Cable Percussion (Shell and Auger) Drilling is carried out by a minimum of a two-man crew, it being one of the most common forms of drilling for site investigations alongside Window Sampling. Boring will normally be carried out using a cable percussion rig (A Dando 150, Dando 175, Pilcon Wayfarer or similar) and tools such as casings, shells, stubbers, chisels and sinker bars are used.
This technique as described in BS5930: Section 20.5 can also be carried out using modular rigs capable of working down to a headroom of 2.5m. The minimum diameter of borings or internal diameter of casing will normally be 150mm, however casings of 200, 250 and 300mm and larger sizes are also used. It is normal for diesel engine powered rigs to be used, however, pneumatic or electric rigs are also available.
Using a two-tonne capacity winch driven by a diesel engine and a tripod derrick, the borehole is formed using a ‘clay cutter’ for cohesive soils or a ‘shell’ for non-cohesive materials. Standard Penetration Tests (SPTs) are carried out every metre and at the base of the borehole after the final depth. The sides of the boreholes can be supported using steel casing which is lowered into the ground as the drilling continues. If the exploratory borehole is within sands or gravels, the casing is used to support the borehole sides to allow in-situ testing and sampling.
Disturbed soil samples will be collected at regular intervals during the drilling in the form of Bulk bags and 1L Tubs. Undisturbed samples may be recovered from any cohesive strata or weak chalk by driving a U100 tube into the ground. In borings where hard strata or obstructions are encountered the drilling crew will continue boring using chisel or similar approval tool for a minimum approved time (normally 1 hour) in an attempt to penetrate the hard strata or obstruction.
Gas and Groundwater installations can be installed in Cable Percussion Boreholes allowing for Gas and Groundwater Monitoring and water sampling. Upon completion of the boring, the borehole will either be backfilled with arisings, grouted up or a monitoring device installed. The borehole will be properly reinstated so that no depression is left; a certain amount of mounding may be required to achieve this in the long term.