01A winch, a tripod and a lot of skill.
The rig is a winch and a folding tripod that can be towed to site. A heavy tool on a steel cable is lifted and dropped to advance the hole: a clay cutter in clay, a shell in sand and gravel, and a chisel to break through hard layers or obstructions. Steel casing follows the hole down to stop it collapsing.
It works in weaker soils and on sites with poor access or low headroom, and it is the method to use when good-quality samples are needed. Cable percussion is a soils method: where we need to go into bedrock, rotary drilling takes over.
Before any borehole starts, the position is checked for buried services, and a hand-dug inspection pit is commonly excavated first.
02Samples and tests as we go.
- Disturbed samples at regular intervals and at every change of strata, for description and laboratory testing.
- Undisturbed samples in clay, using U100 or thin-walled UT100 tubes driven into the ground, for strength and consolidation testing.
- Standard penetration tests (SPT), to BS EN ISO 22476-3. A sampler is driven with a standard 63.5kg hammer and drop, and the number of blows for the last 300mm, the N value, indicates how dense or stiff the ground is.
- Environmental samples, where contamination is suspected.
- Groundwater, recorded where it is struck, with monitoring wells installed in the finished borehole where needed.
03Every metre, logged.
Everything we find goes onto the borehole log: the depth and description of each layer, where each sample and test was taken, the SPT results and any water strikes. It is the record your engineer designs from. Our guide on how to read a borehole log explains it line by line.
Our drillers are qualified to NVQ Level 2 in Land Drilling, the qualification ground investigation specifications commonly ask for.
04When cable percussion is the answer.
- When foundations or piles need information deeper than a trial pit or window sample can reach.
- Through thick glacial clay, sand and gravel, or made ground, which is common across Wigan and Greater Manchester.
- When high-quality samples are needed for strength and settlement testing.
- For commercial units, developments, highways and other larger schemes.
05Questions we get asked.
How deep will you drill?
It depends on the structure and the ground. The borehole needs to go below the depth the foundation loads will affect, and your engineer sets the depth. With cable percussion we can form boreholes up to 60m deep where access and ground conditions allow.
What is an SPT N value?
A count of hammer blows to drive a standard sampler 300mm into the ground. It gives an indication of how dense or stiff the ground is and is used in foundation and pile design.
Why cable percussion rather than window sampling?
Cable percussion goes deeper, takes larger and better-quality samples, and allows SPTs in deeper soils. Window sampling is quicker and suits shallow investigations and tight access.
Can it drill into rock?
Not generally. Cable percussion is a soils method. Where we need to prove or sample bedrock, we use rotary drilling.
