Directional drilling is a trenchless method of installing pipes, ducts and cables underground by steering a bore along a designed path between a launch pit and a reception pit, then pulling the product pipe back through the enlarged hole. In UK utility civils it is almost always called horizontal directional drilling, or HDD.
The point of it is not the drilling. The point is what you avoid: a continuous open trench down a live carriageway, the traffic management that comes with it, and the reinstatement bill at the end. A bore replaces all of that with two small excavations and a controlled path underneath everything in between.
That trade-off is why HDD sits at the heavier end of McFadden’s directional drilling capability, used for the crossings that open-cut cannot sensibly reach. This explainer covers what the technique actually is, where it fits on a UK scheme, and the constraints that decide whether a bore is viable before anyone mobilises a rig.
What directional drilling is (and what it isn’t)
Directional drilling in utility work means a surface-launched, steerable bore. A rig sits at ground level, pushes and rotates a drill string with a steerable head on the end, and the operator adjusts the path in both plan and profile as the head advances. A tracking system on the surface tells the operator where the head is and how deep it is running.
That steering is the defining feature. It separates HDD from every other no-dig technique in the utility contractor’s kit, and it is what allows a bore to duck under a river, a rail crossing or a protected root zone and come up exactly where the design says it should.
It is also not the same as moling. A percussive mole is an unsteered pneumatic tool that punches a hole through soft ground on whatever line you aim it at, which is fine for a short service crossing under a driveway and no use at all for a 120 m curved shot under a river. The full breakdown sits in our guide to moling versus directional drilling, and if the mole is the unfamiliar half of that pair, start with what moling is.
How a directional drill shot works
Every HDD installation runs through three stages, whatever the diameter.
- Pilot bore. The steerable head is driven from the launch pit along the designed alignment to the reception pit. Drilling fluid is pumped through the head to cut, cool, carry spoil back and hold the bore open. The operator steers by orienting the head and tracks its position from the surface.
- Reaming. The pilot hole is pulled back through with a reamer sized to suit the product. On larger installations this happens in stages rather than one pass, stepping the bore up progressively.
- Pullback. The product pipe is fused or assembled in one length on the surface, connected behind the reamer through a swivel, and drawn back through the bore in a single continuous pull.
The pipe never touches an open trench. The only excavation is at each end, plus any trial holes dug to verify existing services on the route.
Where directional drilling is used on UK utility schemes
Directional drilling earns its place wherever the surface above the route cannot be opened. Motorway and A-road crossings, railway crossings, rivers and canals, conservation areas, mature tree root protection zones, and congested urban streets where a permit for a full-length trench simply will not be granted.
On water schemes it is used for main crossings and long service runs in PE. On the electric and telecoms side it installs ducting under obstacles ahead of cable pull. The common factor is a fixed obstacle with a defined start and finish point, not a general preference for no-dig.
One correction worth making, because it circulates widely: gravity drainage is not a natural HDD application. A steered bore controls direction well, but holding a continuous designed fall to the tolerance a gravity sewer needs is a different problem, and that work normally goes to guided auger boring or pilot tube microtunnelling instead. Pressurised systems, water mains and ducts, are where HDD is genuinely at home.
The published UK evidence on the trade-off is better than the marketing usually suggests. The HSE’s good-practice case study on directional drilling as an alternative to trenching records a water main crossing a motorway in northern England installed at roughly 25% of the time and 75% of the cost of full trenching, with average daily hand-arm vibration exposure falling from three hours to 15 minutes. That is one job, not a rule, but it is a published figure from a regulator rather than a contractor’s brochure.
What governs a directional drilling bore in the UK
A bore is governed by what is already in the ground and by who owns the street above it. Two documents set the standard.
The HSE’s HSG47, Avoiding danger from underground services, sets the safe system of work in three elements: plan the work, locate and identify services, then excavate safely. For HDD, the critical point is scope. The hazard is not the bore centreline. It is the pilot bore plus the steering tolerance, plus every staged reamer, plus the final reamer diameter and the product pipe behind it. That whole envelope has to be cleared, not a line on a drawing.
Utility survey quality is specified by PAS 128:2022, which classifies detection work by type. The client specification guide published by CICES sets out the ladder: Type D is a records search only, Type C is site reconnaissance, Type B is geophysical detection at varying confidence, and Type A is physical verification where the service is exposed in a trial hole and surveyed. Records alone never clear a bore. At high-consequence crossings, Type A verification is what a competent contractor will insist on.
What makes a bore viable (or not)
The rig is rarely the limiting factor. The pipe is.
Technical guidance published by PE100+ on horizontal directional drilling sets out the constraints that decide a shot. A bore of roughly 1.2 to 1.5 times the pipe outside diameter is normally required for clearance. The pipe wall thickness may have to be increased, meaning a lower SDR, to survive the pullback load even when a thinner wall would satisfy the operating pressure. Axial pullback capacity becomes a genuine limiter on installations well over 1,000 m. And the allowable installation load is a number the pipe manufacturer provides, not one anybody should assume.
Bend radius is the other hard limit. PE100+ publishes general minimum laying radii for unovalled pipe at 20°C:
PE100+ minimum laying radii for unovalled pipe at 20°C
| Pipe SDR | Minimum bend radius |
|---|---|
| SDR 11 | 15 × DN |
| SDR 17 / 17.6 | 25 × DN |
| SDR 21 | 35 × DN |
| SDR 26 | 45 × DN |
Those are pipe curvature minima, not an automatic bore path specification. The designed radius has to respect whichever constraint is tightest across the pipe, the drill string, any fused joints, the installation temperature and the entry overbend. A DN500 SDR11 main at 15 × DN needs 7.5 m of radius before anything else is considered.
Ground conditions finish the picture. Clean granular or cohesive soils bore well. Cobbles, running sand and rock each change the tooling, the fluid programme and the risk profile, which is why a geotechnical picture and a verified utility survey should exist before anyone prices a shot rather than after.
The short version
Directional drilling is a steered, surface-launched bore that installs a pressurised pipe or duct under an obstacle without opening the ground between the two ends. It is not oilfield drilling, it is not moling, and it is not the right answer for gravity drainage. Where it fits, the published evidence points to real savings in programme, cost and vibration exposure against full trenching.
The decision to bore is made long before the rig arrives. It is made when someone confirms the pipe can take the pull, the radius works, the ground suits the tooling, and the hazard envelope has been verified rather than assumed. If you are weighing up a crossing on a live scheme, the detail on rigs, diameters and typical shot lengths sits on McFadden’s directional drilling page.
Sources and further reading
- HSE, Directional drilling avoids trenching
- HSE, HSG47: Avoiding danger from underground services
- BSI, PAS 128:2022 Underground utility detection, verification and location
- CICES, PAS 128 client specification guide
- PE100+, Horizontal directional drilling technical guidance
Questions on this topic
In UK utility work, yes. Directional drilling, horizontal directional drilling and HDD are used interchangeably for the same technique: a steerable, surface-launched bore that installs a pipe or duct without an open trench. The distinction that does matter is between utility HDD and oilfield directional drilling, which are separate disciplines with different rigs, depths, tolerances and regulations despite sharing the same name.
It needs space at both ends for launch and reception areas, plus room to lay out the product pipe in one continuous length before pullback. Ground conditions can rule it out or push costs up sharply, particularly cobbles, running sand and rock. It is a poor fit for gravity drainage because holding a continuous designed fall is difficult. And on very short crossings the mobilisation cost rarely justifies it against a simpler method.
No. Moling uses an unsteered pneumatic percussive tool that drives through soft ground on a fixed line, suited to short service crossings. Directional drilling uses a steerable head tracked from the surface, so the bore path can be adjusted in plan and profile to follow a designed route around obstacles. Different tools, different scales, different applications.
It is used for pressurised systems such as rising mains, but not normally for gravity sewers. A steered bore controls direction well, yet holding the continuous, tightly toleranced fall a gravity system needs is a different engineering problem, and that work usually goes to guided auger boring or pilot tube microtunnelling. Water mains and ducts are where HDD performs best.
If the entry or reception pits are in the highway, yes. Trenchless installation removes the continuous trench, not the statutory obligations. The pits are excavations in the street like any other, needing the correct notice or permit, compliant signing, lighting and guarding, and reinstatement to the applicable specification.
Depth is set by the design rather than by the rig, driven by required cover, the obstacle being crossed and the clearance needed from existing services. The practical constraint is tracking. Walk-over sonde systems need surface access above the bore and can be affected by interference, so deeper bores or crossings with no surface access typically use wire-line or gyro-based guidance instead.
