Microtunnelling

Microtunnelling is the technique for installing small diameter pipelines to accurate line and level using remotely controlled miniature tunnel boring machines.  The term Microtunnelling generally applies to diameters up to and including 1200mm.  The method provides a structural, watertight, finished pipeline as the tunnel is excavated.

Hydraulic jacks within the drive shaft are used to push specially constructed jacking pipes through the ground.  At the front of the line of pipes is the Tunnel Boring Machine (TBM).  The TBM excavates the ground allowing forward progress of the pipeline.  Excavated material is transported back through the jacked pipeline to the drive shaft and removed from site.  The majority of microtunnelling work is carried out using slurry TBM’s.  These offer the ability to work below the water table, using the slurry as a transport medium for the excavation arisings and to balance the ground water pressure.  Excavated material is separated from the liquid by a specialised plant situated at ground level.  The liquid is then recycled, being sent back to the TBM and forming a closed loop system.  This method of excavation also enables construction of pipelines within challenging ground conditions without any type of pre-treatment or ground water control.  This type of TBM can also be configured to work in rock.

An accurate alignment is maintained by following a laser set to line and grade.  The TBM is steered to the laser line which is displayed on a screen in the control room, situated at ground level adjacent to the drive shaft.  Most of the functions of the microtunnelling operation are computer controlled or assisted.  Pipeline materials are typically pre-cast concrete or extra strength clay, although GRP may be used in specific circumstances.  Drive lengths vary depending upon excavation technique, ground conditions and pipe diameter, although few drives are constructed much in excess of 100m length.

Another of the advantages provided by microtunnelling is the minimal ground movement caused during installation.