What the moling process achieves
Moling is a trenchless method of installing underground pipes by driving a pneumatic tool through the ground to create a borehole, then pulling the new pipe back through the tunnel it leaves behind. The technique avoids digging an open trench along the full route, which means the carriageway, garden, or driveway between the two endpoints stays completely untouched. If you are new to the technique, our article on what moling is and where it is used covers the fundamentals. For the full picture of how we deliver it, see our moling capability page.
This article walks through the moling process step by step, from the moment the survey starts to the point where the pipe is connected and the surface is reinstated. It also covers what separates a clean bore from a problem job, because the technique is simple in principle but relies heavily on getting the setup right. Cost is often part of the decision too. Our moling cost guide covers per-metre pricing in detail, including where larger diameters tip the balance toward directional drilling.
Before the mole goes in the ground
Every moling job starts with a service survey. The contractor scans the route using a cable avoidance tool (CAT) and signal generator (Genny) to locate any existing underground services, including water mains, gas pipes, telecoms ducting, and electrical cables. On congested routes, trial holes are hand-dug at key crossing points to physically confirm what is below the surface. Skipping this step is where problems start. A mole striking a live gas main or fibre optic cable is not something you can undo with a follow-up visit.
The ground conditions along the route are assessed at the same time. Moling works by compacting and displacing soil outward, so it needs ground that will compress. Clay, silt, peat, and soft cohesive soils are ideal. Dense gravel, heavily compacted sand, cobble-heavy fill, and rock will resist the mole, slow its progress, or push it off line. When the ground survey flags unsuitable conditions, a different trenchless method such as directional drilling is used instead. Our article on moling vs directional drilling explains how the two methods differ and when each one fits. The UK Society for Trenchless Technology provides a good overview of the soil types and conditions the technique is designed for.
Once the route is confirmed and services are marked, the entry and exit points are agreed with the client. On highway work, permits under the New Roads and Street Works Act 1991 (NRSWA) are confirmed before any plant arrives on site.
The moling process step by step
1. Excavating the launch and reception pits
Two small pits are dug at either end of the bore route. A typical pit is around one metre square and up to two metres deep, sized to fit the mole and allow the operator to set the alignment. In a garden or verge, these are often hand-dug. On harder ground, a mini excavator does the job in minutes.
The critical point is what happens between these two pits: nothing. No trench, no excavation, no surface disturbance. On a water main replacement, the launch pit sits in the footway or verge, the reception pit sits on the other side of the road, and the carriageway between them is never cut. No temporary traffic management, no road closure notice, no reinstatement of a full trench. This is fundamentally why moling costs less and finishes faster than open-cut methods.
2. Aligning the mole
The mole is a steel, torpedo-shaped cylinder containing a pneumatic piston mechanism. It connects to a surface-mounted compressor via a flexible air hose. Before the bore starts, the mole is placed in the launch pit and aligned towards the reception pit using a sight stick. The sight stick sets both the horizontal line and the gradient. This step is where the job is won or lost. Most impact moles are non-steerable, so once the bore starts, the tool follows whatever line it was set on. An alignment error of two degrees at the launch pit becomes a significant miss at the reception pit 15 metres away.
- Mole diameter range
- 45 mm to 180 mm (matched to pipe size)
- Suitable pipe diameters
- 25 mm to 125 mm (most commonly 25 mm to 63 mm for water services)
- Bore distance per run
- Up to 20 m in ideal conditions; intermediate pits every 8 to 15 m on longer routes
- Bore oversizing
- Minimum 10% larger than pipe OD for clean pullback
- Power source
- Compressed air via flexible hose from surface compressor
- Bore type
- Straight line only (non-steerable moles); steerable variants exist but are uncommon
3. Driving the bore
When the compressor is activated, pulsed compressed air drives the piston inside the mole in a rapid hammering cycle. The mole forces itself forward through the soil, compacting and displacing the earth outward to form a clean, packed tunnel. No spoil is generated. No material needs removing from site. There are no skips, no muck-away lorries, and no excavated material left on site while the job finishes.
The operator monitors progress from the surface. On short runs, the pneumatic hose markings indicate how far the mole has travelled. On longer bores, a radio sonde transmitter inside the mole broadcasts its position to a handheld receiver on the surface, confirming both depth and alignment. In good soil conditions, a mole can travel at up to 1.5 metres per second, though the practical rate on most UK utility jobs is considerably slower depending on soil density and moisture content.
4. Installing the pipe
Once the mole breaks through into the reception pit, the bore is complete. The new pipe, typically blue MDPE for water services, is attached to the rear of the mole and pulled back through the borehole from the reception pit to the launch pit. The bore is deliberately oversized by at least 10% compared to the pipe's outside diameter, which reduces friction and tension during the pullback and allows the pipe to seat cleanly inside the tunnel without dragging against compacted soil.
For telecoms or electrical work, a duct is pulled through instead, and the cable is threaded through the duct afterwards. On pipe replacement jobs where an old, brittle pipe needs removing, a pipe-splitting head or expander can be attached behind the mole to fracture the old pipe outward while the new one is pulled in behind it.
5. Connection, testing, and reinstatement
With the pipe in position, the fittings are made at both ends. Water pipes are pressure tested to confirm integrity before the supply is connected. The two pits are backfilled with compacted material and the surface is reinstated. Reinstatement is minimal and programme-friendly. Turf is relaid over a garden pit. A driveway gets a small patch. A footway gets a paving slab reset. The road between the two points was never opened, so there is nothing to reinstate there.
Moling vs open-cut trenching: site impact comparison
| Factor | Moling | Open-cut trenching |
|---|---|---|
| Surface area disturbed | Two small pits (approx. 1 m² each) | Full trench along the entire route |
| Spoil removal | None (soil displaced in place) | Excavated material removed by skip or grab |
| Traffic management | Minimal or none between pits | Full lane closure or road closure typical |
| Reinstatement scope | Two patches | Full trench backfill plus surface restoration |
| Time on site (typical water main run) | Half a day to one day | Two to three days |
This is also why moling is considered one of the more environmentally friendly installation methods. No excavated material leaves the site, no heavy plant runs the length of the route, and the ground between the two pits is never disturbed.
What separates a clean bore from a problem job
The moling process itself is straightforward, but the technique is unforgiving when the setup is wrong or the ground conditions are misjudged. These are the factors that separate a bore that runs cleanly from one that causes problems on site.
Common moling risks and how experienced contractors prevent them
| Risk | What happens | How it is prevented |
|---|---|---|
| Alignment error at setup | Mole arrives at the wrong position or depth at the reception pit | Precise sight stick alignment checked before the bore starts; operator measures twice, fires once |
| Striking existing services | Damage to gas, electric, water, or telecoms infrastructure | Full CAT and Genny survey with trial holes at crossing points; service plans checked before mobilisation |
| Unsuitable ground conditions | Mole stalls, deviates, or causes surface heave in non-compressible soil | Ground assessment during the survey phase; switch to directional drilling if conditions are not right |
| Surface heave on shallow bores | Visible lifting of turf, paving, or road surface above the bore line | Maintaining adequate depth; adjusting the launch gradient to keep the bore below the zone of influence |
| Bore collapse before pullback | Tunnel closes in before the pipe can be installed, particularly in loose or wet soils | Pulling the pipe immediately after the bore completes; using the mole's own length to support the tunnel during the bore |
The bore collapse point is worth expanding on. Unlike directional drilling, which uses drilling mud to stabilise the bore, moling has no fluid system holding the tunnel open. What it does have is the mole itself. The steel cylinder supports the bore along its full length as it travels, so the tunnel behind it is freshly formed and compacted. The discipline is to pull the pipe back through immediately after the mole arrives at the reception pit, before the soil has any chance to relax or close in. Experienced operators treat this as a single continuous operation, not two separate steps.
How McFadden Utilities delivers moling across the Home Counties
McFadden Utilities operates its own fleet of impact moling machines across Hertfordshire, London, Essex, Surrey, Cambridgeshire, and Berkshire. The team uses moling for water main replacements, new water connections, gas service installations, and telecoms ducting on water company frameworks, developer schemes, and Tier 1 contractor programmes. Every job follows the same sequence: full service survey, ground assessment, alignment setup, bore, pullback, pressure test, and reinstatement. The method is chosen because it keeps disruption to a minimum. The carriageway stays intact, the site stays intact, and the job is completed without the cost and programme time that comes with digging up and reinstating a full trench.
The simplest way to think about moling is this: two small holes, a tunnel between them, and a pipe pulled through. Everything that matters happens underground, and the surface barely knows you have been there.
To find out more about how moling fits into our wider trenchless and no-dig capability, visit our moling service page.
Questions about how moling works
A typical domestic moling job is completed in half a day to a full day. The bore itself runs quickly in good ground, but the overall programme includes the survey, pit excavation, alignment, bore, pipe pullback, connection, pressure testing, and reinstatement. Longer routes that need intermediate pits, or ground conditions that slow the mole, can push the job to two days.
Yes. This is one of the main reasons moling is used. The launch pit is dug on one side of the road and the reception pit on the other. The mole travels beneath the carriageway without the road surface ever being cut. There is no need for a road closure, no trench reinstatement, and no temporary traffic management between the two pits.
Impact moles can install pipes from 25 mm up to approximately 125 mm in diameter, though the most common range for domestic water and gas services is 25 mm to 63 mm. Mole sizes from equipment manufacturers like Hy-Ram range from 45 mm to 180 mm, matched to the pipe being installed. Larger diameters or longer distances typically require horizontal directional drilling instead of moling.
Moling works best in compressible soils that the mole can displace outward as it travels. Clay, silt, peat, and soft cohesive ground are ideal. Dense gravel, heavily compacted sand, cobbles, and rock are not suitable because they resist the mole’s hammering action and can cause it to deviate from the planned line. A ground assessment before the bore starts confirms whether moling is the right method for the specific route.
Moling uses a pneumatic hammer to displace soil in a straight line over short distances, typically up to 20 metres per run. Directional drilling uses a rotating drill head steered from the surface, capable of curved bores over much longer distances and through harder ground. Moling is simpler, faster, and cheaper for short straight runs in suitable soil. Directional drilling handles what moling cannot. We cover this in full in our upcoming moling vs directional drilling article.
