Directional Drilling

What is directional drilling?

Directional drilling is a trenchless method that installs underground pipes, ducts, or cables along a steered, predetermined path, using a surface-launched drilling rig. The technique replaces an open-cut trench with a small entry pit at one end of the run and an exit pit at the other, with a bore driven horizontally through the ground between the two.

Pipes and cables installed by directional drilling sit along a controlled path. Operators can navigate around existing services, cross under roads, rail, rivers, and sensitive surface ground, and install runs that would be impractical with conventional trenching.

In the UK utility civils context, directional drilling is part of the broader no-dig trenchless family, alongside moling, pipe bursting, and thrust boring. The UK Society for Trenchless Technology (UKSTT) promotes and standardises these methods across the construction industry. Its international parent body, the ISTT, runs the annual No-Dig Conference.

How directional drilling works

Directional drilling is delivered in three distinct phases. The first phase drills a pilot bore from the entry pit along a predetermined path to the exit pit, using a steerable drill head. The second phase pulls a larger back reamer back through the pilot bore, enlarging it to the diameter needed for the product pipe. The third phase pulls the product pipe, or a bundle of ducts, back through the enlarged bore into its final position.

Drilling fluid, a mix of water and bentonite or a polymer, is pumped continuously to the drill head throughout the bore. The fluid lifts cuttings from the bore, stabilises the hole against collapse, cools the cutting head, and lubricates the pullback of the product pipe. A reclaimer at the surface filters out spoil and recycles the fluid.

The drill head is tracked in real time from the surface using one of three guidance systems. A walk-over locating system uses a sonde transmitter in the drill head that emits a signal picked up by a handheld receiver at the surface. A wire-line magnetic guidance system runs instrumentation on a hard-wire through the drill pipe and is used on long or deep bores. A gyro-based inertial navigation system is used on the most demanding bores, where a bore must be accurate to within a metre over kilometres.

When directional drilling is used

Directional drilling is chosen when an open-cut trench would be impractical, expensive, or disruptive. The most common UK use cases are:

• Road crossings. Installing a water main, gas main, or cable duct under a live highway without closing or trenching the road surface. The UK’s Health and Safety Executive cites directional drilling as the technique that reduces total job cost to around 75% of full trenching and vibration exposure time from around three hours to fifteen minutes per day.
• Rail crossings. Installing utilities under Network Rail-controlled tracks. HDD avoids disruption to rail operations and removes the need for possessions that open-cut would require.
• River, canal, and waterway crossings. Environment Agency and local drainage board approvals are needed, and the bore typically sits well below any scour depth.
• Environmentally sensitive and protected areas. Sites of Special Scientific Interest, ancient woodlands, conservation areas, and nature reserves. Minimal surface disturbance is essential, and HDD lets the works pass under the site entirely.
• Urban and congested utility corridors. Where existing services lie close to the surface, HDD avoids the risk and cost of trenching through a congested utility map. Route planning requires a full underground utility survey and compliance with HSE guidance book HSG47, Avoiding danger from underground services.

Directional drilling alongside other trenchless methods

Directional drilling sits inside the broader no-dig trenchless technology family, alongside moling, pipe bursting, and thrust boring. It is the longest-distance, most flexible technique of the set, capable of curved paths and navigation around existing services.

For short-distance service connections in soft ground under a driveway or front garden, moling, using a small pneumatic tool, is faster and cheaper than HDD. For replacing an aged pipe along the same route, pipe bursting installs a new pipe through the old. For short, straight, unguided crossings under a road or rail line, thrust boring drives a casing directly through the ground with a hydraulic jack.

HDD is the choice when the bore needs to be steered, the length runs tens or hundreds of metres, or the ground or surface conditions rule out the smaller techniques.

Directional drilling in UK utility civils

Directional drilling in the UK public highway is carried out under the New Roads and Street Works Act 1991, with works notification to the local highway authority and compliance with permit conditions. Entry and exit pits in the carriageway or footway must be reinstated to the Specification for the Reinstatement of Openings in Highways (SROH), the statutory code of practice maintained by the Highway Authorities and Utilities Committee (HAUC).

Safety in route planning is covered by HSE guidance, which points operators to HSG47, Avoiding danger from underground services. Signal detection equipment such as CAT4+ and Genny 4 is used over the planned bore route before drilling starts, cross-checked against utility records, because striking a live service is the biggest risk in any HDD operation.

The UK Society for Trenchless Technology publishes detailed technical guidance on HDD rig classifications and guidance systems, and maintains a register of UK contractors working in the trenchless space.

HDD is routinely used at significant UK infrastructure scale. A current flagship example is the 132kV cable tunnel route between New Cross and Finsbury Market, designed by WSP, where around six kilometres of HDD carries high-voltage cabling at depths of 20 to 30 metres beneath London. Projects at that scale use maxi-class rigs with pullback capacities into the hundreds of tonnes.