Coring in glacial sequences : comparison of cable percussion and window sampler techniques

The glacial sequence of Northamptonshire in the English Midlands is relatively poorly studied, but for over 60 years has been known to include a thin and laterally impersistent chalk-free diamicton beneath a thick and persistent chalky till blanket. During early preparation of new BGS publications in this region, it was deemed necessary to improve knowledge of these deposits by obtaining high quality samples for further testing. This report compares the merits of U100 sampling, using ‘traditional’ cable percussion (aka shell and auger) drilling techniques, and use of a ‘window/windowless sampler’, to acquire near-continuous cored samples through till-dominated (i.e. clay-rich) superficial deposits less than 10 metres thick. In the event, the cable percussion operation was subcontracted at short notice, and this may have compiled the problems of supervising two sites simultaneously (see 4). With hindsight, it seems desirable to forbid subcontracting and to maintain constant supervision on a cable percussion operation. The acquisition of U100 samples by the cable percussion method offers the following advantages: large diameter (100mm) samples, potential for near continuous coring, typically capable of 30 to 40m depth, widely available, simple technology, not easily obstructed, able to operate below water table in sand and gravel. The disadvantages include access restrictions, safety issues, requirement for careful operator/close supervision for good sampling, problems posed by use of water for drilling, costly and time consuming mobilisation, noise, water supply, hole and site restoration. The window/windowless sampler offers the following advantages: ease of access and positioning, speed, fewer safety issues, continuous sampling from surface to terminal depth, no water supply, little restoration necessary, relatively quiet, likely to be cheaper. Its disadvantages include safety issues of solo operation, small diameter samples, limited depth capability, easily obstructed, less robust, limited availability. The sub-sampling and testing strategy undertaken on the samples obtained is also set out.