Microtunnelling and pipe jacking are essentially from the same family of pipeline installation techniques and can be used for installations from approximately 150 mm to 3,000 mm in diameter.
Microtunnelling
The term microtunnelling applies to remotely controlled, steerable trenchless machines using either slurry, auger or vacuum spoil removal to install pipelines. This technique was first introduced into Australia in the mid-late 1980s. Since this time, microtunnelling has been extensively used for the installation of new pipeline infrastructure, particularly for the water industry.
Microtunnelling machines are controlled from the surface, with location and operation of the machine being continuously monitored, usually by means of a laser guidance system. Most microtunnelling drives are straight between shafts, although increasingly in recent years various companies have developed guidance systems that enable curved drives to be completed, particularly on longer length, larger diameter bores.
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Microtunnelling is well-suited to situations where a pipeline has to conform to rigid line and level criteria.
Pipe jacking
A pipe jack is defined as a system of directly installing pipe behind an open-face tunnel boring machine (TBM). The pipe is then hydraulically jacked from a launch shaft, so that it forms a continuous string in the ground. The pipe, which is specially designed to withstand the jacking forces likely to be encountered during installation, forms the final pipeline once the excavation operation is completed. On shorter drives, the pipe jack can also be installed using manual excavation.
Pipe jacking systems are more often than not supplied with jacking frames, which are designed to provide the level of jacking pressure likely to be required by the TBM being used. The requirements for the jacking frame on any project are determined by the ground conditions, length of drive and the type of TBM being used.
An essential feature of pipe for both microtunnelling and pipe jacking is that the entire joint is contained within the normal pipe wall thickness.
Pipe jacking and microtunnelling are both commonly used for mainline or trunk pipelines.
Technical, safety and environmental benefits
The microtunnelling and pipe jacking techniques offer advantages over traditional open-cut construction as well as over other underground construction methods.
Pezzimenti Trenchless Company Director Joe Pezzimenti says that the laser guidance system used in microtunnelling makes this far more accurate than other trenchless construction methods, and that accuracies of between 15 mm and 25 mm have been achieved.
He also says that the method is environmentally friendly, using his own company’s system as an example. “Due to our vacuum extraction system, it is a lot cleaner; only water is used as an additive. We are able to offer continuous ground support by using a jacking pipe or steel sleeve directly behind the drill head, so our setup footprint is quite small compared to other systems,” says Mr Pezzimenti.
Rod Davies Infrastructure Director Rod Davies agrees that there are many environmental advantages to using microtunnelling and pipe jacking over other underground construction methods.
He says “These techniques reduce the quantities of incoming and outgoing materials, reduce the tipping of spoil and quarrying of imported stone backfill, and there is not ‘frac out’ – no liquids surface when injecting bentonite at high pressure as they do when using horizontal directional drilling (HDD). In addition, there is minimal interface with the local and wider environment.”
These methods also offer safety benefits. “Vacuum microtunnelling and pipe jacking are safer methods of working than open-trench construction or traditional segmental tunnelling. There is a significant reduction in the risk of injury as a result of utility strikes and interface with the public,” says Mr Davies.
Züblin Country Manager Josef Kofler says that some other advantages of trenchless tunnelling include minimal visual impact to the local population, weather-proof construction – as tunnels can be built in all weather conditions – and a reduction in landscape ‘scarring’ damage.
Harker Underground Construction Trenchless Consultant Michael Harker says that in some cases, these methods are the only way construction is possible due to depth, obstructions (e.g. roads, rail, waterways, buildings, services, etc.) and geology. He also adds that while traffic disruption is a common disadvantage of above-ground construction, another factor that is often overlooked is the financial stress applied to businesses as a result; microtunnelling and pipe jacking can reduce this disruption.
“Another area that is gaining some traction from local and central government agencies is the carbon tax issue. Of note is a requirement in current and future projects being tendered within the major cities of New Zealand to record plant usage onsite with a forward view of placing a direct cost to the project on carbon production,” says Mr Harker.
“Due to the microtunnelling method requiring minimal excavation of material, there is a drastic reduction in plant movements required and therefore carbon produced in comparison with cut and cover installation. This may turn out to be of considerable value as further studies are done and the environmental benefits and drawbacks are highlighted for various construction methods.”
Recent innovations
Innovations are constantly being developed and incorporated into trenchless technology, with microtunnelling and pipe jacking being no exception.
With a focus on increasing efficiency, safety and reduction of preparation time, Rod Davies Infrastructure has developed the Platypus Vacuum Microtunnelling System. This system allows the operator to quickly adjust boring dimensions by easily changing cutting head diameter without the need to change the costly steering-guide unit.
With the use of modular components, the Platypus minimises costs by eliminating the need of manufacturing a completely new boring head. Platypus also minimises traffic disturbance by positioning all key components of the system within one single lane of traffic.
Pezzimenti Trenchless is currently developing its rock and basalt equipment to tackle increasing demand for installations in these types of ground conditions. In order to cater to the longer distances that are being microtunnelled, the company has also improved its steering systems.
Herrenknecht has developed the Direct Pipe method, which combines microtunnelling and HDD techniques for more efficient pipe installation. Developed as an auxiliary tool for the pullback of pipe, the ‘Direct Pipe’ method involves operating the Herrenknecht Pipe Thruster as a thrust unit, which is a similar technique to that of the jacking frame used for pipe jacking.
The Pipe Thruster holds the prefabricated and laid out pipeline and pushes it into the ground at thrusts of 5 m each. The requisite bore hole is excavated by a slurry microtunnelling machine (AVN), which is placed at the front of the pipeline. The soil excavated by the cutterhead at the tunnel face is mixed with the slurry in the excavation chamber and then pumped through the entire pipeline to a separation plant, using a slurry pump integrated inside the machine. The slurry also provides support at the tunnel face, and after treatment in the separation plant, it is conveyed back into the circuit via a feed line.
This method allows the excavation of the borehole and the simultaneous trenchless installation of a prefabricated and tested pipeline in one single continuous step.
In addition, Züblin has developed a unique desander process, which reduces environmental impact by separating water from the spoil during the tunnel construction process in a closed circuit system. The water is then recycled, eliminating wastage and maximising cost benefits to the client.
According to Mr Harker, pipelines are being installed in ever increasing lengths through a multitude of geological conditions due to the implementation of interjack stations and automated lubrication systems, and with the use of gyro guidance systems within the tunnel boring machine, curved lines down to 200 m radius are now achievable.
The future of trenchless tunnelling
Mr Harker sees more trenchless tunnelling innovations on the horizon.
“I predict that further innovations will include interchangeable machines capable of installing a variety of pipe sizes through a wide range of geological conditions. To achieve this, available torque in a more compact drive unit will be required, and this will be realised as advances in electronics and hydraulics become commercially available,” he says.
Mr Pezzimenti says that his company sees a growing demand for its services due to public safety and convenience. “It is environmentally more viable, and it is becoming more cost effective to bore major roads and rails,” he says.
Mr Davies believes that in the future, much development in trenchless tunnelling will be in small diameter systems and their capability in rock.
“Currently, rock microtunnelling is generally successful from 900 mm diameter. For example, using air hammer technology and smaller cutters will allow small diameter bores to be driven from 300 mm to 600 mm in medium hard rock,” says Mr Davies. “However, in the future, we see improved hardened surfaces and smaller cutter design below 150 mm to drill a rock, improved dewatering methods for fine sand, silt soil units and clay; and real-time navigation systems that can help guide long-distance microtunnelling.”
Trenchless tunnelling technology will continue to develop alongside the growth of pipeline infrastructure.
Herrenknecht Member of the Executive Board Business Unit Utility Tunnelling Peter Schmäh says “Worldwide population growth, ongoing urbanisation and climate change demand suitable underground infrastructure for water and energy supply.
“In its 30 years of tunnelling experience, Herrenknecht has developed a large machine range, which covers AVN, earth pressure balance shields, and hard-rock TBMs for pipe jacking, as well as HDD equipment and Direct Pipe, to install underground pipelines in all kinds of conditions. Providing a technical solution to challenging pipeline projects is our daily business.”
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