A drilled crossing loads the pipe twice, in two entirely different ways. During installation it is pulled through a bore under tension, dragged over ground, and bent around the entry radius. Afterwards it sits there for fifty years carrying pressure. The pipe has to survive both, and the installation case is usually the one that decides what you order.
1. SDR is set by the pull, not by the pressure
On an open-cut main you pick SDR from the operating pressure and stop. On a drilled crossing that is the starting point, not the answer. The pipe has to take a tensile load during pullback and external pressure from the drilling mud, and thin wall handles neither well.
| SDR | Pressure use | Suitability for HDD | Why |
|---|---|---|---|
| SDR 21 and thinner | PN 8 and below | Rarely | Little tensile section; vulnerable to mud pressure and ovality |
| SDR 17 | PN 10 | Short, straight bores | Workable where the pull is modest and the path is clean |
| SDR 13.6 | PN 12.5 | Common choice | Sensible margin for most municipal crossings |
| SDR 11 | PN 16 | The default for demanding bores | Thick wall, high safe pull load, tolerates mud pressure |
| SDR 9 and thicker | PN 20+ | Long or deep crossings | Where the pull force or depth leaves no room |
2. PE100-RC earns its premium here, and almost nowhere else
PE100-RC is PE100 with markedly better resistance to slow crack growth. On a sand-bedded open-cut main the difference rarely justifies the price. On a drilled crossing it does: the pipe is dragged across ground that scratches it, and a scratch is exactly where slow crack growth starts. RC grades are specified for installation without a sand bed for that reason.
3. Know the safe pull load before the rig arrives
The driller computes the expected pullback force from the bore path, the mud and the pipe. What you need from the pipe side is the ceiling: the tensile load the pipe can take without damage. It follows from the wall area and an allowable stress that itself depends on how long the pull takes, because polyethylene creeps.
Safe pull ≈ allowable stress × wall cross-sectional area
Allowable stress falls as the duration of the pull increases — a twelve-hour pull is derated well below a one-hour pull. Thicker wall means more area, which is the other reason HDD crossings run to lower SDR.
4. Check the bend radius against the bore, not the datasheet
PE bends, which is much of why it suits trenchless work, but not without limit. The usual working figure is a minimum radius of 20 to 25 times the outside diameter for the installed condition, tighter for short-term handling. A DN315 pipe therefore wants a radius above about 7 metres.
| Outside diameter | ≈ 20 × OD | ≈ 25 × OD |
|---|---|---|
| DN110 | 2.2 m | 2.8 m |
| DN200 | 4.0 m | 5.0 m |
| DN315 | 6.3 m | 7.9 m |
| DN450 | 9.0 m | 11.3 m |
| DN630 | 12.6 m | 15.8 m |
5. Order lengths that suit the site, not the container
A crossing is pulled as one continuous string, welded up on the surface beforehand. Every joint in that string is a butt weld that has to be made, cooled and tested while the rig waits. Fewer, longer sticks means fewer welds; but the string also has to be laid out somewhere, and a 400 metre lay-down area is not always available.
- Coiled pipe up to DN110 removes joints entirely on smaller crossings — worth asking for even when sticks are the default.
- 12 m sticks are standard; 6 m suits tight sites at the cost of twice the welds.
- Ask how the string will be laid out before fixing the length. On a constrained site the answer sometimes changes the order.
- Where the string cannot be laid out in one line, it can be welded in sections and joined during the pull — slower, and it needs planning rather than improvisation.
What to put on the order
- 1
Give the crossing, not just the pipe
Length, diameter, ground type, and whether it is under a river, a road or a railway. A supplier who knows it is an HDD crossing will flag a class that would be fine open-cut and marginal in a bore.
- 2
Name the grade explicitly
PE100 and PE100-RC are different products at different prices. A quotation that says only PE100 will be PE100.
- 3
Ask for the safe pull figure in writing
For the exact diameter, SDR and expected pull duration. The driller needs it, and it should come from whoever made the pipe.
- 4
Confirm the marking survives the pull
Pipe dragged through a bore loses surface print. Ask how the string will be identifiable afterwards — and keep the delivery documentation, because after installation the pipe cannot be read.
After the pull
Polyethylene that has been pulled is under tension and will recover. Give the string time to relax before tying it in and backfilling the pits, or the recovery pulls the tie-in joints. The usual guidance is to leave it several hours, longer for a long pull, and the driller should have this in the method statement rather than discovering it at handover.