What is a Moling

Moling is a trenchless technique used to install pipes and ducts beneath the ground surface without the need for continuous open trench excavation. It involves using a pneumatic piercing tool, commonly called a mole, to create a narrow underground tunnel through which a pipe or duct can be pulled or inserted. This method is frequently used in utility installations, including water, gas, electricity and telecommunications, as well as in drainage and small-diameter sewer works.

Moling offers a low-disruption, cost-effective solution for crossing driveways, roads, lawns, footpaths and other surfaces where traditional trenching would be destructive or impractical.

How moling works

The moling process begins with the excavation of two small pits: a launch pit and a reception pit. The launch pit contains the mole, which is a pneumatically driven steel cylinder with a pointed head. The mole is connected to an air compressor above ground via a flexible hose.

Once the mole is positioned at the correct depth and direction within the launch pit, it is activated and begins to move forward through the soil, driven by repeated percussive action. As it progresses, it compacts the surrounding soil and creates a narrow tunnel, typically 50 to 70 millimetres in diameter.

When the mole emerges in the reception pit, a pipe or duct is pulled through the newly formed tunnel. In some cases, the mole may be used to directly pull the pipe behind it during its initial pass. Once complete, the pits are backfilled and the surface restored with minimal visible disturbance.

Applications of moling

Moling is used in a variety of settings where subsurface installation is required but full excavation is undesirable or impossible. Common applications include:

• Installing new water supply pipes to domestic properties

• Upgrading gas or electrical service connections

• Laying communication or fibre optic cables across developed areas

• Connecting buildings to foul or surface water drainage systems

• Bypassing tree roots, driveways, paving or mature landscaping

In many cases, moling is the preferred alternative to open-cut methods due to its speed, low environmental impact and reduced surface reinstatement costs.

Advantages of moling

Moling offers several benefits over conventional trench excavation:

• Minimal disruption to surface features such as gardens, roads and paths

• Faster installation, often completed in a single day for small jobs

• Reduced labour and equipment requirements

• Lower reinstatement costs due to limited surface damage

• Decreased risk of damaging adjacent utilities or tree roots

• Environmentally friendly by preserving surface vegetation and soil structure

Moling is particularly suited for use in residential areas and urban environments where access or disruption must be carefully managed.

Limitations and considerations

While moling is a versatile technique, it is not suitable for all ground conditions or project types. Important limitations and considerations include:

• Soil type: Moling works best in cohesive soils such as clay or compacted sand. Loose or stony soils may deflect the mole or cause collapse of the tunnel.

• Depth: Typical moling depths range from 0.5 to 1.5 metres. Greater depths are difficult to achieve without specialised equipment.

• Accuracy: While generally straight, the mole’s path can deviate, particularly in challenging soil conditions. This makes moling less suitable for precision-required installations or close proximity to existing services.

• Obstructions: Underground obstacles such as boulders, old foundations or existing pipes can halt progress and require re-routing or excavation.

• Pipe diameter: Standard moling tools are limited to installing pipes of 25 to 63 millimetres in diameter. Larger pipes require alternative trenchless methods such as horizontal directional drilling (HDD) or pipe bursting.

Pre-site surveys and utility detection are essential before beginning moling operations to avoid service strikes or misalignment.

Equipment and operation

The core of the moling system is the pneumatic mole, typically powered by compressed air supplied from a trailer-mounted compressor. Additional equipment includes:

• Air hoses and connectors

• Lubricants to reduce friction in difficult soils

• Pipe pullers or towing heads

• Launch and reception pit shoring materials

• Inspection cameras or sondes for tracking the mole (optional)

Most moling operations are carried out by two-person crews and require minimal machinery, making them ideal for constrained or sensitive environments.

Health, safety and regulation

Like all underground works, moling must comply with health and safety standards and local regulations. Key safety practices include:

• Conducting a utility survey before excavation

• Using cable avoidance tools (CAT and Genny)

• Securing pits to prevent collapse or entry hazards

• Controlling air pressure and monitoring equipment

• Providing training and supervision for operatives

In the UK, installations involving moling must also meet the standards set out in Water Regulations, Building Regulations (Part H for drainage), and any relevant local authority guidelines.

Comparison with other trenchless methods

Moling is part of a broader family of trenchless techniques, each suited to different project types and scales:

Moling is typically the simplest and most affordable option for small-scale or domestic applications.

Conclusion

Moling is an efficient, low-impact method for installing small-diameter pipes and ducts underground without the need for extensive surface excavation. It is especially valuable in urban or landscaped environments where minimal disruption is essential. While best suited to certain soil conditions and pipe sizes, moling remains a popular and effective trenchless technique for residential and small commercial projects.

With proper planning, equipment and operator training, moling offers a cost-effective, environmentally friendly alternative to traditional trenching, supporting modern utility infrastructure with minimal surface damage and downtime.