Microtunneling Pipejacking

Length of a pipejack

The length of a pipejack is dependent upon variable factors such as the stability and friction characteristics of the geology, the self weight and strength of the pipes, the diameter of pipe, the type of excavation method and the available jacking reaction. Major restrictions can come from the type of ground and the ground water characteristics. Various techniques can be employed to reach an optimised distance.

Intermediate jacking stations

To redistribute the total required jacking force on the pipeline, smaller intermediate jacking stations (interjacks) can be utilised between sections of the tunnel liner.  The interjack separates the piping and due to the friction caused from the launch shaft behind the interjack, the front section of piping is forced forward using a considerably lower force. Interjacks therefore reduce the loads that are transmitted through the shaft structure. This is particularly useful when ground conditions at the launch pit are at a poor or low inherent strength.

Minimizing friction

As the length of the drive shaft increases, so too does the friction of the ground around the pipe. In order to minimise friction, the machine is designed to overcut and produce a small overbreak to the external diameter of the pipeline. By injecting an ecologically friendly lubricant, such as bentonite and/or polymers, into this gap the pipeline can be jacked more readily through a fluid medium. The friction can be reduced however it can never be eliminated as in practice fluid loss may occur. This technique does result in considerable reductions in jacking forces and hence allows for longer jacking lengths.

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