What Is Auger Boring?

Definition: auger excavation, jacked steel casing

Auger boring is a trenchless crossing method in which a steel casing is jacked from an entry shaft towards an exit shaft while the ground ahead of it is cut by a cutting head on the end of a helical auger running inside the pipe. The auger does two jobs at once: it turns the head to cut the ground, and its flights carry the cut material back along the pipe to the shaft. Excavation and spoil removal therefore both happen inside the pipe, with nobody entering it.

The method has been used since the middle of the twentieth century to install steel casings beneath roads and railways, and it is still the first choice for most crossings under highways, railway lines and canals today.

How is it carried out?

1. The shafts are excavated. The entry shaft is dug long enough to hold the machine, the pipe section to be jacked and the thrust wall behind it; the exit shaft is where the head and augers are recovered at the end of the line.
2. The machine is set on its track. A track is installed on the shaft floor at the designed line and grade, and the auger boring machine travels along it. Because the line is fixed by this set-up, the alignment of the track is the most critical measurement of the job.
3. The first pipe and auger are fitted. The cutting head goes on the leading end of the first pipe section, the auger string inside the pipe, and both are coupled to the machine’s drive unit.
4. The cut-and-jack cycle begins. The machine turns the auger while pushing the pipe forward; the head cuts the ground and the auger discharges the spoil into the shaft. When the pipe section is fully in the ground, the machine is retracted, a new section is welded on, the auger is extended and the cycle continues.
5. Breakthrough and clean-out. When the head reaches the exit shaft the augers are removed, the inside of the pipe is cleaned and the carrier line (cable, gas or water pipe) is pulled through the casing.

Unguided and guided auger boring

In the classic form, line is set only by the track set-up; deviation that accumulates along the drive cannot be corrected. On short crossings and on pressure lines where grade is not critical, this is sufficient.

On gravity sewer lines, where the gradient must be held to the millimetre, guided auger boring is used. A small-diameter, steerable pilot bore is drilled first and brought into the exit shaft exactly on target; the cutting head and augers are then coupled to this pilot string, so the casing follows a route that has already been proven. Hitting the target with the pilot bore guarantees the grade of the line from the outset.

In which ground and on which lines?

Auger boring works best in self-supporting clay and stiff ground, preferably above the water table. In flowing sand and wet ground the auger draws water along with the soil and leaves voids around the pipe; in bouldery ground the head cannot pass a boulder and, with no access to the face, the drive can stop. In both cases the geotechnical survey rules the method out from the start and the choice moves to microtunnelling.

Typical applications: steel casing crossings beneath roads, motorways and railways; natural gas and water mains beneath junctions and boulevards; sleeve pipes for power and telecom cables; and, with the guided form, gravity sewer connections.

Auger boring, microtunnelling and HDD compared

Microtunnelling also jacks the pipe from a shaft, but its face is closed and pressurised and the excavation is done by a remotely controlled machine; below water and at fine gradients it is in its element, while its set-up and cost are far greater. Horizontal directional drilling enters from the surface without shafts, reams the pilot hole and pulls the pipe in; it leads on flexible pipe and river crossings. Auger boring’s place is the wide ground between the two: crossings in dry ground, over short to medium distances, with rigid steel casing and at the lowest cost.

Frequently asked questions

Are auger boring and pipe jacking the same thing? Pipe jacking is the general name for methods that push a pipe from a shaft; auger boring is the variant in which the excavation is done by an auger. Because most pipe jacking today is done by auger boring, the two terms are used interchangeably in everyday speech.

Which pipe is used? Almost always steel. It has to carry the jacking load and withstand the auger turning inside it; once the line is complete, the carrier pipe is pulled through the steel sleeve.

What distance and diameter are possible? Distance is set by the ground and the machine’s jacking force, diameter by the machine class. Deltek’s fleet and the diameter and distance limits it works to are given on the pipe jacking service page.

What determines the price? Pipe diameter and wall thickness, crossing length, shaft depth and ground class, whether the guided form is needed, and site access. Request a quotation with the geotechnical survey and the route section.