During an installation, microtunnellers are managing the machine through a variety of ground conditions – soft/hard, sticky/flowing, dry/wet, highly abrasive/highly elastic – all of which are constantly changing and effecting the drilling method. Because of this, pipe selection plays a major role in reducing risk, enhancing accuracy, reducing cost and the overall success of installation. There are a number of options available, and the best option will depend on the prevailing ground conditions.
CC-GRP jacking pipe
CC-GRP jacking pipe would be the preference for most contractors, as it offers a narrow wall with maximal strength and minimal skin friction. The pipe is manufactured with tight tolerances which allow for the accuracy of the installation to be maintained into the final product.
RCP
RCP offers a range of jacking pipe which allow for high jacking loads. The different manufacturers offer slight variations in both tolerances, specifications and wall thicknesses, all of which have an impact on installation costs. The increased weight and higher skin friction increases the required jacking load for installation, which in turn, places greater force on the jacking shaft.
HDPE
HDPE is a pipe typically used for HDD installation and offers low skin friction. This pipe, however, requires a trail out trench which is detrimental to the shaft construction and the overall advantage of the keyhole pipeline. Furthermore, the need to string out the line to reduce installation time would limit the jobsites where this pipe would be practical to install. The limitation of force that the pipe can manage also creates some issues with the minimal overcut common in microtunnel installations. The flexibility of the pipe is ultimately detrimental to ultimate precision.
PVC and PP pipes
PVC and PP pipes are typically limited to either being installed within a casing pipe or requiring consistent, self-supporting ground, as the pipes in their typical format do not allow for jacking, and can deform the joint under load. Furthermore, these pipes have collars and typically require centralisers to eliminate collar wear. Also, with the variation in outer profile, most contractors would also recommend grouting the annulus in order to prevent pipe deflection over time due to point loading. The issue is still present when installed within a casing pipe, however there is also a tolerance stack from installing two pipes then centralising and grouting. There are some new options available in PVC that allow for some jacking and pullback without collars. This does create numerous advantages in terms of accuracy and also reduces the installation risk of PVC significantly in microtunnel styled installations. At this stage, these pipes are limited in diameters available – please check with suppliers.
About Stuart Harrison
Global microtunnelling pioneer Stuart Harrison is the Managing Director of Edge Underground, where he specialises in on-grade microtunnelling installations with millimetre accuracy. Stuart is also the inventor of the Vermeer AXIS Guided Boring system, and he is constantly working to improve the effectiveness of this and other trenchless systems used in the installation of gravity sewers. To discuss your next microtunnelling installation, contact Stuart on 0458 000 009 or at stuart@edgeunderground.co.
