eel drive vehicle. Some cable percussion rigs are fitted to lorry bases and these may offer rotary drilling as well. This system of drilling is very common for geotechnical investigations but is widely used on environmental projects as well. This technique is used mainly for boring into and sampling superficial deposits rather than bedrock. The boring equipment is simple in design and comprises a cable passing from a winch, over a pulley at the top of the rig’s mast to the boring tools. The tools are lifted and dropped successively and, as they advance and the borehole is deepened, more cable is fed from the winch drum.
The boring tools are robust steel tubes with a solid weight fitted above them, around 1500 mm long and of diameters ranging from 140 mm to 300 mm. They are designed to sample the ground in that, as the open end hits the formation on the down stroke; material is forced inside and retrieved when the tool is withdrawn at the surface. Water is sometimes added to the borehole to assist in this. Different boring tools are available to suit different types of ground conditions. A shell (a round cylinder with a flap valve) is used in non-cohesive materials while a claycutter (with windows in its sides) is used in cohesive materials. When the ground contains layers of cobbles or boulders that inhibit the boring tools, a chisel can be used instead to break through and then boring resumes below.
As this technique is used mainly in soft ground conditions, the borehole may need to be lined with “casing” that is advanced at the same rate as the boring tools to maintain stability and minimise the entry of unwanted groundwater into the deepening borehole. In some circumstances, it may be necessary to use several different strings of casing and the more formations that are requiring sealing-off, the larger the diameter of casing that will be needed to start the borehole. Casings of 250 mm, 200 mm and 150 mm are conventional and it is the 150 mm diameter casing that is most commonly used. The technique may be used when there is a need to drill through an aquiclude into an aquifer. In these cases, the borehole is drilled in stages and at each change in casing diameter, a robust bentonite seal is placed at the base and allowed to rehydrate prior to further advancement of the borehole at the narrower diameter.
Soil samples can be taken for environmental and geotechnical purposes. “Bulk” disturbed samples can be taken from the drill cuttings and undisturbed samples can be taken using U38 or U100 sample tubes (denoting 38 mm and 100 mm diameters respectively with the latter also called U4 denoting 4 inch). In situ geotechnical tests such as SPT (Standard Penetration Tests) can also be carried out during drilling if required.
Cable percussion drilling is generally used for depths less than 40 m and typical drilling depths for ground contamination investigations are 5 m to 20 m. Significantly greater depths can be achieved from the rarer, very heavy duty, rigs.
“A” frame cable percussion rigs are fairly maneuverable as they are light but the length of the frame (6 m) prevents access to tight locations. Many contractors have “cut-down” mast rigs for access and operation in limited headroom locations if required. The method can generate considerable mess, particularly in soft clays and when shelling out sands and silts.
Rotary drilling techniques are employed where boreholes are required in very dense gravel or bedrock. Samples of bedrock are recovered in seamless plastic tubes for subsequent logging by a suitably qualified engineer and for laboratory testing.
Rotary drilling techniques are utilised where drilling into bedrock is required to check for shallow mine workings or to determine rock quality. All our drilling rigs can undertake rotary coring to recover rock core samples at various diameters. The cores are then logged by experienced geoenvironmental engineers in accordance with the relevant British and European standards.
Alternatively the borehole can be drilled using rotary open hole methods in which samples are not reco