bre 240 shrinkable clay (1)

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SHRINKABLE CLAYS IN THE UK Firm shrinkable clays, capable of supporting four-storeybuildings on shallow foundations, occur widely in the SE ofEngland — 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 ofEast Anglia, that are derived from these clays by glaciation.

Some shrinkable clays occur further north, for example,those derived from the weathering and glaciation ofCarboniferous shales around Sunderland and north ofShrewsbury. In the North, however, the surface clays aregenerally 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, theKent and Essex marshes alongside the Thames, and theclays of the Firths of Forth and Clyde. They are not shownin Figure 1. All these clays have a firm, shrunken crustwhich is drier than the body of the clay beneath. Thefoundation problems in these areas are not only of clayshrinkage but also of avoiding excessive settlement due toloading the underlying softer clay and peat.

Figure 1 gives only a general indication of the location ofoverconsolidated firm shrinkable clays. More detailedinformation can be obtained from British GeologicalSurvey maps and accompanying memoirs.

Low-rise buildingson shrinkable clay soils: Part 1

The need for additional guidance on building on clay soils wasestablished by the concern at the high incidence of subsidencedamage during the drought of 1975–76. This Digest, whichsupplements Digests 63, 64 and 67 Soils and foundations, is based onthe 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 theshrinking and swelling behaviour of clay soils and shows the generallocation within the UK of the more common shrinkable clays. It alsogives guidance on identifying such clay and assessing its shrinking orswelling potential. Part 2 discusses designs which should providestable foundations in the most adverse circumstances and Part 3describes 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)

BBRREE DDiiggeessttConcise reviews of building technology

Fig 1 Firm shrinkable clay deposits in Britain


NATURE OF CLAY SOILS Clays are characteristically mouldable, and smooth andgreasy 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 aredescribed in BS 5930. As a rough guide, soft clay can bemoulded easily in the hand, but it is only just possible topush a finger nail into stiff clay. The variation inconsistency is largely attributable to the depth of the sampleand geological history of the soils. At similar depths, thestiffer clays tend to be the older soils that were consolidatedby hundreds of metres of subsequent deposition beforebeing exposed by erosion and glacial action. Clays thathave gone through this process are described asoverconsolidated and, near the surface, typically fall intothe ‘firm’ classification.

Although soft and sticky when allowed free access tosurface water, clays shrink and crack as they dry and intactlumps can become very hard to break. When a lump of firmclay is immersed in water it softens only slowly, withoutdisintegrating. If the lump disintegrates quickly, it probablycontains significant amounts of silt and other coarser-grained materials. In the field, firm clays can be identifiedby their highly fissured nature, by the high polish left bydigging tools and by extensive crazing that occurs as theclay dries out on the sunny side of a trench.

It is as much the type of clay mineral in the soil as thequantity that contributes to the behaviour of the clay. Themost common clay minerals are kaolinite, illite andmontmorillonite. All of them hold water attracted to theirmolecular structure and therefore tend to shrink and swellas their water content varies. There are other soils whichcontain clay-sized particles, such as ‘rock flours’, but theyonly 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 aregenerally called high plasticity clays and in nature, undersimilar conditions, they tend to hold more water than thelow plasticity clays which contain fewer clay minerals.

To quantify some of the characteristics of clay soils,engineers measure two index properties (BS 1377), knownas the Liquid and Plastic Limits.

● The Liquid Limit test identifies the water content atwhich the soil starts to lose its plastic properties andbegins to ‘flow’. Originally, the test was performed byprogressively adding water to the soil until a slot, cut ina sample placed in a spoon, closed when tapped on theback of the hand. Nowadays the test can be performedeither using a standard mechanised spoon or bymeasuring the penetration of a cone falling undergravity. The Liquid Limit is a measure of the amount ofwater ‘bound’ to soil particles. So, the greater the LiquidLimit, the more ‘clayey’ the behaviour of the soil.

● The Plastic Limit test identifies the lower bound ofplastic behaviour by measuring the water content atwhich the soil can no longer be moulded without itbreaking up. The test is performed by repeatedly rollinga small sample of soil on a glass plate to form a 3 mmdiameter thread. Each time the soil is rolled out thewater content decreases slightly. The Plastic Limit isdefined as the maximum water content at which it is nolonger possible to form the thread.

When most overconsolidated clays were originallydeposited, their water contents were above the Liquid Limitbut now, owing to overconsolidation, they are close to thePlastic 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, anatural soil is likely to be saturated or nearly saturated withwater; so as the soil dries, the volume change is in directproportion 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 thereduction in volume becomes less than the amount of waterremoved. Ultimately, at very low water contents, thevolume of the sample approaches a constant value. Thisminimum volume that the soil particles can occupy isnormally described by the water content that could just fillall the voids in the sample when it is completely dry; it issometimes known as the Shrinkage Limit.

The difference between the Plastic and Liquid Limits is thePlasticity Index or simply the ‘plasticity’ of the clay. As ageneral rule, the greater the Plasticity Index, the greater willbe the soil’s potential to change volume. Overconsolidatedclays with relatively high Plasticity Indexes are thereforesometimes referred to as ‘firm, shrinkable clays’.Remember, though, that larger volume reductions can resultfrom the shrinkage of normally consolidated soft clays withlower plasticity because of their higher natural watercontents and greater compressibility.

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CLAYIn engineering, the word clay can have three distinct meaningsand it is sometimes used ambiguously. It is important to beclear about which use is intended.

● The most common use of the term clay is to describe a soilwhich contains enough clay-sized material or clay mineralsto exhibit cohesive properties. The fraction of clay sizedmaterial required varies, but can be as low as 15%. Unlessstated otherwise, this is the sense used in this Digest.

● The term can be used to denote the clay minerals. These arespecific, naturally occurring chemical compounds,predominantly silicates.

● The term is often used as a particle size descriptor. Soilparticles which have a nominal diameter of less than 2 µmare normally considered to be of clay size, but they are notnecessarily clay minerals. Some clay minerals are largerthan 2 µm and some particles, ‘rock flour’ for example, canbe finer than 2 µm but are not clay minerals.

The water content of a soil is normally defined as the ratio ofthe 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 befully saturated (ie all the pores are full of water) and it canchange 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. Anincrease in effective stress, and hence a reduction in watercontent, can be brought about in two ways:

● An increase in the imposed loading (eg raising theground level or the addition of foundation loads).

● Reductions in pore water pressure produced byevaporation and/or transpiration through the roots ofvegetation: a process known as desiccation.

Similarly, a decrease in effective stress (and hence anincrease in water content) can result from a decrease in theimposed loading or a reduction in the state of desiccation.Whether the clay is swelling or shrinking, because of itslow permeability, the volume changes occur only slowly,often over months or even many years.

The volume change that occurs isdependent on the magnitude of theeffective stress change. In fact, for manysoils, plotting the volume change againstthe logarithm of the effective stressproduces a straight line. Provided themaximum effective stress does not exceedthe maximum effective stress that the soilhas ever experienced (known as thepreconsolidation pressure), the soil isoften considered to behave elastically andshould therefore return to its originalvolume if the effective stress change isreversed. In normally consolidated softclays, which have never experienced higheffective stresses, the response during theinitial loading will be less stiff than thatduring unloading; initial shrinkage will,therefore, be only partially recoverableand may be much greater than thatexperienced by an overconsolidated claywith similar index properties.

In practice, the additional load applied tothe soil by a low-rise building supportedon strip or trench-fill foundations ismodest, say 20 to 60 kN/m2, and theassociated settlement, particularly for firmor stiff clays, is of the order of a few mm.By contrast, the reductions in pore waterpressure associated with desiccation aremuch larger (typically 300 kN/m2 butpotentially as high as 1400 kN/m2) andcan therefore have a far greater effect onground and foundation movements.Settlement due to loading happens onlyonce but desiccation varies seasonally,resulting in downward movement duringdry summer months, when evaporation

and transpiration are greatest, and heave during the wetterwinter months as moisture is replenished by rainfall and arise in the water table.

The effects of evaporation and transpiration by grass infirm, shrinkable clays are largely confined to the uppermost1 or 1.5 m of soil. Where there are trees or, to a lesserextent, hedges and large shrubs, moisture can be extractedfrom depths of 6 m or more — see Digest 298. In highplasticity clays, which have very low permeabilities, winterrainfall cannot fully replenish the moisture removed bylarge trees during the summer, so a zone of permanentlydesiccated soil develops under the tree. This zone increasesin depth and lateral extent as the tree grows. Although thedegree and extent of the desiccation in this zone variesaccording to the size and species of the tree, prevailingclimate, soil type and groundwater regime, the desiccationcan be fully reversed only by removing the tree.

The volume change is dependent on the ‘volume changepotential’ of the soil. This is an index property unaffectedby the current state of desiccation. The volume changepotential is normally inferred from the index properties ofthe soil.

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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 thePlasticity Index, the greater will be the soil’s potential tochange volume. It is that soil particles with a nominaldiameter greater than 425 µm are removed by sievingbefore the Liquid and Plastic Limit measurements aremade. It follows that the Plasticity Index of a glacial tillderived from a shrinkable clay will be similar to that of theparent clay, despite the fact that, in the ground, the glacialtill 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 someaccount of this factor, the Plasticity Index (Ip) should bemodified by multiplying it by the fraction of the sampletested (the fraction finer than 425 µm). BS 1377 requiresthat the percentage of material passing the 425 µm testsieve (% < 425 µm) is reported for Atterberg Limits tests.The modified Plasticity Index (I'p) is given by:

I'p = Ip ×% < 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 toavoid confusion with the more commonly usedclassification given by the NHBC(1).

To show how Table 1 can be used, the volume changepotential of some examples of clays commonly found in theUK are given in Table 2. Remember that the properties of aparticular clay can vary considerably from site to site. Formost of the overconsolidated clays in the SE of England,modifying the Plasticity Index has little effect on theultimate volume change potential classification. Of theseexamples, the glacial tills are the only soils which would beplaced in a different category if their actual PlasticityIndexes (rather than their modified Plasticity Indexes) wereto be used for the purpose of classification.

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ISBN 0 85125 609 0© Copyright BRE 1993Republished on CD-ROM 1997, with permission of BuildingResearch Establishment Ltd,

by Construction ResearchCommunications Ltd, 151 Rosebery AvenueLondon, EC1R 4QX

Applications to republish all orany part of this publication shouldbe made to ConstructionResearch Communications Ltd, PO Box 202, Watford, Herts, WD2 7QG

Anyone wishing to use theinformation given in thispublication should satisfythemselves that it is not out ofdate, for example with referenceto the Building Regulations

Technical enquiries to: BRE Advisory ServiceGarston, 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 INSHRINKABLE CLAYSThe design of foundations for low-rise buildings must takeaccount of the shrinkage and swelling behaviour of clays,because the water content varies according to climatic andvegetation conditions. In practice, two courses of action areopen:

● Estimate the potential for swelling or shrinkage and try toavoid large changes in water content, for example by notplanting trees near the foundations.

● Accept that swelling or shrinkage will occur and takeaccount of it. The foundations can be designed to resistresulting ground movements, or the superstructure can bedesigned to accommodate movement without damage.

Work carried out by BRE in the 1940s showed the need for aminimum foundation depth of 0.9 m; below this, seasonalwetting and drying, and the influence of minor vegetation,produced no significant ground movement. This depth hasbecome the accepted minimum for foundations on most claysoils. Research also identified the influence of large trees inremoving moisture from clay beneath foundations in thegrowing season. This can cause progressive subsidence andstructural damage. The design of foundations suitable towithstand movements is discussed in Part 2.

REFERENCES AND FURTHER READING1 National House-Building Council. Building near trees.

Standards Chapter 4.2. Amersham, NHBC, 1992.Other BRE Digests63 Soils and foundations: Part l64 Soils and foundations: Part 267 Soils and foundations: Part 3241 Low-rise buildings on shrinkable clay soils: Part 2242 Low-rise buildings on shrinkable clay soils: Part 3251 Assessment of damage in low-rise buildings with

particular reference to progressive foundation movement 298 The influence of trees on house foundations in clay soils318 Site investigation for low-rise building: desk studies322 Site investigation for low-rise building: procurement343 Simple measuring and monitoring of movement in low-

rise buildings. Part l: cracks344 Simple measuring and monitoring of movement in low-

rise buildings. Part 2: settlement, heave and out-of-plumb352 Underpinning381 Site investigation for low-rise building: trial pits383 Site investigation for low-rise building: soil description386 Monitoring building and ground movement by precise

levellingBritish Standards InstitutionBS 1377:—Methods or test for soils for civil engineering purposes

Part 2:1990 Classification testsBS 5837:1991 Guide for trees in relation to constructionBS 5930:1981 Code of practice for site investigationsBS 8004:1986 Code of practice for foundationsBS 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 high40–60 High20–40 Medium<20 Low

This classification applies only to overconsolidated clays. A normallyconsolidated clay may have a considerably greater shrinkage volumechange potential than is indicated by this classification.


bre 240 shrinkable clay (1)

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