Moling and directional drilling are both trenchless methods used to install pipes and cables underground without open-cut excavation. They are not, however, the same technique. Moling uses a pneumatically driven impact tool that displaces soil in a straight line over short distances. Directional drilling uses a steered drill head and drilling fluid to bore along a planned path over much longer runs. The two require different equipment, different crews, and suit different types of project.
Both methods sit within the broader family of no-dig trenchless technology used across UK utility civils. Choosing between them comes down to a handful of practical factors: distance, diameter, ground conditions, the density of existing buried services, and the reinstatement obligations on the route. 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.This article sets out the differences and the decision criteria that determine which method fits a given scheme.
McFadden Utilities operates moling as part of its trenchless capability across Hertfordshire and the surrounding counties, working on water, gas, telecoms, and electrical installations for water companies, developers, and Tier 1 contractors.
What moling is
Impact moling is one of the earliest and most straightforward trenchless techniques. A torpedo-shaped steel cylinder, powered by compressed air, is launched from a small pit and hammers its way through the ground. The soil is displaced sideways rather than removed, creating a bore through which a product pipe or duct is pulled directly behind the mole. The UK Society for Trenchless Technology (UKSTT) records moling diameters from 45 mm to 300 mm with installation lengths of up to approximately 30 metres.
Most impact moles are non-steerable. The mole is aligned on a starting cradle in the launch pit and driven forward on the line and level set at launch. Steerable moles do exist, using a slant-face head similar to directional drilling pilot bores, but they are rare in practice. For the full technical walkthrough, see how moling works.
What directional drilling is
Directional drilling, also called horizontal directional drilling (HDD), is a rig-driven trenchless method that installs pipes and ducts along a planned underground path. A steerable drill head bores a pilot hole while a sonde transmitter allows the operator to track and adjust the bore’s position from the surface. Once the pilot bore is complete, the hole is enlarged using a back-reamer, and the product pipe is pulled through the widened bore.
Unlike moling, HDD removes spoil from the bore rather than displacing it. Drilling fluid (a water, polymer, or bentonite mix) is pumped through the drill string to flush cuttings back to the surface, stabilise the bore walls, and reduce friction during pullback. This fluid management is a practical consideration: the drilling mud needs to be contained, collected, and disposed of, which adds to the site footprint and logistics. The Health and Safety Executive has documented HDD as a method that can reduce total job costs to around 75% of full open-cut trenching while significantly reducing vibration exposure for operatives.
McFadden’s directional drilling capability covers the technical specification for HDD installations on utility civils projects.
Side-by-side comparison
Moling vs directional drilling: key differences
| Factor | Moling | Directional drilling (HDD) |
|---|---|---|
| Distance | Up to approximately 30 m per shot | Tens to hundreds of metres |
| Diameter | 45 mm to 300 mm (UKSTT range) | Small ducts up to large-diameter mains (150 mm+ routine) |
| Steerability | Non-steered (straight line between pits) | Fully steered with sonde tracking |
| Ground conditions | Compressible soils only (clay, silt, peat) | Most soils including some rock; drilling fluid stabilises the bore |
| Spoil handling | Soil displaced sideways, no spoil removal | Drilling fluid flushes cuttings to surface; mud requires containment |
| Equipment footprint | Mole, compressor, hoses. Fits in a van | Drill rig, fluid mixing unit, rods, reamers. Needs carriageway space |
| Typical crew | 2 operatives | 3 to 4 operatives plus locator |
| Typical applications | Service connections, lead pipe replacements, short road crossings | Main runs, highway crossings, rail crossings, river crossings |
When to choose moling
Moling is the right method when the run is short, the ground is compressible, the route is straight, and the carriageway or footway is clear of existing buried services. If there is nothing in the ground to navigate around, moling saves the cost and disruption of opening up the road. The technique is fast to mobilise, runs from small pits, and completes a bore in a fraction of the time an open trench would take. For utility contractors working under NRSWA permits, that means fewer days of road occupation and a smaller reinstatement footprint.
Moling is the default choice for domestic and commercial water service connections, lead pipe replacements under MDPE, short telecoms duct crossings, and gas service installations where the distance from main to property falls within the mole’s working range. Where surface preservation matters (landscaped areas, car parks, finished driveways), moling delivers the bore without cutting the surface between the two pits. For a detailed look at what moling involves, see what is moling.
Moling suits
Straight runs under 30 m in soft ground. Clear route with no known obstructions. Small-diameter services (water, gas, telecoms, electric). Sites where fast mobilisation and minimal kit matter. Budget-sensitive short crossings.
HDD suits
Runs over 30 m or requiring a curved path. Routes with existing utilities to navigate around. Larger-diameter mains or multi-duct installations. Highway, railway, or watercourse crossings. Projects where bore accuracy and as-built records are specified.
When to choose directional drilling
Directional drilling earns its place when the project exceeds what a mole can deliver: longer distances, larger diameters, curved paths, or ground conditions that would stall or deflect a pneumatic mole. Where existing buried services are dense, the ability to steer the drill head around obstacles using real-time sonde tracking makes HDD the safer and more precise option. Highway crossings, rail crossings, and river crossings are almost always HDD territory because the bore length, depth control, and reinstatement constraints rule out a straight-line unguided method.
Both moling and HDD installations fall under the same regulatory baseline. HSG47, the Health and Safety Executive’s guidance on avoiding danger from underground services, governs the ground survey and cable avoidance work around every launch and reception pit regardless of method. Street works under the New Roads and Street Works Act (NRSWA) require notice, coordination, and reinstatement to the Specification for the Reinstatement of Openings in Highways (SROH) whether the bore is made by a mole or a drill rig. The choice of method does not change the regulatory obligations; it changes the practical approach to meeting them.
On schemes where both methods could technically work, the decision often comes down to programme and cost. Moling is cheaper per metre on short runs with simple ground. HDD has a higher mobilisation cost but becomes more economical as the run length increases, because the rig completes longer sections in a single operation without the need for intermediate pits.
The right method is the one that fits the ground, the distance, and the programme. Neither is universally better. A contractor who operates both, and knows where each one earns its place, will deliver the bore more efficiently than one who defaults to a single technique regardless of the site conditions.
Questions about moling and directional drilling
No. Moling uses a pneumatic impact tool that displaces soil in a straight line. Directional drilling uses a steered mechanical drill head with drilling fluid. They require different equipment, different operator skills, and suit different project types. The international standard ISO 21225 and the UK Society for Trenchless Technology (UKSTT) classify them as separate trenchless methods.
Both methods fall under the same regulatory framework. HSG47, the HSE guidance on avoiding danger from underground services, governs the survey and cable avoidance work around every launch and reception pit. Street works on public highways require notice and coordination under the New Roads and Street Works Act (NRSWA), and reinstatement must meet the Specification for the Reinstatement of Openings in Highways (SROH). The method changes the practical approach, not the obligations.
The decision depends on distance, diameter, ground conditions, route complexity, and whether existing buried services need to be navigated around. Short straight runs in soft ground with a clear route suit moling. Longer runs, larger diameters, curved paths, or routes crossing highways, railways, or watercourses suit directional drilling. A contractor who offers both methods can assess the site and recommend the right technique.
HDD stands for horizontal directional drilling. It describes a steered trenchless boring method that uses a pilot bore, back-reaming, and drilling fluid to install pipes and ducts along a planned underground path. The term is sometimes used interchangeably with directional boring, though directional boring more commonly refers to smaller-scale work.
