MAIN FEATURE
drain TRADER
that ground type changes along the proposed routewere fully accounted for before starting any tunnelling operations. Again, earlymachineswere often limited to installation lengths of a fewhundredmetres, often determined by the layout of the pipe network being installed and the effective lengths overwhich any guidance systemcould operate effectively, usually laser line and target guidance systems. Todaymostmachine types are used for longer length installationswith the use of inter-jack stations along the pipe string tominimise the jacking forceswithin specific sections of the pipeline so as not to overstress the individual
pipes.They are also being usedwith long length, usually steel pipe, for very long and often larger diameter bores using specially designed jacking/pipe thruster units as an advancement on traditionalmicrotunnelling operations. In general, pipes that can be installed using this technique include concrete, vitrified clay and polymer (GRP/FRP) and steel as they have to be able towithstand the jacking forces
applied.Thesemay be installed as the final product pipe or as carrier/casing pipes for further utilities and services to be placedwithin. With the advent of requirements to install longer and often curved drives the designers of the guidance systemhave also nowdeveloped significantly improved and specialist systems to ensure that the required path be adhered towithinmillimetres on both line and level despite, sometimes,multiple curves along theway. Finally, there has been an option produced butwhich is nownot often heard ofwhich utilises a two-pass installation technique.With a two- pass installation the cutterhead is advanced using re-usable casing sections to complete the jacking process for the initial
bore.This opens the pathway for the product pipewhich essentially can be jacked into place, replacing the jacking casings, at amuch lower jacking force because the need towithstand high jacking forces required to advance
8 drain TRADER | August 2020 |
www.draintraderltd.com
the cutterhead through virgin ground is
removed.The casing pipe which recovered at the reception shaft side of the bore for use on any further installations.
GUIDED AUGER BORING
Asmicrotunnelling developed and became a popular technique for larger diameter pipeline installations. It became obvious that smaller diameters could also be
installed.However, the relatively high cost of doing so using a fullmicrotunnelling layout oftenmade the option less viable. This led to the development ofGuidedAuger Boring, or as it sometimes known PilotAugerMicrotunnelling or PilotAuger Boring. This is however simply another pipe jacking technique.With this method the installation process is usually amulti-stage operation, although over the years various developments have enabled some applications to require fewer stages than the standard process. It also offers a less costly option and usually amuch smaller surface footprint to achieve an installation as comparedwith fullmicrotunnelling. Themore basic formof operation is for a start shaft to be sunk into which the guided auger bore jackingmachine is positioned.Often the design of themachine is such that there is no requirement for a thrust wall to be constructedwithin the shaft as the shaftwall itself is used for this purpose although theremay be a need for a thrustwall if circumstances require it. Themachine is alignedmore or less along the route of the required drive, then using a suitable guidance systema pilot boremade on the line and grade required of the final installationUsually a laser line systemsteering adjustments aremade as necessary tomaintain the route alignment, aswith full facemicrotunnelling. Once the pilot bore is completed this bore is used as a guide for installation of an auger chainwhich enlarges the pilot bore to the size
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