Table 7.17.

Cable percussion (shell and auger) boring

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  • Heavy tools are repeatedly raised and dropped using a winch to cut, chisel or bail out the soil within the borehole;

  • In unstable soils the borehole is supported by sectional threaded steel casing;

  • 150 –300 mm diameter;

  • Depth 40–60+ m;

  • 10–20 m per day achievable;

  • Collapsible rig is towed behind a suitable vehicle;

  • Working area 2 m × 6 m and 7 m clear headroom.

  • Sites where unrestricted vehicle access can be gained and that are relatively level;

  • Suitable for all natural superficial soils and weathered rock;

  • Can obtain all standard forms of soil samples;

  • All standard in situ tests can be undertaken;

  • All standard forms of monitoring well can be constructed in the completed borehole.

  • Slow progress rates in coarse granular/dense/hard strata and bedrock;

  • Addition of water may be required to penetrate some soils above the water table;

  • Drilling disturbance in gravels, silts/sands, very soft clays and weathered rocks may affect the quality of samples and in situ tests;

  • Vibration and noise;

  • Difficult to set a rig up on or near sloping ground.

Cost factor: medium
Relevance to glacial/periglacial depositsOften seen as the ‘default’ site investigation tool in the UK but demonstrates clear limitations in the quality of samples that can be obtained, making the investigation of complex glaciogenic sequences difficult. Structures and textural variation in glaciogenic and periglacial deposits that might be considered important are often destroyed or deformed by this technique. Management of casing is important as this can often become stuck due to side friction in gravels and in swelling clays. Encounters with granular layers under high hydrostatic pressure can result in running sands and stuck casing. Extremely large quantities of water may be required to maintain a working head when drilling in free-draining granular deposits. Typical driven tube (U100 or U100 (thin wall)) methods of obtaining intact samples struggle with tills containing high clast content or over-consolidation. Progress likely to be very slow in over-consolidated massive diamictons and through cobble beds. Will likely refuse on large boulders, giving a false indication of rockhead. An experienced driller is required to adapt drilling method (casing size, tool weight, tool type, head of water, etc.) to achieve desired depths in variable glacial and periglacial deposits. Tools and weights will need to be more rugged than those typically used by drillers familiar with non-glacial deposits. Figure 7.18 illustrates the complications that can arise when using cable percussion techniques in glacial deposits.