7.3 Sampling by drilling

7.3.1 General

7.3.1.1 Drilling methods and equipment shall be selected as a function of the required sampling category (see Table 5) geological and hydrogeological conditions.

7.3.1.2 The flushing medium should be selected to meet the requirements of the investigation and if necessary appropriate additives can be added to the flushing medium. Hydrogeological requirements shall be considered when selecting flushing medium.

7.3.1.3 In soft rocks, only double-tube or triple-tube corebarrels shall be used.

7.3.1.4 The bit type shall be selected to efficiently cut the rock type (see Table C.16).

7.3.1.5 Cementing can be necessary to stabilise the borehole or to sample when a crushed rock zone shall be passed.

7.3.1.6 The orientation and inclination of boreholes shall be specified, including the maximum acceptable deviation, taking the expected investigation targets and ground conditions into account.

7.3.2 Sampling by rotary dry core drilling

7.3.2.1 In sampling by rotary dry core drilling, a tube system is fitted with a bit at its lower end and is rotated and fed intothe rock mass by thedrill rig via the drillstring. This actionproduces a core sample within the tube system. The sampling tool is a single tube with a borehole diameter of 70 mm to 200 mm. This sampling technique can be used to recover core samples in soft, erodable, water-sensitive rocks. It is less suitable for rocks of medium to high hardness.

7.3.2.2 To prevent overheating of the bit, core runs should not exceed 0,5 m.

7.3.2.4 Sampling by rotary dry core drilling is a category B sampling method (see Table 5).

7.3.3 Sampling by rotary core drilling

7.3.3.1 In sampling by rotary coredrilling, a tube system fitted with a bit at itslower end is rotated and fed into the rock mass by the drill rig via the drill string. This action produces a core sample within the tube system. The sampling tool, i.e. the corebarrel, can be a single tube, double tube or triple tube with a borehole diameter of 70 mm to 200 mm. A flushing mediumis normally used.

7.3.3.2 A single-tube corebarrel consists of a core tube fitted with abit at its lower end and a corebarrel head that attaches to the drill rods at its upper end. A core lifter can be fitted between the bit and the core tube or directly within the bit. The flushing medium passes between the inside diameter of the core tube and the recovered rock core and continuously washes the length of the recovered sample.

7.3.3.3 A double-tube corebarrel consists of two concentric tubes and a bearing arrangement in the corebarrel head which allows the inner tube to remain stationary, whilst the outer and bit is rotated by the drill string. A core lifter is fitted between the bit and the inner tube. The flushing medium passes through the annulus between the inner and outer tubes, thus protecting the recovered sample from erosion.

7.3.3.4 A triple-tube corebarrel is similar in construction to the double-tube design but is fitted with an additional third tube within the inner tube.

7.3.3.5 Both double-tube and triple-tube corebarrels can be fitted with extensions to their inner tubes that pass through the bit, for use in very soft formations.

7.3.3.6 Samples can be obtained by this method as cores/cuttings. The single-tube corebarrel only allows core recovery in consolidated formations, whereas double-tube and triple-tube corebarrels can be used in all rock formations. All these types of corebarrels can be fitted with plastic liners within the inner tube to assist core recovery and protect the recovered core sample.

7.3.3.7 Sampling by rotary core drilling with either single- or double-tube corebarrel is generally a category B sampling method. The sampling method using a triple-tube corebarrel isgenerally category A (see Table 5).

7.3.4 Sampling by wireline core drilling

7.3.4.1 In sampling by wireline core drilling, a double-tube or triple-tube corebarrel with a bit fitted to the lower end is rotated and fed into the rock type to be drilled by the drill rig via the wireline drill rods. This action produces a core sample within the inner tube of the corebarrel. The borehole diameter rangeis from 70 mm to 180 mm. When the coring run is completed, the inner tube containing the core sample is withdrawn through the drill rods by means of a wireline cable and winch. The bit, outer tube and drill rods remain in the borehole during this process.

7.3.4.2 Sampling by wireline core drilling is a category A sampling method.

7.3.5 Sampling of cuttings by rotary open hole drilling

In sampling by rotary open hole drilling a rock roller, drag or button bit is rotated and fed into the rock type so generating cuttings. These cuttings are raised to the surface by the velocity of the flushing medium and collected or sampled at the borehole mouth. The borehole diameter usually ranges from 70 mm to 311 mm. No core samples are produced by this method, only disturbed cuttings, and therefore the sampling category is C.

7.4 Block sampling

7.4.1 In block sampling, samples are obtained from a trial pit, heading, shaft or from the bottom of the borehole by using special samplers with cutting procedure.

7.4.2 This sampling technique is usually a category A sampling method.

7.5 Integral sampling

7.5.1 In integral sampling, complete, orientated and undisturbed core samples can be taken in order to preserve the rock mass characteristics – untainted by the drilling effects – in the core samples, and to determine the primary conditions of the natural discontinuities and their orientation.

7.5.2 In this technique, a perforated central tube shall be placed in a predrilled hole with a minimum diameter of 25 mm. This predrilled hole shall be connected with the surrounding rock material by an appropriate binding material, e.g. cement, over its entire length. The binding material shall be inserted through the central tube without pressure. The sample shall be recovered by over-drilling with an appropriate larger minimum core diameter of 100 mm, after the required setting time of the binding material. Consequently, a sampling method of sampling category B shall be chosen.

NOTE For further information onintegral sampling, see also Reference [23] in the Bibliography.