Site Investigation

Cable percussion drilling

Cable percussion (shell and auger) is the most common drilling method used for geotechnical site or ground investigations in the UK. Depending on access limitations and favourable ground conditions boreholes up to 60m depth can be formed.

Standard light-cable percussion boring uses a two-tonne capacity winch driven by a diesel engine and a tripod derrick approximately 7m in height. The derrick folds down so that the rig can be towed by a four-wheel drive vehicle. If deep or wide diameter boreholes are required a larger three-tonne capacity winch can be used.

In areas where there is low-headroom or access limitations, a modular or demountable percussive rig is operated. This type of rig is either a ‘cut-down’ version of the standard towed tripod or is a modular rig transported in sections on a trailer.

The borehole is formed using a ‘clay cutter’ for cohesive soils or a ‘shell’ (or bailer) for non-cohesive materials. A chiselling tool can be employed to penetrate very hard ground or obstructions.

The sides of the borehole are supported using steel casing which is lowered into the ground as the boring proceeds. If the exploratory borehole is constructed in sands or gravels, the casing is used to support the borehole sides to allow in-situ testing and sampling.

Disturbed samples may be collected from both the clay cutter and the shell. Undisturbed samples may be recovered from any cohesive strata or weak chalk by driving a hollow tube (100mm open tube sampler) into the ground. Thin-walled piston samples are sometimes used for recovering undisturbed samples of soft normally consolidated soils.

Water samples may also be obtained and because the steel casing seals the borehole, it is possible to sample water horizons at different depths with minimal risk of cross-contamination. However, it should be noted that water samples that are fully representative of the groundwater require the installation of a groundwater sampling well.