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SHRINKABLE CLAYS IN THE UKFirm shrinkable clays, capable of supporting four-storey

buildings on shallow foundations, occur widely in the SE of

England — Figure 1. Examples are London, Gault, Weald,

Kimmeridge, Oxford, Woolwich and Reading, Lias, Barton,

and the glacial drift clays, such as the chalky glacial tills of

East Anglia, that are derived from these clays by glaciation.

Some shrinkable clays occur further north, for example,

those derived from the weathering and glaciation of

Carboniferous shales around Sunderland and north of

Shrewsbury. In the North, however, the surface clays are

generally sandy and their potential shrinkage is smaller.

Soft, alluvial clays are found in, and adjacent to, estuaries,

lakes and river courses in the Fens, the Somerset levels, the

Kent and Essex marshes alongside the Thames, and the

clays of the Firths of Forth and Clyde. They are not shown

in Figure 1. All these clays have a firm, shrunken crust

which is drier than the body of the clay beneath. The

foundation problems in these areas are not only of clay

shrinkage but also of avoiding excessive settlement due to

loading the underlying softer clay and peat.

Figure 1 gives only a general indication of the location of

overconsolidated firm shrinkable clays. More detailedinformation can be obtained from British Geological

Survey maps and accompanying memoirs.

Low-rise buildings

on shrinkable clay soils: Part 1

The need for additional guidance on building on clay soils was

established by the concern at the high incidence of subsidence

damage during the drought of 1975–76. This Digest, which

supplements Digests 63, 64 and 67 Soils and foundations, is based on

the findings of continuing BRE research into the performance of low-rise buildings founded on shrinkable clay soils.

This Digest is published in three parts. This part describes the

shrinking and swelling behaviour of clay soils and shows the general

location within the UK of the more common shrinkable clays. It also

gives guidance on identifying such clay and assessing its shrinking or

swelling potential. Part 2 discusses designs which should provide

stable foundations in the most adverse circumstances and Part 3

describes the design of bored pile and beam foundations.

Building Research Establishment

Technical enquiries to:BRE Advisory ServiceGarston, Watford, WD2 7JRTel: 01923 664664 Fax: 01923 664098

Digest 240New edition September 1993

CI/SfB (16)p1(J12)

BRE DigestConcise reviews of building technology

Fig 1 Firm shrinkable clay deposits in Britain



NATURE OF CLAY SOILS

Clays are characteristically mouldable, and smooth and

greasy to the touch. The more clay-sized particles in thesoil, relative to any silt or other coarser-grained material,

then the more pronounced are these characteristics.

In their natural state, clays can vary in consistency from

‘soft’, through ‘firm’, to ‘stiff’. The classifications are

described in BS 5930. As a rough guide, soft clay can be

moulded easily in the hand, but it is only just possible to

push a finger nail into stiff clay. The variation in

consistency is largely attributable to the depth of the sample

and geological history of the soils. At similar depths, the

stiffer clays tend to be the older soils that were consolidated

by hundreds of metres of subsequent deposition beforebeing exposed by erosion and glacial action. Clays that

have gone through this process are described as

overconsolidated and, near the surface, typically fall into

the ‘firm’ classification.

Although soft and sticky when allowed free access to

surface water, clays shrink and crack as they dry and intact

lumps can become very hard to break. When a lump of firm

clay is immersed in water it softens only slowly, without

disintegrating. If the lump disintegrates quickly, it probably

contains significant amounts of silt and other coarser-

grained materials. In the field, firm clays can be identified

by their highly fissured nature, by the high polish left by

digging tools and by extensive crazing that occurs as the

clay dries out on the sunny side of a trench.

It is as much the type of clay mineral in the soil as the

quantity that contributes to the behaviour of the clay. The

most common clay minerals are kaolinite, illite and

montmorillonite. All of them hold water attracted to their

molecular structure and therefore tend to shrink and swell

as their water content varies. There are other soils which

contain clay-sized particles, such as ‘rock flours’, but they

only hold water between particles and do not exhibit clay-

like behaviour; in particular, their dry strength is low. Soils

which contain a high proportion of clay minerals are

generally called high plasticity clays and in nature, under

similar conditions, they tend to hold more water than the

low plasticity clays which contain fewer clay minerals.

To quantify some of the characteristics of clay soils,

engineers measure two index properties (BS 1377), known

as the Liquid and Plastic Limits.

q The Liquid Limit test identifies the water content atwhich the soil starts to lose its plastic properties and

begins to ‘flow’. Originally, the test was performed by

progressively adding water to the soil until a slot, cut in

a sample placed in a spoon, closed when tapped on the

back of the hand. Nowadays the test can be performed

either using a standard mechanised spoon or by

measuring the penetration of a cone falling under

gravity. The Liquid Limit is a measure of the amount of

water ‘bound’ to soil particles. So, the greater the Liquid

Limit, the more ‘clayey’ the behaviour of the soil.

q The Plastic Limit test identifies the lower bound of

plastic behaviour by measuring the water content atwhich the soil can no longer be moulded without it

breaking up. The test is performed by repeatedly rolling

a small sample of soil on a glass plate to form a 3 mm

diameter thread. Each time the soil is rolled out the

water content decreases slightly. The Plastic Limit is

defined as the maximum water content at which it is no

longer possible to form the thread.

When most overconsolidated clays were originally

deposited, their water contents were above the Liquid Limit

but now, owing to overconsolidation, they are close to the

Plastic Limit; a typical value for London Clay is 25 to 30%.

Close to the ground surface, however, the water contentsare influenced by evaporation, transpiration and infiltration,

and very large fluctuations are possible.

At water contents between the Liquid and Plastic Limits, a

natural soil is likely to be saturated or nearly saturated with

water; so as the soil dries, the volume change is in direct

proportion to the amount of water removed — see Figure 2.

However, at water contents lower than the Plastic Limit,

progressively more air enters the soil as it dries and the

reduction in volume becomes less than the amount of water

removed. Ultimately, at very low water contents, the

volume of the sample approaches a constant value. This

minimum volume that the soil particles can occupy is

normally described by the water content that could just fill

all the voids in the sample when it is completely dry; it is

sometimes known as the Shrinkage Limit .

The difference between the Plastic and Liquid Limits is the

Plasticity Index or simply the ‘plasticity’ of the clay. As a

general rule, the greater the Plasticity Index, the greater will

be the soil’s potential to change volume. Overconsolidated

clays with relatively high Plasticity Indexes are therefore

sometimes referred to as ‘firm, shrinkable clays’.

Remember, though, that larger volume reductions can result

from the shrinkage of normally consolidated soft clays withlower plasticity because of their higher natural water

contents and greater compressibility.

240

2

CLAY

In engineering, the word clay can have three distinct meanings

and it is sometimes used ambiguously. It is important to be

clear about which use is intended.

q The most common use of the term clay is to describe a soil

which contains enough clay-sized material or clay minerals

to exhibit cohesive properties. The fraction of clay sized

material required varies, but can be as low as 15%. Unless

stated otherwise, this is the sense used in this Digest.

q The term can be used to denote the clay minerals. These are

specific, naturally occurring chemical compounds,

predominantly silicates.

q The term is often used as a particle size descriptor. Soil

particles which have a nominal diameter of less than 2 µm

are normally considered to be of clay size, but they are not

necessarily clay minerals. Some clay minerals are larger

than 2 µm and some particles, ‘rock flour’ for example, can

be finer than 2 µm but are not clay minerals.

The water content of a soil is normally defined as the ratio of

the mass of water in the soil to the mass of oven-dry soil,

expressed as a percentage.



SHRINKAGE AND SWELLING OF CLAY SOILS

At water contents above its Plastic Limit, clay tends to be

fully saturated (ie all the pores are full of water) and it can

change volume only by the removal or addition of water.

However, the water content can change only as a result of achange in the effective stresses acting on the soil. An

increase in effective stress, and hence a reduction in water

content, can be brought about in two ways:

q An increase in the imposed loading (eg raising the

ground level or the addition of foundation loads).

q Reductions in pore water pressure produced by

evaporation and/or transpiration through the roots of

vegetation: a process known as desiccation.

Similarly, a decrease in effective stress (and hence an

increase in water content) can result from a decrease in the

imposed loading or a reduction in the state of desiccation.Whether the clay is swelling or shrinking, because of its

low permeability, the volume changes occur only slowly,

often over months or even many years.

The volume change that occurs is

dependent on the magnitude of the

effective stress change. In fact, for many

soils, plotting the volume change against

the logarithm of the effective stress

produces a straight line. Provided the

maximum effective stress does not exceed

the maximum effective stress that the soil

has ever experienced (known as the

preconsolidation pressure), the soil isoften considered to behave elastically and

should therefore return to its original

volume if the effective stress change is

reversed. In normally consolidated soft

clays, which have never experienced high

effective stresses, the response during the

initial loading will be less stiff than that

during unloading; initial shrinkage will,

therefore, be only partially recoverable

and may be much greater than that

experienced by an overconsolidated clay

with similar index properties.

In practice, the additional load applied to

the soil by a low-rise building supported

on strip or trench-fill foundations is

modest, say 20 to 60 kN/m2, and the

associated settlement, particularly for firm

or stiff clays, is of the order of a few mm.

By contrast, the reductions in pore water

pressure associated with desiccation are

much larger (typically 300 kN/m2 but

potentially as high as 1400 kN/m2) and

can therefore have a far greater effect on

ground and foundation movements.Settlement due to loading happens only

once but desiccation varies seasonally,

resulting in downward movement during

dry summer months, when evaporation

and transpiration are greatest, and heave during the wetter

winter months as moisture is replenished by rainfall and a

rise in the water table.

The effects of evaporation and transpiration by grass in

firm, shrinkable clays are largely confined to the uppermost

1 or 1.5 m of soil. Where there are trees or, to a lesser

extent, hedges and large shrubs, moisture can be extractedfrom depths of 6 m or more — see Digest 298. In high

plasticity clays, which have very low permeabilities, winter

rainfall cannot fully replenish the moisture removed by

large trees during the summer, so a zone of permanently

desiccated soil develops under the tree. This zone increases

in depth and lateral extent as the tree grows. Although the

degree and extent of the desiccation in this zone varies

according to the size and species of the tree, prevailing

climate, soil type and groundwater regime, the desiccation

can be fully reversed only by removing the tree.

The volume change is dependent on the ‘volume change

potential’ of the soil. This is an index property unaffectedby the current state of desiccation. The volume change

potential is normally inferred from the index properties of

the soil.

240

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Fig 2 Clay soil shrinkage

*Typical values for clay with high volume change potential



There is a drawback to the general rule that the greater the

Plasticity Index, the greater will be the soil’s potential to

change volume. It is that soil particles with a nominal

diameter greater than 425 µm are removed by sieving

before the Liquid and Plastic Limit measurements are

made. It follows that the Plasticity Index of a glacial till

derived from a shrinkable clay will be similar to that of the

parent clay, despite the fact that, in the ground, the glacial

till may have a far higher sand and gravel content.

Intuitively, the volume change potential of the glacial tillmust be less than that of the pure clay. To take some

account of this factor, the Plasticity Index ( I p) should be

modified by multiplying it by the fraction of the sample

tested (the fraction finer than 425 µm). BS 1377 requires

that the percentage of material passing the 425 µm test

sieve (% < 425 µm) is reported for Atterberg Limits tests.

The modified Plasticity Index ( I 'p) is given by:

I 'p = I p ×

% < 425 µm

100%

A simplified classification of volume change potential,

based on the modified Plasticity Index, is given in Table 1.

This classification is different from the one which appeared

in an earlier version of this Digest; it has been changed to

avoid confusion with the more commonly used

classification given by the NHBC(1).

To show how Table 1 can be used, the volume change

potential of some examples of clays commonly found in the

UK are given in Table 2. Remember that the properties of a

particular clay can vary considerably from site to site. For

most of the overconsolidated clays in the SE of England,

modifying the Plasticity Index has little effect on theultimate volume change potential classification. Of these

examples, the glacial tills are the only soils which would be

placed in a different category if their actual Plasticity

Indexes (rather than their modified Plasticity Indexes) were

to be used for the purpose of classification.

240

4

ISBN 0 85125 609 0

© Copyright BRE 1993

Republished on CD-ROM 1997,

with permission of Building

Research Establishment Ltd,

by Construction Research

Communications Ltd,

151 Rosebery Avenue

London, EC1R 4QX

Applications to republish all or

any part of this publication should

be made to Construction

Research Communications Ltd,

PO Box 202, Watford,

Herts, WD2 7QG

Anyone wishing to use the

information given in this

publication should satisfy

themselves that it is not out of

date, for example with reference

to the Building Regulations

Technical enquiries to:

BRE Advisory Service

Garston, Watford,

WD2 7JR

Telephone 01923 664664

Facsimile 01923 664098

Table 1 Clay volume change potential

Table 2 Volume change potential of some common clays

FOUNDATION DESIGN AND PERFORMANCE IN

SHRINKABLE CLAYS

The design of foundations for low-rise buildings must take

account of the shrinkage and swelling behaviour of clays,

because the water content varies according to climatic and

vegetation conditions. In practice, two courses of action are

open:

q Estimate the potential for swelling or shrinkage and try to

avoid large changes in water content, for example by not

planting trees near the foundations.

q Accept that swelling or shrinkage will occur and take

account of it. The foundations can be designed to resistresulting ground movements, or the superstructure can be

designed to accommodate movement without damage.

Work carried out by BRE in the 1940s showed the need for a

minimum foundation depth of 0.9 m; below this, seasonal

wetting and drying, and the influence of minor vegetation,

produced no significant ground movement. This depth has

become the accepted minimum for foundations on most clay

soils. Research also identified the influence of large trees in

removing moisture from clay beneath foundations in the

growing season. This can cause progressive subsidence and

structural damage. The design of foundations suitable to

withstand movements is discussed in Part 2.

REFERENCES AND FURTHER READING

1 National House-Building Council. Building near trees.

Standards Chapter 4.2. Amersham, NHBC, 1992.

Other BRE Digests

63 Soils and foundations: Part l64 Soils and foundations: Part 2

67 Soils and foundations: Part 3

241 Low-rise buildings on shrinkable clay soils: Part 2

242 Low-rise buildings on shrinkable clay soils: Part 3

251 Assessment of damage in low-rise buildings with

particular reference to progressive foundation movement

298 The influence of trees on house foundations in clay soils

318 Site investigation for low-rise building: desk studies

322 Site investigation for low-rise building: procurement

343 Simple measuring and monitoring of movement in low-

rise buildings. Part l: cracks

344 Simple measuring and monitoring of movement in low-

rise buildings. Part 2: settlement, heave and out-of-plumb

352 Underpinning381 Site investigation for low-rise building: trial pits

383 Site investigation for low-rise building: soil description

386 Monitoring building and ground movement by precise

levelling

British Standards Institution

BS 1377:—Methods or test for soils for civil engineering purposes

Part 2:1990 Classification tests

BS 5837:1991 Guide for trees in relation to construction

BS 5930:1981 Code of practice for site investigations

BS 8004:1986 Code of practice for foundations

BS 8103:—Structural design of low-rise buildings

Part 1:1986 Code of practice for stability, site investigation,

foundations and ground floor slabs for housing

Modified Plasticity Index I 'p Volume change potential

%

>60 Very high

40–60 High

20–40 Medium

<20 Low

This classification applies only to overconsolidated clays. A normally

consolidated clay may have a considerably greater shrinkage volume

change potential than is indicated by this classification.

bre240 shrink clay soils part 1 pdf

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