Downloaded from http://egsp.lyellcollection.org/ by guest on October 2, 2021 1. Introduction Clay, noun. Old English Cladg. A stiff viscous earth. of major economic significance, touching virtually every (Blackies Compact Etymological Dictionary. Blackie aspect of our everyday lives, from medicines to cosmetics & Son, London and Glasgow. 1946. War Economy and from paper to cups and saucers. It is very difficult to Standard) over-estimate its use and importance. The treatment of clay in this book is therefore wide ranging to reflect this Clay: The original Indo-European word was 'gloi-" situation. "gli-' from which came "glue' and 'gluten'. In The occurrence of clay is also ubiquitous and diverse Germanic this became 'klai-; and the Old English (see Text Box below) and, with its various mineral spe- 'claeg" became Modern English "clay'. From the same cies, properties and behavioural characteristics, the indus- source came "clammy' and the northern England trial applications of clay are thus manifold and complex. dialect "claggy' all of which describe a similar sticky As well as their traditional major uses for brickmaking, consistency. (Oxford English Dictionary and Ayto's pottery and porcelain manufacture, refractories and the Dictionary of Word Origins, Bloomsbury, 1999) fulling of cloth, clays are now used for refining edible oils, fats and hydrocarbon oils, in oil well drilling and Clay." from Old Greek yRia, y2oia "'glue" 72ivfl synthetic moulding sands, in the manufacture of emulsi- "slime, mucus "" y2oidq "'anything sticky" 'from L-E. fied products in paper and, as noted in Chapters 13, 14 base *glei-, *gli- 'to glue, paste stick together. (Klein and 15, many hundreds of other uses, including medicine, E. A comprehensive etymological dictionary of the cosmetics and, on a larger scale, as fillers, as well as English language. Elsevier, Amsterdam, 1967; Skeat many uses in geotechnical engineering e.g. for grouts, W. An etymological dictionary of the English lan- membranes etc. guage. Oxford University Press, 1961; Mann S.E. An In foundation engineering, clay often provides poor Indo-European comparative dictionary. Buske Verlag, foundation support, and can be responsible for slope Hamburg, 1987) instability. It finds extensive use as a construction mate- rial in embankments and in water-controlling structures as an impermeable barrier and in many other specialist ways 1.1. Clay (see Chapters 10, 11 and 12). Given the worldwide distribution and variability of Definitions of clay are given in Section 1.2. The uses of clay deposits, the production of an authoritative text, clay are ubiquitous and diverse. On a world scale, clay is which is the aim of the Working Party, on all aspects of Downloaded from http://egsp.lyellcollection.org/ by guest on October 2, 2021 2 INTRODUCTION clay, was considered a most daunting task. The original There are many geological dictionaries which seek brief from the Engineering Group committee, the parent to define clay. These are typically in general accord with committee of the Working Party, was that the Working the Working Party views. An example is given in the Text Party report, viz. this book, should exclude the engineer- Box on p. 4, together with closely related terms defined ing aspects of clay in situ (i.e. not consider it as a founda- in the same dictionary. These related terms are also in tion material) but that other engineering aspects, general accord with their use in this book. or example use in embankments or in specialized It is necessary to state, however, that within the follow- engineering applications, should be considered. ing chapters, where the term clay is used in a general In addition to this particular omission conceming the sense as a material, it implies any fine-grained, natural, in situ use of clay, the Working Party decided that in earthy, argillaceous material. This includes clays, shales order to retain depth of discussion it should concentrate and argillites of the geologists, and soils as defined by only on the principal construction and industrial applica- geologists, engineers and agronomists, provided such tions of clay with the omission of other specialist non- material is argillaceous (i.e. it contains clay minerals). engineering uses, and in order to limit the scope of this Any specific definition of the term clay that depends on current publication to a sensible size. the context, in which it is being used, is given within the chapter related to that definition. This is because, as is clear from the above dictionary definitions, the term clay 1.2. Definitions of clay is imprecise and variously defined. Description of clays is discussed in more detail in Chapters 4 and 8. The term clay mineral is described in the second Text Box on p. 4. The term 'clay' has no genetic significance. It is used for material that is the product of in situ alteration, e.g. by 1.2.1. Definitions of clay and clay minerals by the weathering, hydrothermal action or, alternatively, depos- ited as a sediment during an erosional cycle or developed AIPEA Nomenclature and CMS in situ as an authigenic clay deposit. 'Clay' can be used as Nomenclature Committees a rock term and also can be used as a particle size term in mechanical analysis of sedimentary rocks or soils. As a The definitions of clay and clay minerals in the Joint rock term, it is difficult to define precisely because of the Report of the AIPEA Nomenclature and CMS Nomencla- ture Committees should be read in full for the complete wide variety of materials that have been called clays. expression of their views (Guggenheim & Martin 1995), A universal implication of the term 'clay' conveys that it together with the subsequent Discussions on this Report is a natural, earthy, fine-grained material that develops (Moore 1996; Guggenheim & Martin 1996). The follow- plasticity when mixed with a limited amount of water. ing summarises some of the salient points and demon- By 'plasticity', it is meant that within a certain range of strates the difficulties in making a precise definition of moisture content the material will deform under the appli- clay. cation of pressure, the deformed shape being retained when the deforming force is removed. Chemical analysis 'Clay' Definition of clay minerals shows them essentially to comprise The term 'clay' refers to a naturally occurring material silica, alumina and water in variable combinations, composed primarily offine-grained minerals, which is frequently with appreciable quantities of iron, alkalis and generally plastic at appropriate water contents and alkaline earths. will harden when dried or fired. Although clay usually A difficulty is that some material called 'clay' does not contains phyllosilicates, it may contain other materials meet all the above descriptors. A glance at any compre- that impart plasticity and harden when dried or fired. hensive dictionary will show that clay has a plethora of Associated phases in clay may include materials that definitions, scientific and colloquial, often steeped in do not impart plasticity and organic matter." history and clearly demonstrating that the definition of the word 'clay' depends on the context in which it is being In the Discussion to the Definition, the Committees make used. The reasons for this situation undoubtedly lie in the the point that, 'The 'naturally occurring' requirement of clay excludes synthetics and that based on the standard many and diverse industries in which clay is used, each definition of mineral, clays are primarily inorganic mate- having developed, over the years, a definition appropriate rials excluding peat, muck, some soils, etc. that contain to its requirements. A summary of the definitions of large quantities of organic materials. Associated phases, clay and clay minerals, presented in the joint report of such as organic phases, may be present. 'Plasticity' the Association Intemationale pour l'etude des Argiles refers to the ability of the material to be moulded to (AIPEA) Nomenclature Committee and the Clay Miner- any shape. The plastic properties do not require quantifi- als Society (CMS) Nomenclature Committee is given cation to apply the term 'clay' to a material. The fine- in Section 1.2.1; civil engineering definitions of clay in grained' aspect cannot be quantified, because a specific British practice are given in Section 1.2.2 and interna- particle size is not a property that is universally accepted tional civil engineering soil classification by particle size by all disciplines. They say that,for example, most geolo- distribution (grading) is given in Section 1.2.3. gists and soil scientists use particle size less than 2 izm, Examples of common, non-specific dictionary sedimentologists use 4/um, and colloid chemists use 1 I~m definitions are given in the Text Box on p. 3. for clay-particle size.' Downloaded from http://egsp.lyellcollection.org/ by guest on October 2, 2021 INTRODUCTION NO~SD~C~":~i:~*:: " :'::": :::: ~' :": :~: :~: ~: :,t :~: :~ :~,.. ,. iiiii i!ii lipidic!i'!84 ;~ :.': ~: t::., ~:, ::.: al~~ :: :: :;: .::~: :,,:;~ :::: ::~ ~:: :~::: ~;:~ :~:~ :. :~7:~ ~.: :.':~, ~ ~:~, ,:~: :';~ :~:~.~: ::.~:::~: z: ::9 ::~ ~ ~:::~::::~ ~:~ :::~ They also say that 'Plasticity is a property that is Clay mineral greatly affected by the chemical composition of the mate- Definition: rial For example, some species of chlorite and mica can 'The term "'clay mineral" refers to phyllosilicate remain non-plastic upon grinding macroscopic flakes minerals and to minerals which impart plasticity to