33.4.6 Organic soils

COMMENTARY ON 33.4.6

Soils comprising mainly organic materials are termed peats which accumulate in situ in a mire. They are of low density, typically 1,01 Mg/m³ to 1,1 Mg/m³, which is just over half the density of an inorganic soil. Soils with organic contents up to about 30% by weight and water contents up to about 250% behave largely as inorganic soils, albeit with different parameters (see Hobbs, 1986 [52] and Hobbs, 1987 [53]). Such materials are usually transported and would not be described as peat. This morphological distinction might be difficult to recognize, e.g. within a fluvial sequence, and, therefore, a distinction based on engineering behaviour is to be made.

Even small quantities of dispersed organic matter within an inorganic soil can have a marked effect on plasticity and hence the engineering properties and produce a distinctive odour and a dark grey, dark brown or dark bluish grey colour; increasing quantities of organic matter heighten these effects (see Hobbs, 1986 [52]). Soils with a high organic content might oxidize and change colour rapidly.

Soils that consist predominantly of plant remains, either fibrous, pseudo-fibrous or amorphous, should be described as peat according to the degree of decomposition and condition as given in Table 20 and Table 21.

NOTE 1 If the peat forms a horizon of major engineering significance, a fuller description using the schemes of von Post or Troels Smith might be appropriate (see Hobbs, 1986 [52], Hobbs, 1987 [53] and Norbury, 2010 [42]).

NOTE 2 Dark colours and low densities can be associated with inorganic soils such as volcanic materials or loess.

The organic upper soil layer is usually referred to as "Topsoil" and should be described using a simple scheme (see Norbury, 2010, Chapter 13.1 [42]).

NOTE 3 Detailed descriptive schemes for Topsoil are available in BS 3882, ISO 25177 and at www.fao.org/nr/land/soils/soil/EN; these are unlikely to be relevant in engineering projects.

Soils that are mixtures of organic and inorganic constituents should described as indicated in Figure 6. Organic soils containing inorganic matter as a secondary constituent should be described, for example, as slightly clayey or very sandy, these terms being used qualitatively here.

Inorganic soils containing organic matter as a secondary constituent should be described using the qualifying terms in Table 22 (see Swedish Geotechnical Society [54]).

A distinction should be made between the organic and inorganic constituents being disseminated or as discrete inclusions of one within the other.

NOTE 4 Laboratory determinations of organic content are not essential to the description, but can be used to assist consistency of terminology if required. It is not usual for the classification based on laboratory tests to appear on a field record. The previous comments all refer to recent vegetable matter, not to older materials such as coal or lignite.

It should be taken into account that dark colours and low densities can be associated with inorganic soils such as volcanic materials or loess.

33.5 Geological unit, age and type of deposit

The name of the geological unit should be given after the soil description as an indication to the reader of the possible characteristics that the deposit. A guide to naming geological units (senso lato) is given on the maps of the British Geological Survey or its antecedents, and it should be written with at least capital initial letters, e.g. London Clay Formation, Bagshot Formation, Lower Lias. Alternatively, the unit should be given in brackets and/or in upper case letters for clarity. The geological unit should be named where this can be done with confidence, but it might not be easy to tell to which unit a sample belongs, or to locate unit boundaries in a borehole or exposure; conjecture should be avoided, but degrees of uncertainty can be indicated.

The most recently published geological map, memoir or sheet explanation should normally be used as a guide and is often adequate and acceptable, but revisions to stratigraphic nomenclature can cause problems when these publications are of different publication dates.

NOTE 1 The BGS online lexicon provides a useful resource on rock names.¹³⁾

NOTE 2 Specialized geological knowledge of a region might be required if a label other than that published is applied, for instance in the recent subdivisions away from Middle or Upper Chalk. On the other hand, broad nomenclature changes can be applied more readily, e.g. Sherwood Sandstone Group for Bunter Sandstone.

The characteristic lithology is sometimes indicated in the formation name, e.g. London Clay Formation, but it should be remembered that at a particular location or horizon the lithology or material type might be completely different from that indicated in the formation name.

NOTE 3 Formations might be variable in their lithology and knowledge of the formation indicates the possible range of material to be expected. Some indication can be obtained from the key to the one-inch or 1:50 000 and six-inch or 1:10 000, geological maps; from the Sheet Memoir or Sheet Explanation, if published; and from the British Regional Geology Guide (see Annex B).

33.6 Additional information

Any additional information on the composition, structure, behaviour or other characteristics of the soil that would be of value in assessing its nature and properties should be recorded. A special note should be made if the properties of the material are thought to be unusual in relation to the rest of its description. A note should also be made if there is doubt about whether the description is representative of the ground at the level from which it was sampled.

NOTE This might be caused, for instance, by the fracture of particles or a loss of fines during sampling, or by the sample size or borehole diameter being too small in relation to the grading or structure of the material being sampled.

Where relevant, it should be made clear whether the samples on which the description is based were disturbed or undisturbed.