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MICROTUNNELLING


Whenmicrotunnelling first became a recognised useful alternative to open trench installation of buried pipelines, the definition of a microtunnelwas a tunnelling systemthat could install a pipeline, using a guidance systemto keep the bore on a predetermined line and grade,which utilised a boring head to excavate the ground, advancing it using a series of specially designed pipes that could be used to jack the cutterhead forward as the groundwas extracted.The forward jacking processwas achieved using a jacking frame installed in the launch shaft.This shaft is usually constructedwith a thrustwall to the rear of themachine to enable the jacking forces to bemanaged effectively and safelywithout damage to the shaft structure.The directional control ofmicrotunneller’s cutterhead is achieved remotely froma surface control cabin, thereby eliminating any need for the operator or other personnel to enter the pipeline itself.Hence the micro inmicrotunnel, thismeaning non-man-entry.The only requirement is for personnel to add newpipe and handle any spoil extraction equipment at the jacking shaft as the pipeline advances. There has always been a variety of options based around this around this initial concept.Whilst somewhat less commonly used today than in its heyday, the first conceptwas the use of an auger stringmounted inside the jacking pipewhichwas extended as each newpipe section was added as the bore progressed.Themain limitation on this system was usually that it could not always operate effectively in high ground water pressure or volume conditions due to open face presented by the cutting head to the excavated ground. So, ground conditions for this type ofmicrotunnelling had to be fairly stablewith a self- supporting tunnel face.


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Today themost commonly utilised option is that of a slurry based spoil removal system.Here themicrotunnel cutterhead is a closed unit which is isolated fromthe jacking pipe behind themicrotunneller shield.Thismeans it can, provided the ground conditions are known in advance, as is commonwith all these trenchless installation techniques, handle higher groundwater pressures effectively.The slurry systemcomprises a recirculating fluid, usuallywater, awater/ bentonitemix or awater/polymermix.The slurry has two functions.As the cutterhead advances, extracting the ground at the tunnel face, the spoil produced is first cut down to smaller sizes to a then allowthe slurrymixture to gather it up and transport it to surface through the pipeline network feeding the cutterhead.The cutterhead usually has some formof spoil size reduction design to enable the slurry to carry away the extracted ground effectively and prevent clogging in the pipework. This spoil slurrymix is then pumped to a surface arrangementwhere the spoil is separated fromthe slurry for disposal.At this point, the slurry is checked for composition and reconditioned according to the project requirements and ground conditions.This compositionmay vary as the drive advances and ground conditions change. Inmany circumstances the contractorwill have on site a specialist slurry (or mud) engineerwho has specific training to design andmaintain the slurry in the condition that offers best effectiveness to the conditions on any one site.The reconditioned slurry is then recycled through the cutterhead to repeat the spoil removal process.Of course, the necessity to have the control cabin, slurry preparation, cleaning and recycling systems aswell as pipe storage at the site does oftenmean that the site footprint of amicrotunnelling arrangement can be fairly extensive. However, careful planning and layout canminimise this. The other aspect of the slurry systemis that by determining the ground conditions, groundwater pressure/volumes etc. the slurry can be designed to act as a face supportmediumaswell as a spoil removal system, something that the auger-based spoil systemcould not achieve.Again, thismay require the availability of a specialist on site to maintain the slurry composition parameters and change themas necessarywith changing ground conditions.


MT DEFINITIONS


Initially,within the international trenchless community, microtunnelling (MT) sizeswere defined by thembeing in the non- man-entry range as previouslymentioned. In various countries however over a number of years this has changed somewhat. It is now common to seemicrotunnelling definedmore by the fact that the installation is achieved using remote control fromsurface rather than by final bore size.This type of full facemicrotunnelling is therefore available, using the rightmachine, for installations of diameters fromas small as about 200mmup to severalmetres depending on the machinemanufacturer, ground conditions and the availability of jacking pipe suitable for the project in hand. Earlymachinesweremainly aimed at the softer soilsmarketswith clays being themediumthroughwhichmany of the installations took place. In theUK thiswas largely because at the horizons intowhich the pipelineswere being installed comprised this type of ground. Itwas the level of groundwater that often dictatedwhether the auger-based or slurry-based systemswere chosen. However, as the industry and themicrotunnelling engineers became more confident in the systems and their own abilities, a variety of cutterheads for other ground types frommixed ground to solid rock were developed. Each project had to be carefully examined to select the right systemfor the conditions in hand,which also required significant investment in pre-project ground investigations to ensure


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