Microtunnelling expert Stuart Harrison has answered countless questions from the uninitiated about microtunnelling, and maintains there’s no such thing as a silly question. Here, he introduces the science and technology of microtunnelling, addressing some of the most common questions he’s asked on the job site.
First things first
According to Stuart, the first question he’s usually asked by those considering undertaking microtunnelling works is: “How does microtunnelling differ from other installation methods?”
Microtunnelling is one of a number of trenchless installation methods currently available to project owners/builders. Others include horizontal directional drilling (HDD), thrust boring and bed boring.
“All of these techniques vary in their methodology and the accuracy that can be achieved,” says Stuart. “The first major defining detail for trenchless installation techniques is whether the technique is pit-launched or surface-launched.”
The three most common types of pit-launched technologies are microtunnelling, thrust boring (also known as auger boring) and bed boring; while the main surface launched technology is HDD.
When accuracy matters
Of the three main pit-launched technologies, Stuart says that microtunnelling is the top choice when it comes to accuracy.
“The simple reason that we can claim microtunnelling as the most accurate pit-launched installation technique is the fact that microtunnelling machines typically run high end guidance systems with live monitoring. As a result – and this is the critical part – the operator can correct in real time to ensure an accurate installation.”
“Microtunnelling systems are typically capable of installations +/-25 mm, with accuracy of +/-10 mm regularly achieved. Please note this accuracy is a measurement from design line.”
“Thrust boring systems meanwhile, have traditionally been unguided. For an unguided machine, manufacturers state an accuracy of 1 per cent. That means over an installation distance of 100m, a typical result is a less than 1m deviation from the target end point.”
“In more recent times, we have seen a number of steered methods applied with varied levels of success,” adds Stuart. “There is currently potential for these machines to achieve fine tolerance, however at this point there is still quite mixed results.”
The last of the pit-launched techniques, bed boring is traditionally an unguided technology utilised for short shots up to 25m.
According to Stuart, “The accuracy depends upon the accuracy of set up, consistency of the ground and the individual operator’s feel for drilling. In more recent times we are seeing pit-launched HDD and bed borers being combined to try and achieve greater accuracy over longer distances.”
Surface appeal
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.
While pit-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,” notes Stuart. “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 3m deep the manufacturers are stating a possible error of +/-150-300mm. In the field +/-100mm is often possible depending on ground and operators,” said Stuart.
More than just the bottom line
One of the key additionals differences between mictrotunnelling and the other trenchless installation methods is price. Stuart agrees that on many projects (for example, the installation of a pipeline), techniques such as HDD are generally a bit cheaper and they can often be completed more quickly.
However, when viewed over the long term, the variation in price between the methods looks somewhat different.
“While microtunnelling might be a bit pricier, it is typically more accurate and more reliable in the long run,” notes Stuart. “In this way, high-precision microtunnelling installations often more than make up for their initial higher costs through reduced maintenance costs over time.”
“Because HDD lines can flex a bit over the course of an installation, lines installed this way are more likely to fail over time and require more maintenance.”
“HDD’s lower typical accuracy also means that it is not appropriate for some applications. For example, HDD may not be the best choice when building a gravity sewer, as it can only be used on lines with plenty of fall (1-2 per cent or greater). It is, however, suited for pressure applications, such as pump sewer systems, that don’t rely on gravity to transport material.”
“Like many other things in life, you get what you pay for and often it pays to take the long view.”
Taking the long view
So – you’ve taken the long view, and you’ve decided to go ahead with mictrounnelling as your installation method of choice. The next question you should ask is: “What are some of the key considerations I need to take into account before actually appointing a contractor?”
“Before you begin any microtunnelling job, there are several critical factors to consider,” says Stuart.
“Understanding the prevailing ground conditions is paramount in getting successful outcomes from microtunnelling. Your findings in regard to the prevailing ground conditions will then also go a long way to determining the type of pipe to be installed, and the length of the line.”
“The equipment to be utilised will also be somewhat determined by the geotechnical information provided. Because the ground conditions are such a critical aspect of any microtunnelling job, before appointing a microtunnelling contractor, companies should always look into the contractor to try and ascertain the likelihood of success – you wouldn’t want an inexperienced crew in difficult ground conditions.”
According to Stuart, the precision of the accuracy of an installation is a critical factor in any installation, and this comes down to a number of important factors:
- The guidance system – you must have an accurate guidance system in order to get an accurate installation (such as laser or total station).
- The ability of the drill head to steer – if drill heads can’t cut across, they will need to progress to effect a steer, creating a snaking effect in the ground.
- Selection of the cutting face – this needs to match the ground conditions in order to steer effectively.
- Quality checking of equipment – ensuring the equipment is of good repair prior to any launch.
- The ability of the operator – typically a lack of understanding of ground conditions and cutting faces, slow reaction times and poor interpretation of way the head is steering (or not steering) in the ground will have a dramatic impact on the overall accuracy of any installation.
In addition, before appointing any contractor for a microtunnelling job, companies should consider:
- The contractor’s track record in the presumed ground conditions.
- The equipment and method they will utilise.
- The contractor’s contingency plans if things don’t go as expected.
- Their depth of knowledge within the field.
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@www.edgeunderground.co.