What Is Microtunnelling?

Definition: an unmanned tunnel, a jacked pipe

Microtunnelling is a trenchless crossing method in which a pipeline is jacked from an entry shaft towards an exit shaft while the ground ahead of it is excavated by a remotely controlled, closed-face tunnelling machine. The “micro” in the name describes not the size of the pipe but the fact that the tunnel is unmanned: excavation, steering and face support are all run from a control cabin at the surface, and nobody enters the tunnel. The method is commonly used in diameters from 250 mm to 3 metres, above all on gravity sewers and storm-water lines where the gradient has to be held to the millimetre.

How does the method work?

The work runs between two shafts. The main jacking frame is set up in the entry shaft with a thrust wall behind it to transfer the load into the ground; the exit shaft is where the machine is recovered at the end of the drive. The machine (MTBM) is launched from the shaft, new pipe sections are added behind it as it advances, and the whole pipe string is pushed forward together with the machine. There is no separate tunnel lining: the jacked pipes are the lining, and they are selected to carry the jacking load.

The line is read continuously by a fixed laser in the shaft striking a target plate inside the machine. The operator sees any deviation on screen and corrects it with the steering jacks in the machine’s articulated shield; pitch and roll come from inclinometers on board. Being able to see and correct deviation the moment it appears is the main reason microtunnelling is preferred for gravity lines.

As the drive lengthens, friction on the outside of the pipe accumulates. Two measures balance it: continuous injection of bentonite around the pipe, and on long crossings intermediate jacking stations placed along the line. An intermediate station divides the pipe string into segments and pushes each separately, so lengths that a single main frame could never push can be completed in one drive.

Machine types: slurry and EPB

Closed-face machines are divided by how they support the face.

Slurry machines pump a bentonite-and-water suspension into the chamber behind the cutter head; the face is held by the pressure of this fluid. The excavated ground mixes with the fluid, is pumped to the surface through a closed-loop pipe system and separated from the fluid in a separation plant; the cleaned bentonite returns to the circuit. This is the machine for wet, sandy and gravelly ground; the price is the space and water supply the separation plant needs on site.

EPB (earth pressure balance) machines take no support fluid from outside. The excavated soil is turned into a plastic paste with foam and polymer additives, held under pressure in the cutter chamber and removed in a controlled way by a screw conveyor; the pressure in the chamber is set by the conveyor speed. They are efficient in clay and silt, and because no separation plant is needed the set-up is lighter; success depends on getting the soil conditioning right.

On which projects is it chosen?

Microtunnelling exists for the conditions the other pipe jacking methods cannot handle:

• Below the water table and in flowing sand: the face is under constant pressure, so water and soil do not enter the machine.
• On gravity lines: gradient tolerance is at millimetre level and the line can be corrected during the drive.
• On long crossings: with intermediate jacking stations, hundreds of metres are driven in one pass.
• Beneath settlement-sensitive structures: surface settlement is under control below railways, historic buildings and dense utilities.

On a short, shallow crossing above the water table the set-up cost of this system does not pay for itself; there auger boring is far more economical. In bouldery ground where manual intervention at the face might be needed, the closed-face machine has no access, so the geotechnical survey must be done beforehand.

Microtunnelling, auger boring and HDD compared

All three make trenchless crossings, but they are designed for different jobs. Auger boring uses the same shaft arrangement with the excavation done by a cutting head at the end of a helical auger; the face is not pressurised, which makes it fast and cheap in dry, self-supporting ground. Microtunnelling adds pressurised face support and continuous steering to the same arrangement; below water and at fine gradients it is in its element. Horizontal directional drilling (HDD) works without shafts: it enters at an angle from the surface, reams the pilot hole and pulls the pipe in; it leads on cable and pressure lines and river crossings, but cannot deliver the gradient accuracy of gravity lines.

Frequently asked questions

In which diameters is microtunnelling used? The common range is 250 mm to 3 metres. In small diameters the machine cannot be recovered through the pipe, so an exit shaft is essential; in large diameters the method approaches conventional pipe jacking and tunnelling.

What is the difference between microtunnelling and pipe jacking? Both push the pipe from a shaft. In pipe jacking the face is open and the excavation is done by hand or by auger; in microtunnelling the face is closed and pressurised, the excavation is done by a remotely controlled machine, and nobody enters the line.

What determines the cost? Ground type and water level (slurry or EPB, whether a separation plant is needed), diameter, drive length (number of intermediate stations), shaft depth and site access. A price per metre cannot be given without these; request a quotation with the geotechnical survey and the route section.

Deltek carries out pipe jacking projects with all three methods — pipe pushing, auger boring and microtunnelling. For which method suits your project, and for Deltek’s capacity, see the pipe jacking service page.