6.3.2.4 Sampling by reverse circulation drilling
6.3.2.4.1 In sampling by reverse circulation drilling, the flushing fluid passes down the outside of the drill rods over the cutting face of the bit then, carrying the cuttings, passes through a central orifice in the bit and up through the drill rods to the surface.
6.3.2.4.2 The borehole diameter is usually between 150 mm and 1 300 mm.
6.3.2.4.3 This sampling technique is suitable for all soils.
6.3.2.5 Sampling by shell auger drilling
6.3.2.5.1 In sampling by shell auger drilling, a shell auger is used as the sampling tool. Single-edge shell augers shall be used for cohesive soils and double-edge shell augers for non-cohesive soils. Double-edge shell augers with an internal clack are sometimes used for non-cohesive soils. The sampling length into the soil shall not exceed the maximum length of the shell auger. During the penetration of the shell auger, the vertical displacement of the soil in the shell auger shall be minimised. After the screwing, the drilling tool shall be completely removed from the borehole and the sample shall be extracted from the auger.
6.3.2.5.2 Sampling by shell auger drilling shall be only used if the borehole is stable or with a casing.
6.3.3 Sampling by use of hammer driving methods
6.3.3.1 Sampling by percussive drilling
In sampling by percussive drilling, a clay cutter tube device with an internal cutting edge at the lower end is driven into the soil by hammer blows transmitted to it via appropriate drill rods. It is generally suitable for clay, silt and soils with a particle size up to De/31) and with a borehole diameter up to 300 mm. The sample is retained within the clay cutter by a suitable retainer.
6.3.3.2 Sampling by rotary percussive drilling
In sampling by rotarypercussive drilling, a clay cutter tube device with a cutting shoe fitted to the lower end is driven into the soil by hammer blows and the supporting drill rods slowly rotated. It is generally suitable for clays, silt and soils with a particle size up to De/3 and a borehole diameter up to 300 mm. The sample is retained within the clay cutter tube.
6.3.4 Sampling by cable percussion drilling
6.3.4.1 In sampling by cable percussion drilling, appropriate percussive sampling, drilling and bailing tools are suspended on a cable, which is raised and free lowered by a winch so allowing the mass of the equipment to drive the tools into the soil. Boreholes up to 500 mm can be bored and sampled using this method.
6.3.4.2 Sampling by cable percussion drilling can be used in all soils by the selection of the appropriate equipment.
6.3.5 Sampling by hollow stem auger drilling
6.3.5.1 In sampling by hollow stem auger drilling, the hollow stem auger, which consists of a spiral flight wound round a hollow central tube and fitted with an appropriate cutting head, is drilled into the soil in a similar manner to the flight auger (see 6.3.2.3). Additional sections of hollow stem auger are added till the required depth is reached.
6.3.5.2 Once the required depth is reached, a sampling system or corebarrel can be lowered through the centre tube ofthe hollow stem auger to take samplesfrom the bottom of the hole, without removing the hollow stem auger string.
6.3.6 Sampling by grab drilling
6.3.6.1 In sampling by grab drilling, the sampling tool is a cable with grab.
6.3.6.2 The borehole diameter should be between 400 mm and 1 500 mm.
6.3.6.3 This sampling technique is thepreferred method for gravel, cobbles and boulders with a size less than De/2. It isunsuitable for firm, cohesive soils and boulders larger than De/2.
6.3.7 Soil sampling by small diameter drilling
6.3.7.1 Small diameter drilling refers to all drilling with a hole diameter between 30 mm and 80 mm. In principle, the same drilling methods and equipment described in Table 2 can be used.
6.3.7.2 Sampling by small diameter drilling is only suitable in sands and fine-grained soils.
When employing small diameter drilling methods, it shouldbe noted that the samples recovered are sufficient in size and mass, suitable for the scheduled laboratory testing.
6.3.7.3 Generally the quality of a core sample obtained by small diameter drilling is lower than if larger diameter drilling with the same drilling method is used.
6.3.8 Sampling by resonance drilling
In sampling by resonance drilling, a tube fitted with a bit at its lower end is fed into the soil or soft rock by vibration of a frequency variable from 30 Hz to 150 Hz. The frequency is adjusted after each addition of extension rod in order to obtain a resonance.
When the penetration rate is too low, the sampler or corebarrel can be rotated (1 to 5 rotations per metre). The sampler or corebarrel can be equipped with a plastic lining tube.
