Geotechnical drilling

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This topic is part of a series of topics related to geotechnical site investigation. Further information can be found on the main page for that topic.

Diamond drilling to collect rock core samples is commonly used in exploration and mine development programs, and is one of the most expensive forms of geological data collection. Due to this expense, it is prudent that the maximum amount of data possible be collected from the core, and that the resulting hole be put to use, for example by measuring water levels, conducting permeability testing, installing piezometers, and carrying out downhole surveys appropriate for the subsequent design work.

In addition to providing geological and ore grade information, drill core is a valuable source of geotechnical and geomechanical information and at an early stage of study may form the basis for subsequent design work. Depending on the level of study (Table 1), data recording can vary from a summary log of the basic rock characteristics to detailed logs with descriptions of each individual discontinuity.

Hole size and core barrel type are important considerations early in the planning process. The use of a triple tube core barrel is preferred, as it generally results in higher core recovery and less damage to the core when it is being extruded from the core barrel. The hole size is usually either “H” or “N” for exploration diamond drilling, with the larger diameter hole initiated from the surface and then reduced as hole conditions dictate. The hole size required will be determined by the downhole surveys proposed and the installation requirements, and may require tradeoffs depending on the priority for the hole or the geotechnical program in general. For example, a larger hole may be less favourable for televiewer surveys due to problems with image quality that result from centralizing issues, but may be required for multiple piezometer installations. Depending on the ground conditions and the hole depth, the hole diameter may be larger at the top of the hole to allow the hole size to be reduced at greater depth to facilitate drilling. In addition, packer testing equipment may be designed for a certain hole diameter, and this must be considered when planning the drilling program.

Compiling summary information for each drillhole is generally standard procedure, but should include collar coordinates, borehole orientation, borehole survey data, length, core size, etc., as well as any relevant information from drillers logs such as casing depths, areas of additional reaming, locations of hole plugs, etc. Full time supervision is generally recommended at the pre-feasibility level of study and beyond, but if it is not provided during drilling the drillers should be encouraged to record zones where they had to use thicker mud, rods became stuck, areas of difficult drilling, water loss, zones where hole stabilization techniques were used, etc. This will allow the ground conditions to be interpreted in zones where no core was recovered, or where the ground conditions may not have been apparent in the core.

Accurate discontinuity orientation is only possible when the logged orientations are corrected for the hole deviation. The distance between hole orientation measurements can vary, depending on the severity of the deviation of the hole, but should generally be taken every 50 to 100m. Core orientation techniques are described in the section on Geotechnical logging

See also

Discontinuity characterization
Geotechnical logging techniques
Geotechnical model
Geotechnical site investigation

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