Getting a pipe or duct under a road, railway or watercourse without opening it up can be done several ways. The three that come up most are horizontal directional drilling, pipe ramming and auger boring. They all avoid an open trench across the surface, but they work differently and suit different jobs. Here is how we choose, and where each one really fits.
A quick note on what “road” means here, because it changes who has to approve the works. It might be a local authority road, such as a B road or an A road, or a National Highways road, typically a motorway or a major A-road trunk route. The bigger and busier the road, the stricter the approval and the more traffic management a crossing needs, which is one more reason to choose the method that keeps the surface undisturbed.
Horizontal directional drilling (HDD)
Directional drilling is the most flexible of the three, and for most crossings it is where we start. A steerable drill head bores a pilot hole from a launch point to a reception point, the bore is reamed up to size, and the pipe is pulled through. Because it is steered, HDD can control the line and level, go deep, and follow whatever profile the job needs.
Two ways we drill it
HDD does not have to follow a single fixed shape. We use two approaches depending on the job:
- A curved profile. For pressure pipes, ducts and cables, the bore dives down under the obstacle on a designed curve and rises back to the surface on the far side. This is the classic shot for a road, rail or river crossing where the exact gradient in the middle does not matter.
- A straight or graded bore, using a setback. Where a job needs a straight line or a set gradient, such as a gravity drainage run laid to falls, a curve will not do. So we set the launch pit back from where the pipe actually needs to start, drill down from there to a drive pit at the required level, then bore in a straight line or to a set grade from the drive pit through to the reception pit. The pipe is then pulled back from the reception pit to the drive pit. That gives the accurate line and level a gravity system depends on.
Holding the line
How we track and steer the head depends on the accuracy the job needs. For most crossings we use walkover tracking, and with drill-to mode, which locates the head from both sides, it is very accurate. For schemes that demand extreme accuracy, or where there is no safe surface access to walk the line above (a live motorway or railway, for example), we steer down the drill string itself with wireline or gyro guidance.
Pipe ramming
Pipe ramming drives an open-ended steel pipe or casing straight through the ground with a percussive hammer. It is not steered, so it goes in a straight line, but it is tough and reliable in ground that defeats other methods, including made ground, cobbles and mixed fill, and it handles large-diameter steel casings well.
It is a strong choice for a straight, level crossing under a road, railway or watercourse, especially in awkward ground, or where a steel casing is wanted to carry the product pipe. Every ramming job includes us supplying and welding the steel as standard, carried out by a coded welder.
Auger boring
Auger boring is a pit-launched method. A rotating auger inside a casing is jacked forward from a launch pit, cutting the face and carrying the spoil back as the casing advances. It puts in a straight bore and needs a pit at each end.
Of the three, it is the one clients most often move away from once the job is talked through. On the great majority of crossings, directional drilling or pipe ramming does the same job for less, with either the control of a steered bore or the brute reliability of a rammed casing. We are always straight about that. If auger boring genuinely is the right tool for a job we will say so, but more often we would drill it or ram it.
How we choose
For a given road, rail or watercourse crossing, the questions we work through are simple:
- Does the line and level need controlling, or is a straight shot fine? Control, or a set gradient for gravity drainage, points to HDD with a setback.
- What is the ground? Cobbly, made or mixed ground can favour pipe ramming.
- Is a steel casing wanted, or is the product pipe pulled straight in?
- How long, how deep, and is there safe surface access for tracking the bore?
More often than not the answer is directional drilling or pipe ramming. Between them they cover almost every crossing, with the control of a steered bore or the reliability of a rammed steel casing. Send us the crossing details and we will tell you which one fits and why.
For a plain-English overview of all the methods, see trenchless methods explained.