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Arcillas allofanas - 09031-18.qxd 20/Oct/02 1:07 AM Page...

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Geotechnical characterisation and behaviour of allophane clays L.D. Wesley University of Auckland, Auckland, New Zealand ABSTRACT: The clay mineral allophane plays an important role in governing the characteristics of certain clays of volcanic origin. These clays occur when non-crystalline parent material is available and the weathering environment is warm and wet. An overview is given of the current state of knowledge of allophane clays, drawing on both soil science and soil mechanics sources, as well as the author s own experience in geotechnical engineering in such clays. The structure of allophane, and its associated min- eral imogolite, is described in some detail. The very unusual properties of the clays, namely good engi- neering properties despite extremely high natural water contents and Atterberg limits, are described, and an account given of some geotechnical projects in allophane clays. 1 INTRODUCTION The term allophane clay (or allophanic clay) is used here to denote those materials derived from the weathering of volcanic ash (or possibly other volcanic material) whose behaviour is governed predom- inantly by the presence of the clay mineral allophane. The term volcanic ash is often used to denote these materials, but this term covers a much wider range of materials than just allophane clays. It may include material ranging from clay to fine sand, and may be totally free of any allophane content. This paper is therefore restricted to those volcanic ash soils consisting predominantly of the clay mineral allophane, and associated clay minerals such as imogolite. Allophanic clays are found in many parts of the world, including Japan, New Zealand, Indonesia, The Philippines, Central and South America, and Africa. Economic developments world-wide, and especially in developing countries, mean that major engi- neering projects are increasingly encountering allophane clays, and geotechnical engineers need to be more aware of their existence and their unusual properties. The author s experience with allophane clays has been in both New Zealand and Indonesia, and has included earth dams, highway investigations, oil tank farms and geothermal projects. Much of the material in this paper is drawn from practical expe- rience and research in allophane clays in these countries, but considerable additional material is included from other sources to provide a more complete picture. The formation and composition of allophane clays is complex, and most of the research and litera- ture on the subject comes from the discipline of soil science rather than soil mechanics. This research and literature has grown enormously in the last two or three decades since the term allophane first found its way into geotechnical literature, and it shows a number of new and interesting findings. Firstly, it shows that allophane seldom occurs by itself. Instead, it is almost invariably found with other clay min- erals, especially a mineral called imogolite. It seems to be almost inseparably linked to imogolite, and
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