Auger boring and pipe ramming: driving a casing under the road

01 /Why install a casing at all?

A casing is a sleeve, usually steel, that carries the pipe that actually does the work, called the carrier. Road and rail authorities often require casings at crossings so the carrier can be replaced later without disturbing the road, so a leak runs out the ends rather than washing out the embankment, and so the carrier is protected from loads. Auger boring and pipe ramming are the workhorse methods for putting that casing in.

Once the casing is in, the carrier pipe is fitted with casing chocks, spacers that centre it and hold it off the casing wall, and pushed or winched through. The Iowa SUDAS manual notes that the chocks let a contractor correct the carrier's grade inside a casing that is slightly off, and that an oversized casing gives more room to do so where accuracy matters.

02 /How does auger boring work?

1. Dig the bore pit and receiving pit

   SUDAS gives typical bore-pit sizes of 26 to 40 feet long and 8 to 12 feet wide, about 8 to 12 m by 2.5 to 3.5 m, with a crushed-stone base to support the machine.

2. Set the track to line and grade

   The boring machine slides on a master track set to the exact line and grade. This is the critical step, because there is little ability to correct once the bore starts.

3. Brace against a backstop

   Hydraulic rams thrust against a backing plate of steel piling, steel plate or timbers; long or large bores may add a concrete backstop.

4. Bore and jack

   A cutting head on the first auger section turns inside the casing while the machine jacks the casing forward. The auger flights carry spoil back to the pit.

5. Add sections

   The machine backs off, the next casing and auger sections are lowered in, joined and welded, and the cycle repeats.

6. Finish

   At the receiving pit the cutting head is removed, the augers are withdrawn, and the carrier is installed on chocks.

SUDAS mentions a detail that affects friction: the first casing section may have a steel band welded around the top three-quarters of its circumference, which slightly over-excavates the hole so the following sections slide more easily. Because the augers rotate inside the casing, steel is the standard casing material; the carrier inside can be almost any pipe.

03 /What are auger boring's limits?

Those limits explain where auger boring shines: medium casings under two-lane or four-lane roads in reasonably firm ground. They also explain its failure modes. A cobble bigger than the flights can take stops the bore; a sand lens below the water table can run into the casing and leave a void under the road.

04 /How does pipe ramming work?

Pipe ramming drives a steel pipe into the ground with a pneumatic hammer attached to its back end. SUDAS explains that the pipe can be rammed closed-ended in sizes up to about 8 inches, which displaces and compacts the soil like a mole but is harder and not normally recommended. More commonly, and always for larger sizes, the front is left open, so a cylinder of soil equal to the pipe's diameter enters the pipe as it advances.

• Guide rails in the launch pit are set to the line and grade of the bore.
• A steel band around the leading edge slightly over-excavates to reduce friction.
• The hammer drives the pipe along the rails; further sections are welded on as it goes, or the whole length is welded up first when the site has room.
• When the casing reaches the receiving pit, the soil core is removed by augering, compressed air or water jetting.

Steel is the only casing material strong enough to take the hammer's impact. SUDAS notes that ramming is often used under railway and road embankments for medium to large pipes, and that it can be quicker and cheaper than augering or drilling under the right conditions because its pits are smaller and installation is faster; for short bores of 60 feet or less it is generally less costly than directional boring, while drilling suits longer bores.

05 /What are pipe ramming's limits?

06 /Can auger boring or ramming install a gravity sewer?

Yes, through a casing, with care about tolerances. SUDAS works an example of a 12-inch sanitary sewer on a 1% grade bored 200 feet: with the ±0.2% tolerance common for gravity sewers, the job would likely need a significantly oversized casing with auger boring, or microtunnelling, to comply. Allowing ±12 inches instead would likely let a steered auger bore do the work without an oversized casing, at lower cost, while still meeting the sewer's minimum grade.

The lesson for owners is to ask what tolerance is specified and why. Tight tolerances cost money; loose ones risk backfall. SUDAS also suggests building extra slope into gravity sewers through the length of a bore where possible, and placing manholes at both ends of long or difficult bores so small line and grade deviations can be corrected where the bore meets the open-cut sections. For tight gravity work, microtunnelling and pipe jacking is the step up.

07 /What about smaller bores: slurry and compaction methods?

Below the auger range, SUDAS describes slurry boring, which drills a pilot hole with fluid and reams it, most often for pipes of 2 to 12 inches over 40 to 75 feet, and compaction methods such as push rods and impact moles for pipes of about 6 inches or less. It notes that water jetting, which erodes a hole with a high-pressure jet, is rarely allowed by most jurisdictions because it over-excavates and causes settlement, and that some owners prohibit slurry boring because the hole is unsupported until the casing goes in. The small-service end is covered in moling and pipe pulling.

Casing crossings on private property are uncommon; they are mostly municipal, highway, rail and utility work, priced by tender. See municipal and commercial trenchless for how owners choose, and the Method Finder for how these methods rate against your crossing.

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