25 Sampling the ground
The selection of sampling technique should take into account the quality of the sample required (see 25.2, in particular the method for classifying the quality of the sample given in Table 3). It should also take into account the character of the ground, particularly the extent to which it is disturbed by the sampling process.
In choosing a sampling method, it should be clearly understood whether it is the mass properties or the intact material properties of the ground that are to be determined.
NOTE 1 The behaviour of the ground mass is often dictated by the presence of weaknesses and discontinuities (see 20.10). It is, therefore, possible to obtain a good sample of material that is unrepresentative of the mass.
In accordance with BS EN ISO 22475-1, techniques for obtaining soil samples should be divided into three generic groups:
- a) Sampling by drilling. Samples taken from the drill tools or from digging equipment in the course of advancing the borehole or excavation by most of the methods given in Clause 24 are likely to be disturbed, Class 3 to 5 (see 25.3). However, this group also includes rotary core drilling, which is capable of recovering a higher quality sample (see 25.7).
- b) Sampling by sampler. Historically termed drive sampling, in which a tube (or a split tube), with or without a piston, having a sharp cutting edge at its lower end is forced into the ground, either by a static thrust or by dynamic impact (see 25.4 to 25.6). These samples would normally be expected to be relatively undisturbed, Class 1 and 2, but in some circumstances Class 3.
- c) Block sampling. Traditionally in the UK this term was used for cuboid samples specially cut by hand from a trial pit, shaft or heading (see 25.10 and BS EN ISO 22475-1:2006, 6.5.1). However, it is now also used for taking cylindrical samples from boreholes using a large sampler (see BS EN ISO 22475-1:2006, 6.5.2). Both types of sample would be expected to be relatively undisturbed. Class 1 and 2.
NOTE 2 Samples obtained by techniques b) and c) and rotary core drilling can be of sufficient quality to enable the ground structure within the sample to be examined. The quality of such samples can vary considerably, depending on the technique and the ground conditions, and most exhibit some degree of disturbance. 25.4 to 25.10 describe the various sampling techniques and give an indication of the sample qualities that can be obtained.
NOTE 3 Intact samples obtained by techniques b) and c) and rotary core drilling are usually taken in a vertical direction, although this last method can be carried out at any inclination which might be required to investigate particular features.
In addition to its quality, the size of sample required for laboratory test should be in accordance with BS EN 1997-2, BS 1377 or other relevant standards.
The presence or absence of water in an exploratory hole during sampling or testing can have significant influence on geotechnical properties observed including water content and shear strength. This factor should be taken into account when considering sample quality and interpreting test results.
When taking samples for chemical testing and in particular on potentially contaminated sites, cross-contamination and chemical or biological reactions should be avoided, as these can affect the result. The following measures should be taken to reduce the risks of cross-contamination:
- 1) using dry drilling methods for progressing the boreholes;
- 2) using casing to isolate upper layers of soil and groundwater;
- 3) ensuring that all sampling and boring equipment is clean; and
- 4) implementing strict sample handling protocols.
Any sample of ground that might be contaminated by substances hazardous to health should have a warning to that effect on the sample label so that personnel can follow appropriate safety procedures (see 5.1).
NOTE 4 A detailed consideration of investigations on potentially contaminated sites is given in BS 10175.
NOTE 5 Guidance on the cleaning of drilling and other equipment is provided in CIRIA R132 .