What Is Cable Percussion Drilling?
Cable percussion drilling — also known as shell and auger drilling — is a method of advancing a borehole through the ground using repeated impact. A heavy cutting tool is suspended from a steel cable, raised by a winch, and allowed to fall under gravity to break up and collect the soil at the base of the borehole.
The rig consists of a tripod (or A-frame mast), a winch with steel cable, and a set of interchangeable drilling tools. The simplicity and robustness of the equipment is one of its greatest advantages — there are fewer mechanical components to fail, and the system can be operated effectively in almost any conditions.
How Does It Work?
The drilling process follows a well-established sequence:
Starting the Borehole
A steel casing (typically 150–200 mm diameter) is driven into the ground to support the borehole walls and prevent collapse. In softer ground, the casing may advance ahead of the drilling tools. In firmer ground, the tools lead and the casing follows.
Advancing Through Cohesive Soils
In clay and silt, a clay cutter is used — an open-ended cylindrical tool that cuts into the soil and retains it inside the barrel. The tool is raised to the surface and the soil is knocked out for examination and sampling. This allows the driller to produce a detailed log of the ground as the borehole deepens.
Advancing Through Granular Soils
In sand and gravel, a shell bailer (or sand shell) is used. This is a tube with a flap valve (clack) at the base that opens as the tool is driven down, allowing water and soil to enter, and closes as the tool is lifted, retaining the material inside. Water may be added to the borehole to help fluidise loose sands and aid recovery.
Sampling and Testing
At specified intervals, the driller stops advancing and carries out sampling and in-situ testing. Disturbed samples (bulk bags or jars) are taken for classification and chemical testing. Undisturbed samples are recovered by pushing or driving a thin-walled tube (U100 sampler) into the soil at the base of the borehole — these preserve the soil's natural structure for laboratory strength testing.
Standard Penetration Tests (SPTs) are carried out at regular intervals, typically every 1.0–1.5 metres. The SPT provides a direct measure of ground resistance that is used worldwide for foundation design. See our FAQ for more detail on SPT testing.
What Are the Advantages?
Cable percussion drilling has endured as the industry standard for good reasons:
- Versatility — effective in virtually all soil types, from soft alluvium to dense gravel and stiff clay
- Depth capability — can reach 50 metres or more in suitable ground conditions
- Sampling quality — allows recovery of high-quality disturbed and undisturbed samples at any depth
- In-situ testing — permits SPTs, permeability tests, and vane shear tests within the borehole
- Monitoring installations — boreholes can be fitted with standpipes, piezometers, and gas monitoring wells
- Portability — the equipment can be made modular and carried into confined spaces where other rigs cannot reach
- Reliability — simple, robust equipment with minimal mechanical complexity
- Ground logging — the driller can observe and log the ground continuously as the borehole advances
Modular Rigs for Confined Access
One of the key advantages of cable percussion drilling over rotary or tracked rigs is that the equipment can be fully disassembled into individual components. At KF Drilling, we have taken this further by engineering custom modular rigs specifically designed for the most restricted sites.
Our rigs can be hand-carried through doorways, corridors, stairwells, and narrow walkways, then reassembled on site. Modular mast extensions allow drilling in low-headroom environments such as basements and undercrofts. This capability opens up investigation sites that would be inaccessible to any other drilling method.
For real-world examples of confined access drilling, see our project case studies including investigations in Tower Hamlets, South London, and behind Claridge's Hotel in Mayfair.
When Is Cable Percussion Drilling Used?
Cable percussion drilling is the method of choice for the majority of geotechnical and environmental site investigations in the UK. It is specified in BS 5930 and Eurocode 7 as a standard investigation method and is used for:
- Foundation design investigations for buildings and structures
- Contaminated land assessments and environmental monitoring
- Basement and underground construction investigations
- Infrastructure projects including roads, railways, and bridges
- Groundwater monitoring and dewatering assessments
- Slope stability and retaining wall investigations
Need Cable Percussion Drilling?
KF Drilling has been providing specialist cable percussion drilling services for over 30 years, working with consultants, engineers, and developers across the UK. Whether your site is straightforward or presents complex access challenges, get in touch to discuss your requirements.