Foundation Engineering in Difficult Ground

602 Pages·1978·11.913 MB·English

by  F. G. Bell (Eds.)

Foundation Engineering
in Difficult Ground
Edited by
F.G.BELL
With 17 specialist contributors
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First published 1978
reprinted 1981
© Butterworth & Co (Publishers) Ltd, 1978
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British Library Cataloguing in Publication Data
Foundation engineering in difficult ground.
1. Foundations
1. Bell, Frederic Gladstone
624'.15 TA775 77-30301
ISBN 0-408-00311-1
Typeset and produced by Scribe Design . Medway . Kent
Printed in Great Britain by W & J Mackay Ltd, Chatham
Preface
The basis of this book was a conference on Foundation Engineering which was
held in Sheffield in September 1976. However, rather than publish the papers
in the form of proceedings it was decided to produce a text, which would be of
value to the practicing foundation engineer on the one hand and the teacher and
student on the other. This involved expanding the number of topics considered
in order to provide a more balanced treatment of the subject. The topics were
suggested to the individual authors, many of whom have international reputa-
tions, who, of course, developed their own contributions in their own way.
Their style and philosophy has been retained as much as possible, within the
concept of the text.
Although it is assumed that the reader has a basic knowledge of soil mecha-
nics, the major emphasis of the book is placed on ground conditions, especially
difficult ground conditions. In this way, it differs from most other works on
foundation engineering. Poor ground conditions arise from lack of strength,
excess ground water or appreciable ground movement, generally in some combi-
nation rather than in isolation. It is poor ground conditions which present the
engineer with problems. In addition it must be borne in mind that the heavy
demand placed on land means that sites previously considered unsuitable for
development are now used for building purposes. In order to overcome these
problems at the design stage the engineer must first investigate the subsurface
conditions in order that some attempt at understanding their likely behaviour
may be made. With such information the engineer can now choose either to
enhance the performance of the ground or to adopt special foundation struc-
tures, or to combine the two. This method of attack should allow the structure,
which is to be erected, to perform satisfactorily.
The book deals with foundation engineering in the context outlined above. It
starts with an examination of ground conditions, focusing attention on geology,
soils, rocks and ground water, and their influence on foundations; foundations
of all types, from those of small dimensions such as necessary for houses, to
those required for concrete dams. Consideration is then given to how ground
conditions are investigated. Then follow chapters on ground movements; settle-
ment, subsidence and, worse still, earthquakes, all provide appreciable problems
for the foundation engineer. If ground conditions prove difficult then they can
be subjected to some form of treatment to improve their engineering perfor-
mance. Ground treatment is therefore dealt with in chapter 14. The problem of
slope stability, whether in soils or rocks, is present during excavation. As a
consequence slopes in soils and rocks are considered before open excavation.
The latter part of the book deals with foundation structures, particularly piles,
and failures.
Time involved in production and the size of the work place restrictions on the
number of topics which can be covered. No treatment can ever be exhaustive
and the topics are accordingly selected for their intrinsic importance. One is
conscious that certain topics may have been included by others, for instance,
retaining walls springs to mind. But this is not a textbook of design methods,
which a treatment of retaining walls mainly would have involved. For the same
reason, no detailed attempt is made to cover the various aspects of design of
foundations structures. These are included in most texts of foundation engi-
neering, indeed, one of the authors, M.J. Tomlinson, is responsible for a first
class textbook on design and construction of foundations.
Finally, the editor would like to express his warm appreciation to the authors
for helping to make this text a reality, and in particular acknowledges the kind
help given by Dr. Ian Farmer who commented upon chapter 9.
F.G. Bell
Blyth, Notts
The design on the jacket is a diagram of the Shell building, London showing raft
on piles. The piles extend beneath the London Underground Bakerloo line.
(Reproduced by courtesy of the Institution of Civil Engineers)
Chapter 1
Introduction
SOIL CLASSIFICATION
Soil is an unconsolidated assemblage of solid particles which may or may not
contain organic matter, the voids between the particles being occupied by air
and/or water. It derives mainly from rock material broken down by physical or
chemical weathering* and by water, wind and ice which are responsible for the
varying amounts of transport which particles may undergo prior to deposition.
Gravity likewise has a hand in particle transport. The type of breakdown and
amount of transport have a significant influence on the character of a deposit
(Table 1.1), as has the parental material. In addition changes occur in a deposit
after it has been laid down. Accordingly different types of soil evolve, with
different grain size distributions, with differing degrees of sorting and packing,
and with differently shaped particles.
Classification of soils is made on a basis of certain fundamental properties
and provides an ordered framework for their systematic description. Such a
classification ideally should indicate the engineering performance of the soil
type and should provide a quick means of identification.
The fundamental property upon which most engineering classifications of
soils are based is particle size distribution, since it is readily measurable and has
an important influence on soil behaviour. Boulders, cobbles, gravels, sands, silts
and clays are distinguished as individual groups, each group being given the fol-
lowing symbol and size range:
1. Boulders (B), over 200 mm
2. Cobbles (Cb), 60 to 200 mm
3. Gravel (G), 2 to 60 mm (20 to 60 mm, coarse; 6 to 20 mm, medium; 2 to
6 mm, fine).
4. Sand (S), 0.06 to 2 mm (0.6 to 2 mm, coarse; 0.2 to 0.6 mm, medium;
0.06 to 0.2 mm, fine)
5. Silt (M), 0.002 to 0.06 mm (0.02 to 0.06 mm, coarse; 0.006 to 0.02 m,
medium; 0.002 to 0.006 mm, fine)
6. Clay (C), less than 0.002 mm.
The gravel, sand and silt size ranges are further divided up into coarse, medium
and fine categories. Sands and gravels are granular materials ideally possessing no
*Peat, being an organic soil, does not fall within this context
1
2 Introduction
Table 1.1 EFFECTS OF TRANSPORTATION ON SEDIMENTS
Gravity Ice Water Air
Size Various Varies from clay to Various sizes from Sand size and less
boulders boulder gravel to muds less
Sorting Unsorted Generally unsorted Sorting takes place Uniformly sorted
both laterally and
vertically. Marine
deposits often uni-
formly sorted. River
deposits may be well
sorted
Shape Angular Angular From angular to well Well rounded
rounded polished
surface
Surface Striated Striated surfaces Gravel: rugose Impact produces
texture surfaces surfaces. frosted surfaces
Sand: smooth,
polished surfaces.
Silt: little effect
cohesion whereas silts and clays are cohesive materials. Mixed soil types can be
indicated as follows:
1. sandy GRAVEL With sand sized or gravel sized material as an impor-
gravelly SAND tant second constituent of the coarse fraction
2. clean GRAVEL/SAND With under 5% fines (G; S)
3.GRAVEL/SAND with With 5 to 20% fines (can be distinguished as silt or
some fines (F) clay: G-F, G-M, G-C; S-F, S-M, S-C)
4. GRAVEL/SAND with With 20 to 50% fines (can be distinguished as silt or
much fines clay: GF, GM, GC; SF, SM, SC)
5. SILT/CLAY with With 30-50% gravel or sand (MG, MS; CG, CS)
gravel or with sand
These major groups are divided into subgroups on a basis of grading and the
1 2
plasticity of the fine material (Table 1.2). In the proposed British Soil Classifi-
cation , which is based upon the Casagrande classification, granular soils are
described as well graded (W) or poorly graded (P). Two further types of poorly
graded granular soils are recognised, namely, uniformly graded (Pu) and gap-
graded (Pg). Silts and clays are generally subdivided according to their liquid
limits (LL) into low (LL = less than 30. L), medium (LL = 35 to 50. M) and high
(LL = over 50. H) subgroups. Very high (LL = 70 to 90. V) and extremely high
(LL = over 90. E) categories have also been recognised. Each subgroup is given a
combined symbol in which the letter describing the predominant size fraction is
written first (e.g. GW = well graded gravels; CH = clay with high liquid limit).
Silty soil has a restricted plastic range in relation to its liquid limit whilst clay is
2
fully plastic. The plasticity chart for use in soil classification (Table 1.2c) was
introduced by Casagrande. Silts tend to plot below, and clays above the A-line.
Organic soils also plot below the A-line.
Any group may be referred to as organic if it contains a significant propor-
tion of organic matter, in which case the letter O is suffixed to the group symbol
(e.g. CVSO = organic clay of very high liquid limit with sand). The symbol Pt is
given to peat.
In many soil classifications boulders and cobbles are removed before an
Introduction 3
attempt is made at classification, for example, their proportions are recorded
separately in the proposed British Soil Classification. Their presence should be
recorded in the soil description, a plus sign being used in symbols for soil mix-
tures, for example, G + Cb for gravel with cobbles. The British Soil Classification
has proposed that very coarse deposits should be classified as follows:
1. BOULDERS Over half of the very coarse material is of boulder
size (over 200 mm). May be described as cobbly
BOULDERS if cobbles are an important second con-
stituent in the very coarse fraction.
2.COBBLES Over half of the very coarse material is of cobble
size (200-60 mm). May be described as bouldery
COBBLES if boulders are an important second con-
stituent in the very coarse fraction.
Mixtures of very coarse material and soil can be described by combining the
terms for the very coarse constituent and the soil constituent as follows:
1. clean BOULDERS/COBBLES , up to 5% soil
2. BOULDERS/COBBLES with some SOIL* 5-20% soil
3. BOULDERS/COBBLES with much SOIL* 20-50% soil
4. SOIL* with many BOULDERS/COBBLES 50-20% boulders
5. SOIL* with some BOULDERS/COBBLES 20-5% boulders
6. SOIL* with occasional BOULDERS/COBBLES up to 5% boulders
*Give soil name in brackets, e.g. Cobbly BOULDERS with some SOIL (sand with some fines)
The British Soil Classification also has proposed that classification can be
made either by rapid assessment or by full laboratory procedure (Tables 1.2a
and 1.2b). It was recommended that if classification was by means of rapid
assessment, brackets should enclose the group symbol indicating a lower degree
of accuracy, whereas if full laboratory procedure was used for classification the
group symbol should not be bracketed. For a comparison with the Unified Soil
Classification which is used in the United States see Table 1.2c. A note on the
engineering uses of these soils is provided in Table 1.2d.
COARSE GRAINED SOILS
The composition of a gravel deposit reflects not only the source rocks of the
area from which it was derived but is also influenced by the agents responsible
for its formation and the climatic regime in which it was or is being deposited.
The latter two factors have a varying tendency to reduce the proportion of
unstable material. Relief also influences the nature of a gravel deposit, for
example, gravel production under low relief is small and the pebbles tend to be
chemically inert residues such as vein quartz, quartzite, chert and flint. By
contrast high relief and rapid erosion yield coarse, immature gravels.
Sands consist of a loose mixture of mineral grains and rock fragments. Gene-
rally they tend to be dominated by a few minerals, the chief of which is quartz.
3
There is a presumed dearth of material in those grades transitional to gravel on
the one hand and silt on the other (see Glossop and Skempton). Sands vary
appreciably in their textural maturity.
The engineering behaviour of a soil is a function of its structure or fabric,
which in turn is a result of the geological conditions governing deposition and
the subsequent stress history. The macro-structure of a soil includes its bedding,
laminations, fissures, joints and tension cracks, all of which can exert a domi-
nant influence on the shear strength and drainage characteristics of a soil mass.
Table 1.2a THE BRITISH SOIL CLASSIFICATION SYSTEM FOR ENGINEERING PURPOSES (BSCS). FIELD OR RAPID IDENTIFICATION
[First remove material coarser than 60 mm and record as COBBLES (60-200 mm) or BOULDERS (over 200 mm)]
Remoulded at suitable moisture
SOIL GROUPS content
GRA VEL and SAND may be qualified SYMBOL TEXTURE COHESION PLASTICITY
sandy GRA VEL and gravelly SAND Ability to Ability to deform
where appropriate stick together without rupture
COARSE SOILS GRAVELS Little or no Fines. GW has wide
More than half More than half GW range of grain sizes, well
coarser than 60 M of coarse material Clean GRA VEL distributed. GP has one size None None
(i.e. visible to is of gravel size GP predominating, GPu, or
eye or gritty (coarser than 2 mm missing, GPg
to feel)
GRA VEL with some silt G-M Gravel with some silt None to low None
G-F
GRA VEL with some clay G-C Gravel with some clay Low to medium None
GRA VEL with much silt GM Gravel with much silt None to low None
GF
GRA VEL with much clay GC Gravel with much clay Medium to high Low to medium
SANDS Little or no fines. SW has wide
More than half Clean SAND SW range of grain sizes, well Low None
of coarse SP distributed. SP has one size
material is of predominating, SPu, or
sand size missing SPg
(finer than
2 mm) SAND with some silt S-M Sand with some silt Low None
S-F
SAND with some clay S-C Sand with some clay Low to medium None to low
SAND with much silt SM Sand with much silt Low to medium Low to medium
SF
SAND with much clay SC Sand with much clay Medium to Medium to
very high very high
FINE SOILS SILTS and 5/LTwith gravel MG Coarse material mainly over
FC
less than half CLAYS with CLA Y with gravel CG 2 mm Similar to SILT and CL/1Y groups
coarser than gravel or sand (see below), but with properties
60 M (i.e. 50-70% fines •S/ITwith sand MS Coarse material mainly under modified by a considerable pro-
FS
visible to eye CLA Y with sand CS 2 mm portion of coarse material
or gritty to feel
First remove coarser particles by hand
DRY STRENGTH TOUGHNESS
Dried from plastic Consistency at DILATANCY
limit plastic limit
SILT M ML Dries moderately None to low None to low High to
CLAY quickly and can be moderate
SILTS and of low liquid limit CI CL brushed off the
CLAYS fingers. Inorganic Medium Medium Low
70-100% fine of intermediate liquid limit CI High Medium None
(finer than 60 M) of high liquid limit CH Sticks to fingers High High None
and dries slowly.
Shrinks appreciably
of very high liquid limit CV on drying, usually
showing cracks Very high High None
ORGANIC 'Organic' prefixed to any group name and letter O, suffixed Organic matter suspected to be a significant constituent,. Dark colour,
SOILS to symbol, e.g. CO distinctive odour, moisture content may be very high
PEAT Pt Peat soil consists predominantly of plant resins, which may be fibrous or amorphous. Dark colour, distinctive odour
low bulk density, moisture content may be very high
ON
Table 1.2b BRITISH SOIL CLASSIFICATION SYSTEM FOR ENGINEERING PURPOSES (BSCS)
[First remove material coarser than 60 mm and record as COBBLES (60 mm-200 mm) or BOULDERS (over 200 mm)]
Soil Groups Sub-Groups and laboratory identification
GRA VEL and SAND may be qualified Sandy GRA VEL Group Sub-Group Fines % less
and Gravelly SAND where appropriate Symbol Symbol than 60 n Sub-Group name
n cUA i LeK v at i
CCOOAARRSSEE GGGRRRAAAVVVEEELLLSSS Clean GRA VEL G GW GW 0-5 Well-graded ri>Ai/m
SSOOIILLSS MMMooorrreee ttthhhaaannn 555000%%% GP GP GPu GPg Poorly/uniformly/gap graded
MMoorree tthhaann ooofff cccoooaaarrrssseee mmmaaattteeerrriiiaaalll
5500%% ccooaarrsseerr iiisss ooofff gggrrraaavvveeelll sssiiizzzeee GRA VEL with some silt G-M GWM GPM
G-F 5-20 Well/poorly graded GRA VEL with some ^
tthhaann 6600 MM (((cccoooaaarrrssseeerrr ttthhhaaannn GRA VEL with some clay G-C GWC GPC
222 mmmmmm)))
GRA VEL with much silt GM GM 20-50 GRA VEL with much silt: sub-divide like GC
GF
GRA VEL with much clay GC GCL GRA VEL with much clay of low/intermediate/
GCI high/very high plasticity
GCH
GCV
n c lMe ay v
SSSAAANNNDDDSSS Clean SAND S SW SW 0-5 Well graded QAMH
MMMooorrreee ttthhhaaannn 555000%%% SP SP SPu SPg Poorly/uniformly/gap graded ^
ooofff cccoooaaarrrssseee mmmaaattteeerrriiiaaalll
iiisss ooofff sssaaannnddd sssiiizzzeee SAND with some silt S-F S-M SWM SPM 5-20 Well/poorly graded SAND with some ^
(((fffiiinnneeerrr ttthhhaaannn SAND with some clay S-C SWC SPC
222 mmmmmm)))
SAND with much silt SM SM 20-50 SAND with much silt: sub-divide like SC
SF
SAND with much clay SC SCL SAND with much clay of low/intermediate/
SCI high/very high plasticity
SCH
SCV