Laterite Soil Engineering: Pedogenesis and Engineering Principles PDF
Preview Laterite Soil Engineering: Pedogenesis and Engineering Principles
Further titles in this series: 1. G. SANGLERAT THE PENETROMETER AND SOIL EXPLORATION 2. Q. ZARUBA AND V. MENCL LANDSLIDES AND THEIR CONTROL 3. E.E. WAHLSTROM TUNNELING IN ROCK 4A. JR. SILVESTER COASTAL ENGINEERING, I Generation, Propagation and Influence of Waves 4B.Ä. SILVESTER COASTAL ENGINEERING, II Sedimentation, Estuaries, Tides, Effluents, Modelling 5. R.N. YONG AND B.P. WARKENTIN SOIL PROPERTIES AND BEHAVIOUR 6. E.E. WAHLSTROM DAMS, DAM FOUNDATIONS, AND RESERVOIR SITES 7. W.F. CHEN LIMIT ANALYSIS AND SOIL PLASTICITY 8. L.N. PERSEN ROCK DYNAMICS AND GEOPHYSICAL EXPLORATION Introduction to Stress Waves in Rocks Developments in Geotechnical Engineering 9 LATERITE SOIL ENGINEERING Pedogenesis and Engineering Principles M.D. GIDIGASU Senior Research Officer and Head, Soil Mechanics and Foundations Council for Scientific and Industrial Research, Building and Road Research Insitute, Kumasi, Ghana ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam Oxford New York 1976 ELSEVIER SCIENTIFIC PUBLISHING COMPANY 335 Jan van Galenstraat P.O. Box 211, Amsterdam, The Netherlands AMERICAN ELSEVIER PUBLISHING COMPANY, INC 52 Vanderbilt Avenue New York, New York 10017 Library of Congress Card Number: 74-21856 ISBN 0-444-41283-2 Copyright © 1976 by Elsevier Scientific Publishing Company, Amsterdam All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Scientific Publishing Company, Jan van Galenstraat 335, Amsterdam Printed in The Netherlands The book is dedicated to the memory of Dr. Kwame Nkrumah, who laid the foundation for Scientific Research in Ghana. FOREWORD "Latente Soil Engineering; Pedogenesis and Engineering Principles" is one of the most outstanding scientific works which has emanated from any of the new African territories; it also has the distinction of being one of the very few books on the solving of engineering problems by the aid of engineering pedology. When the term "pedological engineering" was first introduced it raised consider able mirth amongst civil engineers but those days have passed. It may be defined, as an extension of J.S. Joffe's description of pedology, as "the science of investigating engineering foundation problems making use of all the natural laws known to be operative under the particular and specific conditions prevailing". In pedology is to be found an understanding of the secrets of soil properties as those of cohesion, of their resistance to stress, their moisture relationships, their susceptibility to volume change and their reaction to the various kinds of additives incorporated for the purpose of moisture or strength stabilisation — only some of which are eve"r likely to be effective for any specific soil material. In conventional soil mechanics that understanding is missing though that restricted outlook is rapidly changing. Having spent the leisure hours of my professional life as a freelance in the study of foundation soils and their problems, having served in one of the African territories visiting others to study their difficulties, and having served in other tropical and near-tropical climates as well as in temperate England, I am well aware of the prob lems facing the soil engineer and designer in the tropics, of the limitations of standard specifications for suitability of materials and of the approaches to design developed in the northern states of the U.S.A. and in Europe. I can thus sympathise with any person or country trying to develop an understanding and approach suitable for the areas of their involvement. I can also sympathize with the chagrin of those appreciating that the specifications for a construction project do not always keep in mind the particular properties and problems of the materials likely to be encoun tered. A closer liaison between the designer, the specification writer and the resident engineer would often lead to a reduction in the number of claims. In many parts of the world it is not the safe bearing pressure which controls the design of foundations but the reverse one of soil swelling pressure. On a world-wide basis that swelling pressure is one of the most important of all the soil properties. The western engineer may not always be conversant with the implications of the various water relationships and appreciate the reasons for setting the control limits VIII FOREWORD at such values as given in the standard specifications of his homeland. When he is faced with working under tropical conditions he is lost. This is not because there is anything wrong with the specifications with which he is familiar but because the materials for which they were tailored are just not available. He has to realize that western standards are framed to suit western countries and that a new form of tailoring may be necessary in the tropics. Materials tested to give high LL's and Pi's may be suitable for use as road sub- grades or even for use in base courses. The implications of these constants and the interpretation of the data, with the responsible clay minerals in mind, must be understood and used to assess the suitability of such materials. To avoid unnecessary and costly failures and waste of time, this may mean that the emphasis on the precautions to be taken may have to be changed. A similar argument may be advanced in respect of the grading requirements for base course materials. Soil structure and cementation, not usually of importance in the U.K., may play a useful part in increasing resistance to stress providing other conditions are favour able. In such instances it is the properties of the colloidal constituents which are of importance; whether they are likely to be chemically reversible or not and if so under what conditions; whether they are likely to be thixotropic under stress or variations in internal climate. It is because phenomena and materials not commonly encountered under temperate climates are commonplace in tropical zones, and because of my strong belief in the value of pedology in solving the problems posed by such phenomena and materials that I, now retired, feel so deeply honoured to have been invited to write this Foreword to such a well written and comprehensive treatise on these variable latente soils. Dr. Gidigasu has zealously done his homework — a rare occurrence these days. He has read extensively, given his subject deep thought and has been involved in much research. Of the many variables responsible for soil properties, some appear, under climatic conditions normal to a specific zone, to be constants and are thus ignored. Their possible existence and importance may even be unknown to the engineer. Change the climate and those constants may become variables of importance. In the U.K., severe and prolonged droughts when the natural moisture content falls appreciably below the commonly encountered minimum value in the subsoil approximating the PL, are but seldom experienced. When they do occur and are accompanied by severe desiccation to be followed, in due course, by reswelling of the soil then light structures are bound to suffer. Buildings crack and road surfaces may be distorted to suggest signs of incipient failure. This is not a rare occurrence in the tropics and is likely to be a yearly feature. In some soil profiles any natural or unnatural impediment to drainage, equivalent FOREWORD IX to the soil being subjected to a different weathering system, may lead to the ac cumulation or to the formation of clay minerals not typical of the area as a whole. The soil properties in such, perhaps only micro, areas will be different or change. The clay minerals formed in the tropics may be very different to those found in England. Soon after my arrival in Rangoon, 1929 (monsoon type of rainfall 100 inches, high humidity, mean annual temperature about 82°F) I was taken to see a large nearby latente quarry extending for the full depth of a long hillside face cutting. The quarry was similar to any hard stone quarry in that the material was blasted and the products had to be broken down to acceptable size. To me, latente did not begin to appear to be a problem material till years later when, during the construc tion of RAF airfields near Rangoon at the outbreak of war, the consolidation of this clinker-like material was found to be as difficult as the compaction of phonolite. The presence of other varieties of latente all around me escaped my attention which was given to the determination of an explanation for a more obvious prob lem — the phenomena responsible for the severe damage suffered by light buildings constructed in the high swelling soils of the central Burma semi-desert area; and in the prevention of such damage. It was not till many years later, after experience in India, Africa, in the U.S.A. and Australia that I fully realized how diverse and problematical these latente soils could be. The word latente describes no material with reasonably constant properties. It can signify a different material to people living in different parts of the world. Whereas the Oxford Dictionary defines latente as a red friable ferruginous sur face clay, I would, in my early days, have described it as a very hard homogeneous vesicular massive clinker-like material, found in hilly tropical countries, in which the structural framework is of red hydrated ferric oxides and the vesicular infill is of soft aluminium oxides of a yellowish colour; or a similar profile of softer material which hardens on extraction and exposure. Alternatively, and in less hilly country, the massive structure may be absent and the material be found to exist as a very hard, or soft, coarse angular red gravel. Laterites as a soil group rather than a well defined material, are most commonly found in the leached soils of the humid tropics where they were first studied. They are formed under weathering systems productive of the process of laterization, the most important characteristic of which is the decomposition of ferro—alumino silicate minerals and the permanent deposition of sesquioxides within the profile to form the horizon of material known to the engineer and builder as laterite. Another feature of the process as encountered in the tropics is the leaching of silica, by an effectively alkaline soil solution, part of which may form a complex with the sesquioxides to accentuate the formation of a concretionary or massive structure. The remainder of the silica may form secondary clay silicate minerals or be completely removed by soil drainage. χ FOREWORD If this leaching of silica is minimal or does not take place, as in the formation of the soils generally referred to as podsols, then the process of laterization may be considered to occur under climates other than tropical. For this reason there are many soils which can be classified as having been formed by the process of lateriza tion. Hence other terms as lateritites, lateritic and laterized soils have been in troduced. At one end of the series of materials we have hard latente, some podsolic pans formed by the less intensive weathering of less permeable soils or of soils with a fluctuating high water table, and the but partly laterized parent rock of less humid regions. These are followed by other granular materials as fractured sheet formations of hard latente, pisolithic fine graded gravels and lateritic sands. At the other extremity are the non-granular tropical red earths, mottled red-yellow or red-brown clays and, possibly, the terra rossa, the marls and even the treacherous krasnozems and podsolic soils along the northeast seaboard of Tasmania. Treacherous because some 2—3 ft. down the profile of red or red-brown sandy top soil is a whiteish permeable layer of so-called "fine sand and silt" which encourages slips and slides to occur. The lower the silica-sesquioxide ratio of such materials, the more advanced the laterization process is likely to be. The factors encouraging and effecting the results of laterization are: A basic or intermediate parent rock material containing ferro—alumino silicates. The amount and kind of vegetation and of organic matter and humic residues formed. A permeable profile. A heavy rainfall, preferably of a monsoon type; high humidity. A hot climate with coolish nights involving topography (high temperatures accelerate decomposition; the contrast between day and night temperatures and even changes during the day may be considerable; these encourage not only moisture content changes within the profile but a reversal of the direction of moisture flow although such changes may only be of a micro-nature). A fluctuating water-table especially where the topography is flattish and of a savanna type. A combination of alternate leaching by an effectively acid and an effectively alkaline soil solution (silica is removed by an alkaline medium and the sesquioxides are mobilized by an acid medium). Variations in the applicability of these factors, as caused by the involvement of different parent materials and differing external and internal climates, reflect them selves in the many forms of lateritic materials found in the various parts of the world. There will be variations in the products of decomposition and particularly in the secondary clay minerals formed. Instead of being of a reasonably constant nature and behaviour their profiles, grading, structure, constituent minerals and properties FOREWORD XI are highly variable. According to these variations the soil material, even within relatively small areas, may be found to be characterized by the dominating presence of montmorillonite, hydrated halloysite, metahalloysite or even allophane all of which may coexist in any sample. THE KENYA LATERITIC AND LATERIZED SOILS In contrast with the Burma latentes which are the matured, generally rock-hard, end products of the laterization of various parent materials, the Kenya latentes are found in the young profiles of soils belonging to the tropical red earth group. They exist as horizons of "murrains" (concretionary hardpans rich in sesquioxides) or of partly decomposed tuffaceous rock. Their LAA values, at the best, vary between about 80 and 50. During the decomposition of basic tuffs one of the first of the secondary clay minerals to be formed is montmorillonite though it does not appear to be an end product. Under conditions of an effectively high rainfall and a permeable profile, it is either washed out, vertically or laterally, by drainage water or is converted, in time, to halloysite. But if the drainage is impeded in micro or macro areas, then that mineral will tend to accumulate. Thus in quarried material it is sometimes found as a small dark mass surrounded by a red coating of iron oxide which is suspected to act as a water-proofing agent. Similarly, it may collect, though its presence will not be visible, in the red-brown soils and impart to them a swelling pressure of at least 3 tons/sq.ft. or three times that which can be exerted by suspect red halloysite soils. An apparent characteristic of the halloysite soils was that the hydrated form was only detected, as the sole clay mineral, in samples taken from an altitude above about 7,000 ft. With decrease in altitude metahalloysite began to appear until the detected mineral was entirely of the dehydrated variety. For still lower altitudes, as near the coastal seaboard, halloysite seemed to be replaced by kaolinite. As the analysis of data proceeded, a suspicion arose that allophane might be present and particularly in association with hydrated halloysite. Two samples of soil from the Sasamua Dam site1 were sent to the State Geological Survey Division, Univ. of Illinois, for testing by W.A. White, internationally known as an authority on this clay mineral. Both samples were reported to contain allophane (Wooltorton, W.A. White, correspondence March 1961). Not very much is known about the properties of allophane soils. The clay mineral is amorphous. The soil is fine grained and yet permeable despite its high base ex change capacity. 1 Terzaghi, 1958. Report on the design and performance of the Sasamua Dam". Proc. Inst. Civ. Eng., Lond., April, pp. 369-394 (in which the problems encountered during the use of hydrated halloysite fill are described). XII FOREWORD They are of low bulk density with a high water holding capacity said to increase with compaction. This suggests, with their difficulty in being dispersed, a phenom enon similar to that referred to below as characterizing some halloysites. They evidence high shrinkage on drying but this volume change appears to be but slowly reversible. Their cohesion is low. Some of their constants are given by M. Grad well and K.S. Birrell2 as: LL = 144; PL = 48; LL/c = 1.78; mean thickness of water film at LL = 3.79 X 1CT6 cm for a mixture of halloysite and 5% allophane. The cor responding values for allophane alone were 231, 136, 3.61, and 1.34 X 1CT6 respec tively. The organic matter content of mature latente soils is low; that in the halloysites may be as high as 15% whilst that in the allophane is higher still. Its residues ap parently surround the aggregated clay mineral particles of the friable halloysites to act as a water repellant when appreciable work must be done on the material before its moisture content can be sensibly increased. This is also a characteristic of the allophanes but whether, in that instance, it is a result of the presence of organic matter residues or is a characteristic of the amorphous clay mineral is not clear. The halloysite soils are generally good engineering materials whilst in a struc tural state achieved naturally or after compaction but may cause trouble by flowing if that structure be destroyed as by the direct application of traffic in the presence of free water. Material containing the hydrated form may be found to exist at a moisture content apparently in excess of its Proctor OMC, but whether this is effectively so is not so certain. Though it gave cause for alarm during the construction of the Sasamua Dam, the difference was too small to hold up the roadworks projects in which I was concerned. If left in the excavated state and allowed to desiccate during the hot dry weather, it will change rapidly to metahalloysite or to some intermediate crystalline form not yet identified. The existence of an intermediate form was suggested by some of the X-ray patterns. This means that if the initial compaction testing is done on site and the CBR's are determined in some distant laboratory, involving a truck journey in sacks and the passage of some time, then the moisture content used for making up the test samples will be too high and a contractural claim may follow. This is illustrated by the following test data: Seemingly general values of LL PL PI OMC(P) Notes Hydrated halloysite 47-65 34-48 10-16 26-28 7 samples Metahalloysite 62-91 35-59 14-38 38-49 6 samples 2 Physical properties of certain volcanic soils. N.Z. J. Sei. Tech., 36,1954-55 (pages not now known).
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