Directional drilling works in three underground stages. A steerable head drills a small pilot bore along a designed path, the bore is enlarged by pulling a back-reamer through it, and the product pipe or duct is pulled back through the enlarged hole. The ground surface between the launch pit and the reception pit is never opened.
That three-stage sequence is the easy part to describe. The part that decides whether a bore succeeds is everything fixed before the rig turns: the depth the water company has specified, the clearance to the services already in the ground, and how much the bore can be enlarged without losing the hole. McFadden’s directional drilling capability runs bores under carriageways, railways, rivers and mature landscaping across Hertfordshire, London and Surrey, and the pattern is consistent: the drilling is rarely what causes trouble. The survey work is.
This guide walks the process end to end, with the UK figures that govern each stage.
What a directional drill is doing underground
A directional drill is a surface-mounted rig that pushes a string of drill rods into the ground at a shallow angle. At the front is a steerable head with a slanted face. When the driller pushes the rods without rotating them, the slanted face bites into the soil and deflects the head in whichever direction the slant is pointing. When the driller rotates the rods while pushing, the slant averages out and the head runs straight.
Steering direction comes from a sonde, a battery-powered transmitter housed just behind the head. It broadcasts depth, pitch and clock position to a walkover locator carried by a second operative on the surface, who tracks the head metre by metre and radios corrections back to the rig. The driller is flying an instrument approach, not looking down a hole.
Before the rig starts, the bore path is already fixed
HSE guidance HSG47 breaks safe work near buried services into three elements: plan the work, locate and identify services, and excavate safely. The point that gets missed on trenchless jobs is that HSG47 applies to the entire bore trajectory, not just the two pits at either end. Plans, locating devices and trial holes are required for directional drilling exactly as they are for open-cut, and the bore route is then planned to avoid what has been found.
That matters because records are incomplete and locators have limits. Plastic water services and unmarked private apparatus frequently do not show up on a conventional cable and pipe locator, which is why critical crossing points get proved with trial holes or vacuum excavation before a rod goes in the ground.
Depth is not the contractor’s decision either. Streetworks UK positioning guidance recommends a minimum of 750 mm of cover for water apparatus in footways, verges and carriageways, and notes there is no general statutory obligation fixing apparatus depth in every highway. Water UK’s best practice design principles set a wider envelope of 750 to 900 mm minimum and 1,200 mm maximum, measured from finished surface to the crown of the pipe, and leave the specific figure to the adopting water company. On adoptable work the water company issues the approved design and the contractor bores to it.
How directional drilling works, step by step
- Excavate the launch and reception pits. The launch pit sits at the rig end and gives working room plus somewhere for drilling fluid returns to collect. The reception pit sits at the far end where the head exits.
- Set and anchor the rig. The rig is aligned to the planned bore and anchored against the push and pull forces it will generate. UKSTT gives a typical rack angle of 8 to 16 degrees from horizontal for a surface-launched rig. A pit-launched rig can start at 0 degrees. The angle is chosen from the required cover, the bend radius the drill rods and product pipe can tolerate, and the space available, not picked off a chart.
- Drill the pilot bore. The head is pushed forward in rod lengths, steered on the slant, and tracked continuously from the surface. Each rod added is a fresh chance to correct line and level before the deviation compounds.
- Circulate drilling fluid throughout. A bentonite-based fluid is pumped down the rods and out through the head. UKSTT sets out its four jobs: it supports the bore, carries cuttings back to the pit, lubricates and cools the tooling and sonde, and forms a filter cake that stabilises the bore wall.
- Swap the head for a back-reamer. Once the head surfaces in the reception pit it is removed and a reamer is fitted to the rod string.
- Ream back towards the rig. The reamer is rotated and pulled back through the pilot bore, enlarging it. UKSTT gives a normal final diameter 30 to 50 per cent larger than the installed product, so roughly 1.3 to 1.5 times the product outside diameter. Large bores are enlarged over successive passes rather than in one go.
- Pull the product through. The pipe or duct is connected behind the reamer through a swivel, which stops the rotation of the reamer transferring into the pipe. Pipe and reamer travel back to the launch pit together on the final pass.
- Tie in, test and reinstate. The new main or duct is connected, pressure tested to the water company’s requirements, and the two pits are reinstated to specification.
The numbers that govern a UK utility bore
Key parameters for a UK utility bore
| Parameter | Typical UK figure | Set by |
|---|---|---|
| Entry angle, surface-launched rig | 8 to 16 degrees from horizontal | UKSTT, adjusted for cover and bend radius |
| Entry angle, pit-launched rig | 0 degrees | Site constraints |
| Final reamed bore diameter | 1.3 to 1.5 times product outside diameter | UKSTT |
| Cover to water apparatus in the highway | 750 mm recommended minimum | Streetworks UK positioning guidance |
| Water main cover design envelope | 750 to 900 mm minimum, 1,200 mm maximum | Water UK best practice, water company specification |
| Clearance to existing services, trenchless work | 250 mm or 1.5 times the diameter of the pipe being laid, whichever is greater | HSE HSG47 |
| Clearance for electricity cable maintenance | Approximately 300 mm | HSE HSG47 |
What goes wrong on a bore, and how it is prevented
Inadvertent returns are the failure most people outside the trade have never heard of. Drilling fluid under pressure will always take the easiest route back to the surface, and if that route is a fissure, a gravel band, an old borehole or a stretch of shallow cover rather than the pilot bore, bentonite breaks out at ground level. It is messy rather than dangerous, but on a carriageway, near a watercourse or in someone’s garden it is a problem worth designing out in advance. The controls are a contingency plan written before mobilisation, sensible cover near the entry and exit, and fluid pressures matched to the ground.
Collisions come from tracking the wrong thing. The pilot head is small, but the reamer that follows is up to half as wide again as the product pipe, and it is the reamer that does the damage. The clearance envelope has to be built from the reamer diameter plus locator tolerance plus steering deviation, not from the centreline of the pilot bore.
Pullback tension is the third one. Every metre of enlarged bore, every degree of bend and every under-lubricated section adds load to the pipe being pulled, and polyethylene has a finite tensile limit. Getting the reaming right is what keeps that load inside the pipe’s capability.
Ground loss is the quiet one. Over-reaming relative to the product, or losing fluid into permeable strata, removes more material than the pipe replaces, and settlement follows weeks later. That is the reason the 1.3 to 1.5 ratio exists at all.
Where the ground is straightforward, the run is short and no steering is needed, impact moling is often the better tool for the job, and the comparison between moling and directional drilling sets out where the line sits.
Trenchless does not mean permit-free
A bore that never breaks the carriageway surface still counts as street works. Boring or tunnelling under a street sits squarely within the works that require a street works licence under Section 50 of NRSWA for anyone without statutory undertaker powers, and the launch and reception pits need the same notices, traffic management and permit conditions as any other excavation.
The pits also need proper reinstatement. Two small excavations in a footway are still highway openings, and they are inspected against the SROH reinstatement specification like everything else. Plenty of otherwise clean bores generate defects at the pits because the reinstatement was treated as an afterthought.
Finally, record what was actually installed rather than what was planned. The as-built line and level of the bore, not the design profile, is what protects the next contractor who drills across that street.
The short version
Directional drilling works by drilling a steered pilot bore, enlarging it with a back-reamer, and pulling the product pipe back through. The three mechanical stages are the same on every job. What changes from job to job is the survey quality, the depth the water company has specified, the clearance available to existing apparatus and the ground the bore has to hold open, and those four things decide whether the bore is a half-day job or a fortnight of remedial work.
If you are weighing up a crossing under a road, a railway or a mature landscape, the useful question to ask a contractor is not how fast they can bore. It is what their reamed diameter will be, what clearance they have proved to the services on the route, and what their plan is if fluid comes up somewhere it should not. Our directional drilling capability page sets out the rig range, the ground conditions covered and how bores are planned and tracked.
Sources and further reading
- UKSTT, Horizontal Directional Drilling: process stages, entry angles, reamed diameter ratio and the role of drilling fluid
- HSE, HSG47 Avoiding danger from underground services: planning, locating and clearance requirements, including for trenchless methods
- Streetworks UK, Positioning and Colour Coding of Underground Apparatus: recommended depths for utility apparatus in the highway
- Water UK, Best practice design principles for water mains: cover depth envelope for new water mains
Questions on this topic
Steering comes from the shape of the drill head, not from a rudder. The head has a slanted face, so pushing the rods without rotating them deflects the head in the direction the slant points, while rotating the rods as you push makes it run straight. A sonde behind the head transmits depth, pitch and clock position to a walkover locator on the surface, and the operative tracking the head radios corrections to the driller between each rod.
The pilot bore is the first, small-diameter pass that establishes the line and level of the route. The final bore is what you get after back-reaming, where a cutting head is pulled back along the pilot bore to enlarge it. UKSTT gives a normal final diameter of 30 to 50 per cent larger than the product being installed, and large bores are usually enlarged over several passes rather than one.
For water apparatus in a UK highway, Streetworks UK positioning guidance recommends a minimum of 750 mm of cover, and Water UK’s best practice design principles set an envelope of 750 to 900 mm minimum with a 1,200 mm maximum. On adoptable work the adopting water company specifies the figure in the approved design and the contractor bores to it. Depth is not chosen on site.
HSE guidance HSG47 gives a general minimum of 250 mm or 1.5 times the diameter of the pipe being laid, whichever is greater, with roughly 300 mm allowed where electricity cables need maintenance access. Critical or high-pressure apparatus often needs more, agreed with the asset owner. The clearance has to be measured from the reamer diameter rather than the pilot head, because the reamer is the widest thing that passes.
Yes. Boring or tunnelling under a street is included in the works that require a street works licence under Section 50 of NRSWA for anyone without statutory undertaker powers, and the launch and reception pits need the usual notices, traffic management and permit conditions. Leaving the carriageway surface intact does not remove the obligation.
A frac-out, also called an inadvertent return or bentonite breakout, is drilling fluid escaping to the surface somewhere other than the launch or reception pit. It happens when the fluid finds an easier path back up than the bore itself, typically through fissured clay, gravel bands, old boreholes or a stretch of shallow cover. It is managed with a contingency plan written before mobilisation, appropriate cover at entry and exit, and fluid pressures matched to the ground.
The limit is rarely the rig. On UK utility work the practical constraint is usually where you can legally and physically put a launch pit and a reception pit, then the bend radius the product pipe will tolerate, the tensile load the pipe can take on pullback, and whether the ground will hold the bore open long enough to get the product through. A short bore in poor ground can be harder than a long one in good ground.
