Cable percussive or shell and augur boreholes are the most common ground investigation technique in Ireland and the UK. It is well suited to Irish soils and offers a flexible and economic method for formation of exploratory holes.
Standard light-weight rigs consist of a tripod mast structure and diesel-powered winch unit mounted on a wheeled frame base. The tool string is attached to a wire rope which passes over a crown sheave at the top of the tripod mast structure and then to the winch drum on the base unit. The tool string is raised using the winch and dropped by freefall action thus causing cutting, crushing or fragmentation of the formation.
It is generally used to retrieve Class 3 to 5 disturbed samples for classification using a clay cutter for fine soils or a bailer in coarse soils whilst supporting the walls with casing. The samples are logged by the driller and / or sent to the laboratory for testing. For harder formations such as rock, rotary drilling can be completed. Field tests include the Standard Penetration Test (SPT) where a thick-walled sample tube or solid cone is driven into the ground by blows from a 63.5kg hammer over 300mm. The N-value offers an indication of soil strength, relative density and other parameter after some corrections are applied. On completion, a piezometer can be installed to monitor pore pressures