Engineering Properties of Soils and Rocks PDF
Preview Engineering Properties of Soils and Rocks
Engineering Properties of Soils and Rocks Third edition F. G. Bell - 1 U T T E R W O R TH E I N E M A N N Butterworth-Heinemann Ltd Linacre House, Jordan Hill, Oxford OX2 8DP (Q? PART OF REED INTERNATIONAL BOOKS OXFORD LONDON BOSTON MUNICH NEW DELHI SINGAPORE SYDNEY TOKYO TORONTO WELLINGTON First published 1981 Second edition 1983 Reprinted 1985 This edition 1992 © Butterworth-Heinemann Ltd 1981,1983,1992 All rights reserved. No part of this publication may be reproduced in any material form (including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication) without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act 1988 or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1P 9HE. Applications for the copyright holder's written permission to reproduce any part of this publication should be addressed to the publishers British Library Cataloguing in Publication Data Bell, F. G. (Frederic Gladstone) Engineering properties of soils and rocks. I. Title 624.15136028 ISBN 07506 04891 Library of Congress Cataloguing in Publication Data Bell, F. G. (Frederic Gladstone) Enginnering properties of soils and rocks/F. G. Bell.—3rd ed. p. cm. Includes bibliographical references and index. ISBN 0 7506 04891 1. Soil mechanics. 2. Rock mechanics. I. Title. TA710.B424 1992 624.1'513—<ic20 91-39429 CIP Composition by Genesis Typesetting, Laser Quay, Rochester, Kent Printed and bound in Great Britain Preface As was stated in the Preface to the first edition, civil engineers, mining engineers and engineering geologists require a working knowledge of the engineering properties and behaviour of soils and rocks. Although this text provides such information and cites actual values of particular soils and rocks, the reader is reminded that these should be taken only as guides. Natural materials are variable, some highly variable; they are not made to specification like manufactured materials. The properties and behaviour of individual soil and rock types vary according to many factors, notably their composition, texture, degree of weathering, presence of fissures or discontinuities, etc. The text pays regard to these factors. In fact it is intended to complement texts on soil and rock mechanics. The author has taken the opportunity to revise the text fully, with the result that this edition is twice as long as its predecessor. It is the author's hope that readers will find this volume, with its increased coverage, a valuable addition to their bookshelves, both now and throughout their professional lives. F. G. Bell Chapter 1 Properties and classification of soils 1.1 Origin of soil Soil is an unconsolidated assemblage of solid particles between which are voids. These may contain water or air, or both. Soil is derived from the breakdown of rock material by weathering and/or erosion and it may have suffered some amount of transportation prior to deposition. It may also contain organic matter. The type of breakdown process(es) and the amount of transport undergone by sediments influence the nature of the macro- and microstructure of the soil, which in turn influence its engineering behaviour (Table 1.1). Probably the most important methods of soil formation are mechanical and chemical weathering. The agents of weathering, however, are not capable of transporting material. Transport is brought about by gravity, water, wind or moving ice. If sedimentary particles are transported, then this affects their character, particularly their grain-size distribution, sorting and shape. For instance, stream channel deposits are commonly well graded, although the grain size Table 1.1 Effects of transportation on sediments Gravity Ice Water Air Size Various Varies from clay to Various sizes from Sand size and boulders boulder gravel to muds less Sorting Unsorted Generally unsorted Sorting takes place both Uniformly sorted laterally and vertically. Marine deposits often uniformly sorted. River deposits may be well sorted Shape Angular Angular From angular to well Well rounded rounded Surface Striated Striated surfaces Gravel: rugose surfaces. Impact produces texture surfaces Sand: smooth, polished frosted surfaces surfaces. Silt: little effect 1 1 Properties and classification of soils characteristics may vary erratically with location. On the other hand, wind-blown deposits are usually uniformly sorted with well-rounded grains. Changes occur in soils after they have accumulated. In particular, seasonal changes take place in the moisture content of sediments above the water table. Volume changes associated with alternate wetting and drying occur in cohesive soils with high plasticity indices (see below). Exposure of a soil to dry conditions means that its surface dries out and that water is drawn from deeper zones by capillary action. The capillary rise is associated with a decrease in pore pressure in the layer beneath the surface and a corresponding increase in effective pressure. This supplementary pressure is known as capillary pressure and it has the same mechanical effect as a heavy surcharge. Therefore surface evaporation from very compressible soils produces a conspicuous decrease in the void ratio of the layer undergoing desiccation. If the moisture content in this layer reaches the shrinkage limit then air begins to invade the voids and the soil structure begins to break down. Moreover if the plasticity index of the soil exceeds 20% then the seasonal variations in moisture content of the upper layers are accompanied by ground movement. The decrease in void ratio consequent upon desiccation of a cohesive sediment leads to an increase in its shearing strength. Thus if a dry crust is located at or near the surface above softer material it acts as a raft. The thickness of dry crusts often varies erratically. Chemical changes which take place in the soil due, for example, to the action of weathering, may bring about an increase in its clay mineral content, the latter developing from the breakdown of less stable minerals. In such instances the plasticity of the soil increases whilst its permeability decreases. Leaching, whereby soluble constituents are removed from the upper, to be precipitated in a lower horizon, occurs where rainfall exceeds evaporation. The porosity may be increased in the zone undergoing leaching. During sediment accumulation the stress at any given elevation continues to build up as the thickness of the overburden increases. As a result the properties of the sediment are continually changing, the void space, in particular, being reduced. If, subsequently, overburden is removed by erosion, or for that matter by extensive excavation, the void ratio tends to increase. With continuing exposure soil develops a characteristic profile from the surface downwards. This development involves the accumulation and decay of organic matter, leaching, precipitation, oxidation or reduction and further mechanical and biological breakdown. The profile which forms is influenced by the character of the parental material but climatic conditions, vegetative cover, groundwater level and relief also play their part and the time factor allows the distinction between immature and mature soils. 1.2 Basic properties of soil As remarked, a soil consists of an assemblage of particles between which are voids, and as such can contain three phases - solids, water and air. The interelationships Properties and classification of soils 3 of the weights and volumes of these three phases are important since they help define the character of a soil. One of the most fundamental properties of a soil is the void ratio, which is the ratio of the volume of the voids to that of the volume of the solids. The porosity is a similar property, it being the ratio of the volume of the voids to the total volume of the soil, expressed as a percentage. Both void ratio and porosity indicate the relative proportion of void volume in a soil sample. Water plays a fundamental part in determining the engineering behaviour of any soil and the moisture content is expressed as a percentage of the weight of the solid material in the soil sample. The degree of saturation expresses the relative volume percentage of water in the voids. The range of values of phase relationships for cohesive soils is much larger than for granular soils. For instance, saturated sodium montmorillonite at low confining pressure can exist at a void ratio of more than 25, its moisture content being some 900%. On the other hand saturated clays under high stress that exist at great depth may have void ratios of less than 0.2, with about 7% moisture content. The unit weight of a soil is its weight per unit volume, whilst its specific gravity is the ratio of its weight to that of an equal volume of water. In soil mechanics the specific gravity is that of the actual soil particles. The density of a soil is the ratio of its mass to that of its volume. A number of types of density are distinguished. The dry density is the mass of the solid particles divided by the total volume, whereas the bulk density is simply the mass of the soil (including its natural moisture content) divided by its volume. The saturated density is the density of the soil when saturated, whilst the submerged density is the ratio of effective mass to volume of soil when submerged. In fact the submerged density can be derived by subtracting the density of water from the saturated density. The density of a soil is governed by the manner in which its solid particles are packed. For example, granular soils may be densely or loosely packed. Indeed a maximum and minimum density can be distinguished. The smaller the range of particle sizes present and the more angular the particles, the smaller the minimum density. Conversely, if a wide range of particle sizes is present the void space is reduced accordingly, hence the maximum density is higher. A useful way to characterize the density of a granular soil is by its relative density (D ) which is r defined as * = T ^ T- (1.D where e is the naturally occurring void ratio, e is the maximum void ratio and mxa e is the minimum void ratio. Five degrees of density have been distinguished min (Table 1.2). 4 Properties and classification of soils Table 1.2 Grades of relative density and their description Class Relative density Description 1 Less than 0.2 Very loose 2 0.2-0.4 Loose 3 0.4-0.6 Medium dense 4 0.6-0.8 Dense 5 Over 0.8 Very dense 1.3 Particle-size distribution The particle-size distribution expresses the size of particles in a soil in terms of percentages by weight of boulders, cobbles, gravel, sand, silt and clay. The United Soil Classification (Wagner, 1957) and Anon (1981) give the limits shown in Table 1.3 for these size grades. In nature there is a deficiency of soil particles in the fine gravel and silt ranges, and boulders and cobbles are quantitatively speaking not significant. Sands and clays are therefore the most important soil types. The results of particle-size analysis are given in the form of a series of fractions, by weight, of different size grades. These fractions are expressed as a percentage of the whole sample and are generally summed to obtain a cumulative percentage. Cumulative curves are then plotted on a semi-logarithmic paper to give a graphical representation of the particle-size distribution. The slope of the curve provides an indication of the degree of sorting. If, for example, the curve is steep as in curve A Table 1.3 Particle size distribution of soils Types of material Sizes (mm) Boulders Over 200 Cobbles 60-200 Coarse 20-60 Gravel Medium 6-20 Fine 2-6 Coarse 0.6-2 Sand Medium 0.2-0.6 Fine 0.06-0.2 Coarse 0.02-0.06 Silt Medium 0.006-0.02 Fine 0.002-0.006 Clay Less than 0.002 Properties and classification of soils 5 e g a Uniformly graded nt medium grainedy Well graded e rc sand H sand-gravel - pe silt soil e Gap graded v ati silty sand- ul m u C Figure 1.1 Grading curves in Figure 1.1, then the soil is uniformly sorted, whilst curve B represents a well sorted soil. The sorting or uniformity of a particle size distribution has been expressed in a great many ways but one simple statistical measure which has been used by engineers is the coefficient of uniformity (U). This makes use of the effective size of the grains (Dio), that is, the size on the cumulative curve where 10% of the particles are passing and is defined as U = — (1.2) ^10 Similarly D&) is the size on the curve at which 60% of the particles are passing. A soil having a coefficient of uniformity of less than 2 is considered uniform whilst one with a value of 10 is described as well graded. In other words the higher the coefficient of uniformity, the larger is the range of particle sizes. The coefficient of curvature (C) is obtained from the expression c 2 £>30 C = — ( 1 . 3) c AKAO A well-graded soil has a coefficient of curvature between 1 and 3. 1.4 Consistency limits The Atterberg or consistency limits of cohesive soils are founded on the concept that such soils can exist in any of four states depending on their water content. These limits are also influenced by the amount and character of the clay mineral content. In other words a cohesive soil is solid when dry but as water is added, it 6 Properties and classification of soils first turns to a semi-solid, then to a plastic, and finally to a liquid state. The water content at the boundaries between these states is referred to as the shrinkage limit (SL), the plastic limit (PL) and the liquid limit (LL) respectively. The shrinkage limit is defined as the percentage moisture content of a soil at the point where it suffers no further decrease in volume on drying. The plastic limit is the percentage moisture content at which a soil can be rolled, without breaking, into a thread 3 mm in diameter, any further rolling causing it to crumble. Unfortunately the inadequacy of control involved in the test means that the results obtained are not consistent for any particular clay. Turning to the liquid limit, this is defined as the minimum moisture content at which a soil will flow under its own weight. Clays may be classified according to their liquid limit as shown in Table 1.4. Table 1.4 Plasticity according to liquid limit Description Plasticity Range of liquid limit Lean or silty Low plasticity Less than 35 Intermediate Intermediate plasticity 35-50 Fat High plasticity 50-70 Very fat Very high plasticity 70-90 Extra fat Extra high plasticity Over 90 The consistency of cohesive soils depends on the interaction between the clay particles. Any decrease in water content results in a decrease in cation layer thickness and an increase in the net attractive forces between particles. For a soil to exist in the plastic state the magnitudes of the net interparticle forces must be such that the particles are free to slide relative to each other with cohesion between them being maintained. The plasticity of fine grained soils refers to their ability to undergo irrecoverable deformation at constant volume without cracking or crumbling. The numerical difference between the liquid and plastic limits is referred to as the plasticity index (PI). This indicates the range of moisture content over which the material exists in a plastic condition. The plasticity index has been divided into five classes which are as shown in Table 1.5. Table 1.5 Plasticity of soils (after Anon, 1979) Class Plasticity index (%) Description 1 Less than 1 Non-plastic 2 1-7 Slightly plastic 3 7-17 Moderately plastic 4 17-35 Highly plastic 5 Over 35 Extremely plastic Properties and classification of soils 1 The liquidity index of a soil is defined as its moisture content in excess of the plastic limit, expressed as a percentage of the plasticity index. It describes the moisture content of a soil with respect to its index limits and indicates in which part of its plastic range a soil lies, that is, its nearness to the liquid limit. The consistency index is the ratio of the difference between the liquid limit and natural moisture content to the plasticity index. It can be used to classify the different types of consistency of cohesive soils, as shown in Table 1.6. Table 1.6 Consistency of cohesive soils Description Consistency Approximate Field identification index undrained shear strength (kPa) Hard Over 300 Indented with difficulty by thumbnail, brittle Very stiff Above 1 150-300 Readily indented by thumbnail, still very tough Stiff 0.75-1 75-150 Readily indented by thumb but penetrated only with difficulty. Cannot be moulded in the fingers Firm 0.5-0.75 40-75 Can be penetrated several centimetres by thumb with moderate effort, and moulded in the fingers by strong pressure Soft Less than 0.5 20-40 Easily penetrated several centimetres by thumb, easily moulded Very soft Less than 20 Easily penetrated several centimetres by fist, exudes between fingers when squeezed in fist The plasticity of a soil is influenced by the amount of its clay fraction, since clay minerals greatly influence the amount of attracted water held in the soil. With this in mind Skempton (1953) defined the activity of a clay as Plasticity index Activity = (1.4) % by mass finer than 0.002 mm He suggested three classes of activity, namely, active, normal and inactive which he further subdivided into five groups as follows: (1) inactive with activity less than 0.5, (2) inactive with activity range 0.5-0.75, (3) normal with activity range 0.75-1.25, (4) active with activity range 1.25-2, (5) active with activity greater than 2.
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