18 Frequency of sampling and testing

18.1 General principles

The geotechnical advisor should decide on the frequency of sampling and testing in an exploratory hole, taking account of the following factors:

a) the guidance given in BS EN 1997-2;
b) the information that is already available about the ground and groundwater conditions;
c) the information obtained by any geophysical or other non-intrusive methods forming part of the investigation; and
d) the technical objectives of the investigation.

NOTE In the case of a phased investigation (see 17.2), a different sampling and testing programme might be appropriate given that:

the preliminary phase concentrates on the determination of the character and structure of all the strata and the groundwater conditions; and
the design phase concentrates on the determination of the properties of the various strata whose locations have been determined in the preliminary phase.

Irrespective of phasing, the overall sampling regime should be designed to ensure that:

the range (numbers and depths) of samples of various quality classes is such as to permit an appropriate suite of tests to be carried out for the anticipated design;
samples are available for geoenvironmental testing if contamination of the ground proves to be an issue; and
samples are preserved and handled to maintain their condition as required by the sample type (see 25.11).

18.2 Determination of character and structure of the strata

COMMENTARY ON 18.2

This Subclause only relates to characterizing the ground in relation to stratigraphy and not to obtaining samples for laboratory tests to determine strata properties.

In areas where suitable information about the ground conditions has been built up from the results of previous investigations, it might be possible to omit this aspect.

The location, character and structure of each stratum should be determined, so far as is practicably possible.

NOTE Some of the strata might be quite thin, and continuous sampling through the full succession might be required in order that the necessary information can be obtained.

Where highly variable ground conditions are expected, it might be advantageous initially to sink one or more boreholes by rotary core drilling or another continuous sampling method such as resonance/sonic drilling or by percussion boring with consecutive tube sampling. The cores or tube samples should then be examined to give guidance for sampling at selected depths in boreholes subsequently sunk close to the initial boreholes.

In fine soil, and some silty sand, consecutive tube samples may be obtained using a variety of techniques; these samplers, appropriate to different soil types, are given in BS EN ISO 22475-1:2006, Table 3.

In coarse soil, such as gravel, tube sampling is rarely successful and in their absence disturbed samples should be taken from the drill tools (see 25.3), together with any split barrel standard penetration test samples, although in gravels it is likely that the solid cone is used (see 25.4.5).

Sampling procedures should be adopted that allow the recovery of representative samples including the retention of fines.

In some soils and in rock, continuous rotary core sampling (see 25.7) should be undertaken to give the appropriate sample quality; if the core recovery is poor, the use of alternative methods or equipment might be necessary.

A selection of samples obtained by drive sampling or rotary coring should be split along their longitudinal axis and carefully examined and described first in their fresh condition and then again later in a semi-dried state when the fabric is more readily identified.

Groundwater conditions should be determined from the water levels in the exploratory holes, from the identification of water bearing strata and from observations in groundwater monitoring installations set at the appropriate depths (see Clause 26 and Section 8).

18.3 The determination of strata properties

After the strata, whose properties are likely to be relevant to the technical objectives of the investigation, have been identified, the properties of those strata should be assessed using appropriate techniques (see BS EN ISO 22475-1). These techniques should include a combination of field observations, field testing, monitoring and laboratory testing (see Section 7 to Section 9).

The programme and frequency of sampling and testing in soil and rock should be designed for the requirements of a particular investigation (for example, geotechnical category of the structure, anticipated ground conditions and depth requirements of the structure) in accordance with BS EN 1997-2.

The sampling and testing scheme should be tailored specifically to the requirements of each ground investigation, taking into account the requirements given in BS EN 1997-2:2007, 3.4.3.

NOTE The UK specification for ground investigation [8] gives a default sampling regime related to material types, which might be useful in the early stages of an investigation.

18.4 Planning for laboratory testing

COMMENTARY ON 18.4

Laboratory tests are used to measure parameters and to complement field observations, field testing and back analysis of the behaviour of existing structures that have been monitored. In some cases, a field test might provide more realistic results because of the differences in scale of the tests, the in-situ test generally testing larger amounts of material without the effects of sample disturbance. However, there is a large body of practical experience behind most of the more common tests and when the data derived from them are integrated and used with skill and experience, reliable predictions usually result

The geotechnical advisor should design the laboratory test programme, taking into account the geological and lithological variation, the expected behaviour and the data and parameters required for the geotechnical design. Samples for laboratory testing should be of the quality and size required by the proposed tests.

The programme of laboratory testing and the specification of each test should be determined by the geotechnical advisor, taking account of the requirements of BS EN 1997-2 and the NA to BS EN 1997-2 and other documents as required. The details and number of the tests required to determine the parameters needed for design should be assessed and specified. Each test, or series of tests, should address one or more of the purposes listed below. The principal factors that should be taken into account include:

a) the nature of the ground and the type of soil or rock being tested;
b) the quality of the sample and whether it is representative of the characteristics of the ground in situ;
c) the method of analysis proposed as required in BS EN 1997-1; this might be, for example, an empirical method, a limit equilibrium calculation or a finite element analysis; and
d) the requirements of the structure and the temporary works, including whether the designs are controlled by stability or by the need to limit deformations, and whether short-term or long-term conditions are most critical.

Discontinuities and weak regions are usually the critical elements in the mass and their shear strength and deformability can be investigated in the field or by large-scale laboratory tests on block samples. The influence of changes in water content on the strength and deformation should be investigated as part of the laboratory test programme.

Planning of the ground investigation and the use of laboratory test results should take into account that recovery of representative samples of some materials can be difficult, for example:

1) extremely weak to very weak rock might be poorly represented in borehole core or might be difficult to prepare and test in the laboratory; in these circumstances only the stronger sections of rock core are tested and the results of the laboratory tests are biased;
2) materials that contain weaker and stronger components, such as flint within chalk, and gravel and cobbles within clay might be disturbed during sampling.

Strata that could introduce these types of problems should be identified during the desk study. The likely effects on the samples and laboratory test results should be taken into account during the planning of the sampling and laboratory test programme.