vacuum blockage which can be encountered in high plasticity clay or sand
Thus, eliminating the need to salvage a bore via an intercept or head excavation
Retractable microtunnelling heads and cutter are suitable for boring when:
- ‘Freeboring’ (a microtunnelling method that requires no outer casing to support the ground) is not possible due to ground conditions or authority regulations
- Pipejacking a sleeve up to a manhole with no disturbance around the manhole
Retractable Head Methodologies
Retractable Microtunnelling heads can be utilised for both the Pipe in a Pipe, and Close-Fit Pipe Bore Sections. Where required, a temporary outer casing can be used for both bore sections, which is then Pipejacked out later.
The Pipe in a Pipe Bore Section methodology is shown as an example below. Clicking the blue buttons below each bore section will allow you to view the other methods for each section.
+ Methodology shown below is for the Pipe in a Pipe Bore Section
The retractable microtunnelling head is launched from the entry shaft inside the first sleeve.
02. An Obstruction Encountered
In the event that an an obstruction is encountered, ground conditions change or the head requires repair – the microtunnelling head can be retracted – while the outer casings remain in place to support the ground.
03. Situation Remedied
Once the Obstruction has been removed, the Cutters replaced to match ground condition or the microtunnelling head has been repaired – the microtunnelling head is pushed back to the face and microtunnelling recommenced.
04. Bore Completed
The microtunnelling head can be removed via the exit Shaft OR retracted back to the launch shaft. Outer casing is now in place.
05. Slipline the Carrier Pipe
The carrier can is now sliplined into the outer casing and the annulus grouted as required. Microtunnel complete.
+ Retractable Microtunnelling Head Projects
Ground Condition Change
A change of ground condition from Clay to Shale at this RMS Road crossing in Marsden Park required the microtunnelling head to be retracted. The cutters were replaced and microtunnelling recommenced with minimal project downtime.
Retractable Heads were used on both bores at the simultaneous Rail Crossings at Wyee on the NSW Central Coast during one 48 hour weekend shutdown. Both 440mm diameter bores involved pipejacking a DN427 GRP Jacking Pipe.
No Exit Shaft
A 26m, 830mm diameter tunnel with no retrieval access available at the exit end was required at this project in Austinmer. Retractable Cutters were fabricated, with the design enabling the microtunnelling head to be retracted to the launch shaft after breakthrough at the exit end leaving behind the carrier pipe.
Into Manhole Base
Retractable Cutters and laser guided accuracy were utilised to core into a manhole base on this project at Pymble. After microtunnelling 19m in clay and fill the head was retracted back to the launch shaft. A corer then replaced the cutters and was used to break into the manhole.
Book a Presentation
For futher information on Retractable Microtunelling Head Capabilities and Methodologies book a presentation for your design team with Pezzimenti today. Call Scott Wells on: +61 (2) 4735 6676 or email firstname.lastname@example.org