Section 6: Description of soils and rocks

32 The description process


Accurate descriptions of exposures (natural, in excavations, cores or samples) of soil and rock are an important aspect of ground investigation. The results of a ground investigation might be required long after the disposal of the samples when the descriptions are, in many cases, the only evidence remaining of what was discovered. In addition, designers often make use of past experience for materials of similar age, origin or condition based on the descriptions.

For some soils it might be difficult to obtain representative samples or to carry out field tests and, in such materials, the descriptions might form the main, or only, information on which the assessment of the behaviour of the ground is based.

The words cohesive and non-cohesive, or granular, are often used to distinguish soils that contain a significant proportion of fine grains and behave in a cohesive manner when subjected to quick loading from coarse-grained soils, which have no apparent cohesion. The word cohesive usually describes a soil which has an undrained strength measured in an unconfined compression test. In this test the strength arises from a combination of friction and negative pore pressure; cohesion, therefore, describes the ability of a soil to sustain a pore-water suction when unconfined.

All soils can be considered granular so, in terms of effective stress, true cohesion in any un-cemented soils is very small (most un-cemented soils slake when immersed in water). Moreover, any un-cemented soil can behave in either a cohesive manner or in a non-cohesive manner depending on the response of the pore pressure to loading. For example, saturated and dry clean sand is non-cohesive and can be poured, but partially saturated sand can behave in a cohesive manner and a cohesive strength can be measured. Under long-term, fully drained conditions, clays behave in a frictional, non-cohesive manner and it is the frictional properties of the soil that are relevant when assessing the long term stability of clay slopes.

The description of soils and rocks should be carried out in accordance with BS EN ISO 14688-1 and BS EN ISO 14689-1 (further guidance is also given in Norbury, 2010 [42]).

The descriptive terms apply primarily to natural soils and rocks but may also be applied to many types of man-made materials (see 33.4.5 for further details).

Each category of natural and man-made materials outlined in Figure 5 should be described using the appropriate approach and terminology.

Figure 5 Selection of descriptive procedure for different materials
Selection of descriptive procedure for different materials

The process shown in Figure 6 for the identification and description of soils should be followed.

Descriptions should be made on samples recovered from boreholes and excavations and/or from examination of in-situ materials. The borehole log should aim to be as objective a record as possible of the ground conditions at the borehole position before the ground was subjected to disturbance and loss by the boring or excavation process. The degree of interpretation should be kept to a minimum. If interpretation is necessary to provide information, it should be clearly identified.

The level of detail that is required or appropriate in descriptions might vary; the level of detail should be established at the start of an investigation.

NOTE 1 Adjectives such as "probably" or "possibly" are useful in this regard.

The reliability of sample descriptions in reflecting the in-situ characteristics depends greatly on the quality of the samples and the level of detail in the description should reflect this quality. Any doubts as to the representativeness or reliability of the sample should be stated. An accompanying report should give the origin, type and quality of each sample.

Following description, soils and rocks may also be classified in accordance with BS EN ISO 14688-2 and BS EN ISO 14689-1. These classification terms should not generally appear on field records.

NOTE 2 Classification (see Clause 34) provides additional guidance on the behaviour of a particular soil or rock by allowing comparison with behaviour encountered on other sites.

Figure 6 General identification and description of soils
General identification and description of soils