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 recovered; rock cuttings are returned to the surface during the drilling and this allows a rapid examination of the strata. This method is often appropriate to locate shallow coal seams or mine workings and allows a number of boreholes to be completed in a short space of time.
Continuous monitoring of gases such as carbon monoxide, methane and carbon dioxide can be carried out while drilling through old mine workings.
Dynamic (Window) Sampling / Dynamic Probing
Window sampling is a percussive method of creating small diameter boreholes. Mostly the technique is achieved using hand-held equipment but some small drilling rigs have the capacity for window-sampling and the power of the rig enables window samplers to be driven to, and extracted from, greater depths compared to the manual technique.
A window sampler is a high tensile steel tube with a hardened cutting shoe to penetrate hard materials. Each sampler is usually 1 m or 2 m long with a series of “windows” or slots cut in the wall of the tube through which to view or extract soil samples. Samplers are driven down into the ground using a percussive hammer. A full set of window samplers usually consists of around four samplers ranging in diameter from 80 mm down to 35 mm. These are used systematically, starting with the widest and subsequently at reducing diameter to the required depth or limit of the technique. The depth limit tends to be around 8 m to 10 m although the technique is usually used to a maximum depth of around 5 m. In practice, the limiting factor tends not to be how deep the samplers can be driven in but rather whether the ground conditions are such that they may then be pulled back out. Coarse dense gravels, in particular, can grip the samplers and make extraction extremely difficult. The driller will assess the conditions at each location individually.
The start diameter is dictated by the ground conditions i.e. the softer the conditions, the wider the sampler set that can be driven in. Typically, however, a 60 mm to 80 mm diameter start sampler w