A microtunnelling machine is a small, remotely controlled tunnel boring machine used to install pipes underground without open trenches. The machine is launched from a shaft and pushed forward by powerful hydraulic jacks, which also push a string of pipes behind it. As the cutting head excavates the ground, the pipes follow, so that when the machine reaches the reception shaft the pipeline is already in place. The whole operation is steered from a control container on the surface, and nobody works inside the pipe during the drive.
Microtunnelling is a form of pipe jacking and belongs to the family of trenchless technologies, together with horizontal directional drilling, auger boring and pipe ramming. Its strength is precision: it can install gravity sewers and drains to tight line and level tolerances, under roads, railways, rivers and buildings, even in soft ground below the water table. That precision, combined with minimal disruption at the surface, has made it a standard method for urban sewer and utility work in many countries.
Brief History of Microtunnelling Machines
Pipe jacking itself is much older, with pipes being jacked under railway embankments in the United States from the late nineteenth century. Microtunnelling as a remotely controlled method was developed in Japan in the 1970s. Rapid urban growth, soft and water-bearing ground and the need to build sewers in crowded cities encouraged Japanese companies such as Iseki to develop small slurry shields that could be steered from the surface.
In the early 1980s the technology spread to Europe, where Germany and the United Kingdom became important markets, and the first projects followed in North America in the mid-1980s. European manufacturers developed their own machines and pushed the method to longer drives and larger diameters. Laser guidance, more powerful jacking frames, better lubrication systems and intermediate jacking stations made longer and curved drives possible. Today microtunnelling is used worldwide for sewers, water mains, culverts and utility crossings, and the boundary between microtunnelling and small TBM tunnelling has become less sharp.
How a Microtunnelling Machine Works
- Launch and reception shafts. The drive starts from a launch shaft that contains the jacking frame and ends at a reception shaft where the machine is recovered.
- Jacking frame. A steel frame with hydraulic cylinders pushes the machine and the pipe string forward. After each push, the cylinders retract and a new pipe is lowered into the shaft.
- Cutting head. A rotating head fitted with tools suited to the ground, from soft soil to rock. Many machines include a crusher cone behind the head that breaks cobbles and boulders to a size that can be pumped away.
- Face support. In slurry machines, a pressurised fluid balances the groundwater and earth pressure at the face, preventing settlement and inflow.
- Spoil removal. Slurry machines pump the excavated material to a separation plant on the surface. Auger machines carry it back through a screw conveyor inside the pipes.
- Steering and guidance. A laser or gyro-based guidance system measures the position of the machine, and hydraulic steering cylinders articulate the head to correct line and level.
- Lubrication. Bentonite or polymer lubricant is injected around the pipes to reduce friction and jacking forces.
- Intermediate jacking stations. On long drives, extra jacking stations along the pipe string push sections of the pipeline forward in sequence.
Types of Microtunnelling Machines
Slurry Microtunnelling Machines
The most common type. A slurry circuit supports the face and removes the spoil. These machines work in a wide range of soils, including water-bearing sands and gravels, and can be fitted for rock.
Auger Microtunnelling Machines
Machines that use a screw auger inside the pipes to remove spoil. They are simpler and suit stable ground above the water table and shorter drives.
Earth Pressure Balance Microtunnelling Machines
Small EPB machines that support the face with conditioned soil, used mainly in cohesive ground.
Pilot Tube Systems
A guided method in which a small pilot tube is first installed on line and level, then enlarged in one or more passes. It is used for small-diameter gravity pipes and is sometimes grouped with microtunnelling.
Rock Microtunnelling Machines
Machines with disc cutter heads for hard rock, often based on slurry machines with a different cutting head.
Curved and Long-Distance Drives
Machines and pipe systems designed for curved alignments and drives of several hundred metres to more than a kilometre, depending on ground, diameter and equipment.
Size Classes
| Class | Typical pipe inside diameter | Typical use |
|---|---|---|
| Small | about 250 to 600 mm | Sewer connections, small gravity sewers |
| Medium | about 600 to 1,500 mm | Sewers, water mains, culverts |
| Large | about 1,500 to 3,000 mm and more | Trunk sewers, storm drains, utility tunnels |
Figures are indicative. The upper end overlaps with pipe jacking TBMs, and many countries define microtunnelling as work where people do not normally enter the pipe.
Equipment on Site
A microtunnelling spread consists of more than the machine itself. The control container on the surface holds the operator’s console, the guidance display and the data logging system. Hydraulic power packs drive the jacking frame and steering cylinders, and a lubrication plant mixes and pumps bentonite or polymer to the pipe string. Slurry machines need feed and return pumps, pipelines and a separation plant with screens, hydrocyclones and sometimes centrifuges or filter presses to clean the slurry for reuse. A crane or excavator lowers pipes into the launch shaft, and a generator or grid connection supplies power. On larger drives, intermediate jacking stations and their control systems are added.
Main Manufacturers
- Herrenknecht, the German manufacturer, with its AVN slurry microtunnelling machines.
- Akkerman, the American manufacturer of microtunnelling and pipe jacking equipment.
- Iseki, the Japanese company that pioneered the technology.
- Robbins, the American tunnelling company, which also builds small-diameter and microtunnelling machines.
- Chinese manufacturers, which supply machines in large numbers for domestic sewer and utility work.
Main Uses of Microtunnelling Machines
- Gravity sewers and stormwater drains that need precise gradients.
- Water mains and pressure pipelines.
- Crossings under roads, railways, rivers and runways.
- Culverts and utility conduits.
- Outfalls and intakes for water treatment and power plants.
- Casing pipes for cables and other services.
Planning a Microtunnelling Project
- Know the ground. Soil type, groundwater, boulders and contaminated ground all affect machine selection and cutting head choice.
- Space for shafts. Launch and reception shafts must be sized for the jacking frame and pipe length, and located where the surface allows.
- Choose the right pipe. Jacking pipes of concrete, clay, steel, glass-fibre reinforced plastic or polymer concrete must be designed for the jacking loads.
- Plan drive lengths. Longer drives reduce the number of shafts but increase jacking forces and risk. Lubrication and intermediate jacking stations help.
- Allow for obstructions. Old foundations, piles and boulders can stop a drive, and rescuing a stuck machine from a new shaft is costly.
- Consider the separation plant. Slurry machines need a separation plant and space to handle and dispose of spoil.
Safety
One of the main safety benefits of microtunnelling is that operators do not work in the pipe. Hazards remain in the shafts, where workers deal with lifting operations, heavy pipes, hydraulic systems and possible ground or water ingress. Shafts must be properly supported and protected against falls, and confined space procedures apply when entering them. Utility strikes are a concern if services are not properly located. On the surface, the separation plant, slurry lines and electrical equipment need safe layouts and regular inspection.
Trends and Technology
Microtunnelling machines are being used for longer drives, curved alignments and harder ground, supported by gyro guidance systems that work without a line-of-sight laser. Automation of steering, jacking and slurry control, together with real-time data monitoring, is making drives more consistent. There is growing use of microtunnelling in rehabilitation, for example replacing old sewers on the same line, and of larger machines that blur the line between microtunnelling and small-diameter TBMs.
Key Terms
- Pipe jacking: installing pipes by pushing them through the ground behind a shield.
- Jacking frame: the hydraulic system in the launch shaft that pushes the pipe string.
- Slurry circuit: the pumped fluid system that supports the face and removes spoil.
- Intermediate jacking station: a jacking unit built into the pipe string on long drives.
- Crusher cone: a device behind the cutting head that breaks cobbles and boulders.
- Separation plant: equipment that removes spoil from the slurry for reuse.
Frequently Asked Questions
What is the difference between microtunnelling and horizontal directional drilling? Microtunnelling jacks rigid pipes behind a steerable machine and achieves precise line and level, which suits gravity sewers. Horizontal directional drilling pulls a flexible pipe into a drilled bore and is usually used for pressure pipes and cables.
How long can a microtunnelling drive be? Drives of a few hundred metres are common. Longer drives are possible with good lubrication, intermediate jacking stations and suitable ground.
Does anyone go inside the pipe? Not during normal operation. The machine is controlled from the surface, which is one of the main safety benefits.
Can microtunnelling machines cut rock? Yes, with cutting heads fitted with disc cutters, although rock drives are usually slower and more costly.
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