25.2 Sample quality

The sampling procedure should be selected on the basis of the quality of the sample that is required, which depends on the suitability of the sample for the types of laboratory tests to be carried out (see BS EN 1997-2:2007, Section 3 and BS EN ISO 22475-1). Table 3 shows the basis for assessing quality class together with the category of sampler capable of achieving the class.

NOTE 1 In general, Classes 1, 2 and 3 are achieved with tube samplers, rotary coring or block samples (see 25.1); these are often known as intact samples. Samples from the drill tools (see 25.3) are Class 3, 4 or 5.

NOTE 2 Laboratory tests cited in Table 3 are relevant only to soils, but the quality classification may usefully be applied to rock samples in respect of other properties. Laboratory testing of samples is discussed in Section 9.

Where Category A sampling methods are used, it is sometimes only possible to obtain samples with some degree of disturbance, i.e. Class 2 at best rather than Class 1. The results of any strength or compressibility tests carried out on such samples should be treated with caution.

NOTE 3 A further consideration in the selection of procedures for taking Class 1 samples is the size of the sample in relation to the likely soil particle size and by the structure of the ground, which for soil is often referred to as "the fabric" (see Rowe, 1972 [27]).

NOTE 4 The presence of coarse particles is likely to adversely affect the quality of the recovered sample. This is due to the disturbance that can occur during sample taking.

Where the ground contains discontinuities of random orientation, e.g. in a clay fill, the sample diameter or width should be as large as possible in relation to the spacing of discontinuities. Alternatively, where the ground contains strongly orientated discontinuities, e.g. in jointed rock, it might be necessary to take samples that have been specially oriented. For fine soils that are homogeneous and isotropic, samples as small as 35 mm in diameter may be used. However, for general use, samples 100 mm in diameter should be used because the results of laboratory tests can be more representative of the ground mass. In special cases, samples of 150 mm and 250 mm in diameter or block samples of larger size may be used (see Rowe, 1972 [27]).

NOTE 5 Specifying the use of appropriate samplers is not a guarantee that, for example, Class 1 samples can be obtained. There are a large number of factors that can produce a non-Class 1 sample despite the best endeavours of the designer and qualified operator.

The quality of recovered samples should be assessed and recorded at certain key times during the investigation and by the following key personnel, as follows:

• a) on site immediately following the taking of samples by the qualified operator of the plant being used and/or the site supervisory staff by assessing the following:
  • 1) soil particle size in relation to sampler in use;
  • 2) voiding in undisturbed samples;
  • 3) partial core recovery;
  • 4) mixing of strata types;
  • 5) breaking up of cobbles or boulders into smaller particles using sampling tools/chiselling;
  • 6) disturbance by stronger inclusion;
  • 7) condition of sample tube;
  • 8) over drilling of previously cored ground to recover sample or core; and
  • 9) recovery of collapsed or squeezing ground.
• b) at the time of scheduling samples for test by the person responsible for scheduling by assessing:
  • 1) use of inappropriate sampler for material type;
  • 2) excessive blows to drive samplers;
  • 3) partial recovery in samplers; and
  • 4) appropriateness of test for soil sample.
• c) during extrusion or preparation of the sample in laboratory by laboratory technicians by assessing material disturbance as exhibited by:
  • 1) laminations turned down on sample margins;
  • 2) disturbance by stronger or weaker inclusions;
  • 3) softening of sample;
  • 4) open fissures;
  • 5) wax ingress into soil fabric;
  • 6) incomplete waxing or sample preservation;
  • 7) obvious drying of sample; and
  • 8) presence of ice crystals (indicating freezing of sample).

This is not an exhaustive list and the geotechnical advisor should satisfy themselves that appropriate checks are in place for each investigation.

25.3 Disturbed samples from boring tools or from excavating equipment

COMMENTARY ON 25.3

The quality of the sample depends on the technique used for sinking the borehole or excavation in combination with the nature (particle size range) of the ground (see 20.2 to 20.8) and on whether the ground is dry or wet. For example, when disturbed samples are taken from below water in a borehole or excavation, there is a danger that the samples obtained might not be truly representative of the deposit. This is particularly the case with cable percussion boring in coarse soils containing fines, which tend to be washed out of the tool. This can be partly overcome by placing the whole contents of the tool into a tank and allowing the fines to settle before decanting the water.

The following classes of sample should be taken during sampling by drilling (using the cable percussion method) or by excavating pits:

• a) Class 3: disturbed samples from dry excavations and from dry boreholes;
• b) Class 4: disturbed samples obtained in fine soil from excavations or from boreholes in conditions where water is present; and
• c) Class 5: disturbed samples in coarse soil from wet excavations or from cable percussion boreholes sunk using a shell and also from any borehole sunk by a method in which the drill debris is flushed out of the borehole, e.g. rotary open hole drilling, wash boring.

NOTE 1 The quality of samples obtained from the dynamic sampling and resonance drilling tools (rather than cable percussion boring) are considered in 25.8 and 25.9 respectively.

The mass of sample required for various purposes should be determined by the character of the ground and the tests that are to be undertaken (see Table 4 for a guide to the mass of soil sample required for various purposes). Care should be taken that the sample is representative of only the stratum from which it comes, and has not been mixed with other strata.

NOTE 2 Field samples are sometimes larger than required by the laboratory. Whilst sub-sampling is usually better done in the laboratory under controlled conditions, guidance on pre-treatment in the field to obtain a representative sample from a larger amount of material, for example material excavated from a trial pit, is provided in BS ISO 18400-201 (in preparation).