Questions answered by Stuart Harrison, Edge Underground
There are a number of trenchless installation methods available today, and they vary in terms of their methodology and the accuracy that can be achieved. I’ll start by explaining some of the differences between methodologies, then look at the differences in accuracy that can be obtained.
The first major defining detail for trenchless installation techniques is whether the technique is pit/shaft launched or surface launched.
The three most common types of pit/shaft launched technologies are microtunnelling, thrust boring (also known as auger boring) and bed boring.
Microtunneling is the top of the tree when it to comes to accuracy. These machines typically run high-end guidance systems with live monitoring so that the operator can correct in real time to ensure an accurate installation. Microtunneling systems are typically capable of installations +/-25mm with common accuracy of +/-10mm. Please note this accuracy is a measurement from design line.
Thrust boring systems have traditionally been unguided. For an unguided machine manufacturers state an accuracy of one per cent, i.e. over 100m, less than 1m deviation. In more recent times we have seen a number of steered methods applied with varied levels of success. There is currently potential for these machines to achieve fine tolerance, however at this point there is still quite mixed results.
Bed boring is traditionally an unguided technology utilised for short shots up to 25m. The accuracy would depend upon the accuracy of set-up, consistency of the ground and the operator’s feel for drilling. In more recent times we are seeing pit launched horizontal directional drilling (HDD) and bed borers being combined to try and give greater accuracy over longer distances.
The main surface launched technology is HDD. When completing a surface launched HDD installation, a setback area is also required from which a team can set up for the installation.
Shaft launched technologies are fundamentally very rigid, both in machinery and tooling. Surface launched technologies such as HDD need to be flexible in order to flatten out to horizontal and to navigate bends. The consequence of a rigid versus flexible system is that one is wanting to go straight by design, whilst the other is wanting to flex. These differences can be balanced by a number of factors, including the steering system used, guidance system used, the ground conditions and the operator’s competency.
HDD rigs are run in conjunction with a range of systems to assist accuracy. Most common is a walk over system with senders and receivers. These systems show depth, pitch (angle) and a clock face (for steer direction). The manufacturers of the guidance systems state an accuracy of 5-10 per cent. With this in mind, at 3 m deep the manufacturers are stating a possible error of +/-150-300mm. In the field +/-100mm is often possible depending on ground and operators.
Do you have a burning question about microtunnelling, need a practical answer to a problem you’ve encountered, or even just curious? Then, ask your expert! Send your questions to Stuart care of expert@utilitymagazine.com.au
