Cable percussion drilling (shell and auger) is the most common drilling method used for geotechnical site or ground investigations in the UK. Depending upon 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 used. This type of rig is either a ‘cut-down’ version of the standard towed tripod or is modular rig which is transported in sections on a trailer and the component parts are assembled at the borehole location.
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 used 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 hole is formed in sands or gravels, particularly within the saturated zone below the water table, the steel casing will be driven into position to support the borehole sides to allow in-situ testing to be carried out and the soil to be recovered using the shell.
The material sampled from the borehole is generally sufficiently representative to determine the depth and description of the geological strata. Disturbed samples may be collected from both the clay cutter and the shell. Undisturbed samples may be recovered from any cohesive strata or from weak chalk by driving a hollow tube (100mm open tube sampler) into the ground and withdrawing the resultant core for examination and laboratory analysis. Thin-walled piston samples are sometimes used for recovering undisturbed samples of soft normally consolidated soils.
Water samples may also be obtained during the drilling process and because the steel casing seals the borehole from the surrounding ground, 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.