I noted that the Missouri Department of Transportation has seen a gradual sea change in small diameter borings for electrical services. Twenty years ago they were writing contracts where HDD could only be used with written authorisation from the project engineer when interconnecting intersection signals and lighting. Ten years ago, the option was made available for the contractors to choose between open cut and HDD in their bidding. Today, nearly all of this work is done with HDD.
The owner of Trenchless Flowline Inc. also advised that the price for pipe bursting and cured-in-place-pipe is now 60 per cent less than the open cut price, but at the start of each of these technologies the price per metre was equal to or more than the open cut price.
Their experience after installing over 60,000 feet of close tolerance HDD (CTHDD) over the past 14 years is that this method can be used for half the combined open cut price, whilst providing all the trenchless benefits and meeting the open cut specifications. CTHDD, which is also referred to as ArrowBore directional boring, or guided horizontal boring for gravity mains, was developed to match open excavation methods. It is understood that the tight fit bored hole allows CTHDD to install large diameter pipe at very shallow depths and prevents pipe floatation when installing gravity sewer mainlines.
ArrowBore allows licensees to use standard HDD equipment to install pipelines on-grade and on-line with help from laser technology and vertical sight holes drilled along the bore path, allowing grades of 0.28 per cent to be drilled.
Bored vertically at 30-foot intervals, 16-inch inspection holes let engineers verify grade accuracy during, rather than after installation. Pilot stem depth is checked at each hole with a laser sight and a measuring rod dropped into the hole. If the drilling head is off-grade, it’s realigned with another rod that’s inserted down the hole and hooked onto the pilot stem. The inspection holes also serve as slurry outlets, as the pipe forces the slurry up through them during pullback.
The process uses back reamers to widen the bore a quarter-inch larger than the pipe’s outside diameter. This prevents the pipe from floating within annulus space around the pipe, another feature that contributes to on-grade installation.
Trenchless Flowline offers training, tools, and equipment support for people interested in the method for sewer and water mainline projects.
About Charles Stockton
UK-born Charles Stockton has been a part of the HDD sector in Australasia since 2003. He is the Managing Director of Stockton Drilling Services, a leading engineering consultancy specialising in HDD and other trenchless pipeline installation methods.