Cable percussion (shell and auger) still remains one of the most popular drilling method used for geotechnical site or ground investigation though in recent years it has been replaced in many jobs by hand held percussive window sampling or increasingly by window sampling carried out using tracked mini drills and rigs . The reason is simply that window sampling is cheaper, more productive, causes less disturbance and can be brought into restricted access or difficult access sites. In addition the window sampler can be transported in a van operated by 1 person and does not require a yard for storage. However the drawback of window sampling is that it is restricted to much shallower depth typically 5m to 8m though Geoinvestigate has achieved 11m on occasion.
Depending upon access limitations and favourable ground conditions boreholes up to 60m depth can be formed and even deeper though drilling time and costs tend to rise steeply as the depth increases.
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
Water samples may also be obtained during this 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. Gas wells can also be installed in holes formed by this technique as well as flexible tubes and instruments to measure the vertical and/or horizontal deflection of the ground and movements of slopes and cliff tops in soil.
A heavy chisel is used to break-up obstructions such as boulders and can penetrate a short depth into weak rock. Where a core sample is required of the rock a rotary follow on pendant drill attachment can be used suspended from the tripod or the rig can be pulled back or removed altogether and rock drilling carried out using a rotary drill rig.