Table 7.19.

Soft ground rotary sampling

Soft ground rotary drillingMethodApplicationsLimitations
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  • Borehole advanced by rotary drilling with modified core barrels which incorporate a cross-cut plate on the full diameter;

  • Fully contained re-circulated water flush with low exit velocity to remove cuttings and support sides of uncased borehole;

  • Depth <20 m;

  • 10–15 m per day;

  • Working area to suit form of rotary rig used.

  • Minimal disturbance of soft sensitive soils ahead of the borehole through elimination of percussive action or water jetting effects inherent with other boring techniques;

  • Usually used in combination with piston samplers to obtain a near-continuous soil samples;

  • Often carried out in conjunction with CPTs.

  • Only applicable to very soft or soft clays/silts;

  • Boreholes cannot be advanced past obstructions or non-cohesive deposits. Other forms of drilling are required where these are encountered;

  • The technique will not by itself guaranty high-quality samples and it should be considered in combination with other factors.

Cost factor: medium to high
Relevance to glacial/periglacial depositsRarely applicable for glacial deposits other than detailed sampling of soft glaciolacustrine silts and clays.