Cable Percussion Boring
Light-cable percussion is the most common drilling method used for geotechnical site or ground investigations and depending upon access limitations and favourable ground conditions deep boreholes up to 50m depth can be formed.
Light-cable percussion drilling uses a mobile rig with a winch of between one to two tonne capacity driven by a diesel engine and a tripod derrick of about 7m height. The derrick folds down so that the rig can be towed by a four-wheel drive vehicle. In areas where there is restricted headroom or access limitations a demountable or low-headroom drilling rig can be used. This rig is brought to site in sections on a trailer and the equipment is re-assembled on site.
A borehole is formed using either 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 conditions 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 pushed into position to support the borehole sides to allow any insitu testing to be carried out and the material to be removed using the shell.
The material recovered 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 used for recovering undisturbed samples of soft normally consolidated soils.
Water samples may also be obtained during the drilling process and because the 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 monitoring well.