61 Laboratory tests

61.1 Sample suitability

The samples selected for testing should be appropriate for the test or tests; this includes material type, sufficient size to carry out the test or tests and suitable quality as required for the test(s) (see BS EN 1997-2 and the test method or methods). Where the sample is not appropriate for a test due to the lithology being unsuitable or to sample disturbance or insufficient material for the test(s), the geotechnical advisor should be informed and they should make the decision as to whether the test should proceed or other samples might be used. If the test is carried out on a non-conformable sample it should be reported with the reasons in the results.

NOTE The re-use of material for more than one test is permissible for some tests and might be included as a part of the test programme by the geotechnical advisor.

The laboratory should provide guidance to the geotechnical advisor on the largest sample that can be handled and tested at the facility.

Undisturbed samples for testing the effects of the discontinuities on strength and deformation characteristics should be sufficiently large to include a representative pattern of these discontinuities relevant to the test required. This might require use of large "undisturbed" samples.

61.2 Conditions of test

The conditions of the test should be those appropriate to determine the parameters required in Clause 55 and Clause 59 and the test methods and test conditions as specified by the geotechnical adviser.

61.3 Relevance of test results

All laboratory test results should be examined critically by a suitably qualified and experienced person, usually the laboratory supervisor. They should be examined to check the following:

  • a) that the results are reasonable, correctly reported and refer to the correct sample;
  • b) that the results from similar samples are consistent (i.e. results from samples with similar descriptions are comparable);
  • c) that results from a particular sample are consistent with other data relating to the sample;
  • d) that the results of the laboratory tests are consistent with the results of relevant borehole log descriptions and in-situ tests if available; and
  • e) that the test results are consistent with previous knowledge and accepted data for similar materials.

Whenever test results appear to be anomalous, the reason should be established by re-examination of the specimen, original results or confirmed by further testing, which might include other types of investigation such as petrological or mineralogical testing. If the reasons for an unusual result cannot be established, the geotechnical advisor should be notified and the anomaly should be clearly stated in the report on the laboratory tests.

61.4 Reporting test results (p. I)

Laboratory test results should be reported clearly, as required by the test standard or the specified test method and interpretation; this includes how the test was conducted and how the raw data obtained from the test were analysed and interpreted. The original laboratory worksheets, analogue or digital, should be kept for future reference as required by the project specifications. Data from tests should be provided in a form as required by test method and the data management plan in accordance with BS 8574.

The origin of the sample, its size and class and all procedures carried out prior to testing should be reported. In some cases, the test results should be reported factually; in other cases the test results may be interpreted and values for design parameters given.

The appearance of samples tested for strength, deformation or permeability should always be sketched and/or photographed after they have been tested. Visible surfaces of failure should be noted and their angle to the direction of externally applied direct loads recorded. Samples should be broken open to assess their uniformity and to identify any obvious reason for their behaviour.

Table 36 Categories of test specified in BS 1377 with the BS EN ISO 17892 equivalent tests
BS 1377 BS EN ISO 17892A) equivalent or near equivalent Tests
Part 1B) General requirements and sample preparation
Part 2 Part 1, Part 2, Part 3, Part 4, Part 12 Classification tests
Part 3 Chemical and electro-chemical tests
Part 4 Compaction related tests
Part 5 Part 5 C) Compressibility, permeability and durability tests
Part 6 Part 11C) Consolidation and permeability tests in hydraulic cells with pore pressure measurement
Part 7 Part 7, Part 8, Part 10 Shear strength tests (total stress)
Part 8 Part 9 Shear strength tests (effective stress)
D) Part 6 Fall cone
A) Not all parts of BS EN ISO 17892 were published at the time of publication of this British Standard. They are in preparation.
B) No equivalent in BS EN ISO 17892. The specific requirements for the tests are in each part of BS EN ISO 17892.
C) Partial equivalent in BS EN ISO 17892.
D) No equivalent in BS 1377.
Table 37 Common laboratory tests for soils (1 of 10)
Category of test Name of test or parameter measured Where details can be found Remarks
Classification tests Head, 2006 [131] Index tests Water content (previously known as moisture content) BS EN ISO 17892-1 Generally carried out as a test on its own or with other tests. When carried out in conjunction with liquid and plastic limits (liquidity index or consistency index), it can give an indication of undrained strength.
Liquid and plastic limits BS 1377-2 (BS EN ISO 17892-12) Used to classify fine grained soil and the fine fraction of mixed soil. The derived value (plasticity index, Ip) is used with the percentage of less than 0,425 mm fraction to classify the swell and shrinkage (volume change) potential of clays. The relative consistency or liquidity index is calculated with water content and indicates consistency of the sample. With the clay-size proportion it can give an indication of the activity of the clay minerals.
Linear shrinkage BS 1377-2 An indicator of the shrinkage potential on desiccation.
Density Particle density (previously known as specific gravity) BS 1377-2 (BS EN ISO 17892-3) Values reflect the proportions of different minerals present. It should be measured on samples used in compaction tests. Otherwise values commonly range between 2,55 Mg/m3 and 2,75 Mg/m3. There are methods for fine-grained and for coarse grained soils. Many organic soils and some minerals might have particle densities very different to the values above.
Bulk and dry density or unit weight BS EN ISO 17892-2 Used in the calculation of forces exerted by soil. Dry density is used to calculate voids ratio, porosity and saturation.
Limiting and relative density, maximum, minimum density index of sand BS 1377-4 To identify the relative density of a coarse grained soil in comparison with the minimum and maximum density of that soil. Results can be used to indicate the stiffness and peak strength of coarse-grained soils. A number of different methods are available and need to be clearly identified as they are known to give different results on the same soils. The methods are sensitive to particle size distribution and particle shape.
Table 37 Common laboratory tests for soils (2 of 10)
Category of test Name of test or parameter measured Where details can be found Remarks
Classification tests (continued) Particle size analysis Coarse grained - sieving BS 1377-2 (BS EN ISO 17892-4) Sieving methods give the grading of soil particles coarser than silt (<0,063 mm). When the sample contains silt or clay the test is done by wet sieving. The relative proportions of silt and clay are determined by means of sedimentation tests.
Fine-grained - sedimentation BS 1377-2 (BS EN ISO 17892-4) Hydrometer or pipette analysis.
Automated methods primarily for fine grade Equipment manufacturer methods Automated methods including x-ray sedigraph, laser or granulometer methods are only to be used after they have been shown to give equivalent results as the standard methods. It is important to ensure that the sample sizes used are relevant for the particle sizes tested.
Chemical tests Chemical tests for civil engineering purposes might be carried out in geotechnical laboratories. Most of the methods are described in BS 1377-3. These methods do not generally include modern analytical techniques. Analytical methods may be used if they are shown to give equivalent results to standard methods. The sample preparation should follow the methods in BS 1377-3 and the results reported as specified in this British Standard.
pH value BS 1377-3 Measures the acidity or alkalinity of the soil or water. Needs to be performed as soon as possible after sampling. Values of pH might change during storage when pyrite oxidizes as the disturbed materials are exposed to the air. These materials are generally grey and dark grey clays and mudstones.
It is often carried out in conjunction with sulfate tests.
Excessive acidity or alkalinity of the groundwater in soils can have detrimental effects on buried concrete, cause corrosion of metals and use of resinous materials is unsuitable in alkaline soils.

BS 5930:2015 Code of practice for ground investigations