Unit 8 Expansive Soils

8/13/2019 Unit 8 Expansive Soils

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EXPANSIVE SOIL

T.Sandeep GuptaAsst.prof,Dept.Of.Civil Engg

MREC


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Properties of Expansive soils

Expansive soils are soils that experiencesignificant volume change associated with

changes in water contents.

These volume changes can either in the form ofswell or in the form shrinkage and this is why

they are sometime known as swell/shrink soils Expansive soils plastic clays( Claysoil)


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Water Content in expansive Soils

Expansive soils can be found in humidenvironments where expansive problems occur

with soils of high Plasticity Index (Ip)

In general, expansive clays have liquid limitsand plasticity indices greater than about 40 and

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Occurrence in Pan India

In arid/semi arid soils where soils of evenmoderate expansiveness can cause significant

damage Soils that experience swell/shrinkproblems in the India are typically found in thesouth and east of the country, notably in theBlack Cotton Zone of India

aridwhen it is characterized by a severe lack ofavailable water


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PROBLEMS CAUSED BY THE EXPANSIVESOIL Swelling pressures can cause heaving, or

lifting, of structures whilst shrinkage can cause

Differential settlement For example, water content changes in the soil

around the edge of a building can cause swelling

pressure beneath the perimeter of the building,while the water content of the soil beneath thecentre remains constant. This results in a failureknown as end lift


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The opposite of this is centre lift, where swelling isfocused beneath the centre of the structure or where

shrinkage takes place under the edges. Damage to foundations in expansive soils commonly

results from tree growth This occurs in twoprincipal ways physical disturbance of theground and shrinkage of the ground by

removal of water. Physical disturbance ofthe ground caused by root growth is oftenseen as damage to pavements and brokenwalls


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Structural damage to house caused by endliftExample of differential settlement due toinfluence of trees


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Understanding swell behavior

The shrinkswell potential of expansive soilsis determined by its initial water content; void

ratio Clay particles are very small and their shape

is determined by the arrangement of the thin

crystal lattice layers that they form, with manyother elements which can become incorporatedinto the clay mineral structure


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In an expansive clay the molecular structureand arrangement of these clay crystal sheets has

a particular affinity to attract and hold watermolecules between the crystalline layers in astrongly bonded sandwich

Because of the electrical dipole structure of

water molecules they have an electro-chemicalattraction to the microscopic clay sheets. Themechanism by which these molecules becomeattached to each other is called adsorption


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Molecular Sandwich


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When potentially expansive soils becomesaturated, more water molecules are absorbed

between the clay sheets, causing the bulk volumeof the soil to increase, or swell. This sameprocess weakens the inter-clay bonds andcauses a reduction in the strength of thesoil


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When water is removed, by evaporation orgravitational forces, the water between the clay

sheets is released, causing the overall volumeof the soil to decrease, or shrink. As this occursfeatures such as voids or desiccation crackscan develop

Clay sized particles are considered to be lessthan 2m


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Occurrence In GroundExpansive soil problems typically occur due towater content changes in the upper few metres,

with deep seated heave being rare (Nelson andMiller 1992). The water content in these upperlayers is significantly influenced by climatic andenvironmental factors and is generally termed thezone of seasonal fluctuations or active zone

Thus it is important to determine the depth of theactive zone during a site investigation. This can varysignificant with climate conditions with depths 5 to6m


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Water content profiles in the active zone (Nelson and Miller,1992)


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As potential changes occur as a result in climate change, these effects arelikely to become

more significant. The term Active Zone can have different meanings. Nelson etal. (2001) provide

four definitions for clarity: 1. Active Zone: The zone of soil that contributes to soil expansion at any

particular time. 2. Zone of Seasonal moisture fluctuation: The zone in which water content

change due to climatic changes at the ground surface. 3. Depth of wetting: The depth to which water contents have increased due to

the introduction of water from external sources.

4. Depth of potential heave: The depth at which the overburden vertical stressequals or exceeds the swelling pressure of the soil. This is the maximumdepth of the active zone. The depth of wetting is particularly important as itis used to estimate heave by integrating the strain produced over the zone inwhich water contents change (Walsh et al., 2009).


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Laboratory Testing

An important aspect of the laboratory testingof expansive soil is to classify them according

to their degree of potential expansiveness. Themost commonly used system is to classify soilsas having either a very low, low, medium, high,or very high expansion potential. There aremany different ways to classify expansive soilsand some of the more commonly used methodsare discussed in the following subsections.


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Index Properties: Clay Content :- The clay content is defined as the percentage of

soil particles that are finer than 0.002 mm, based on dry weight. In

essence, the clay content is simply the percent clay in the soil. Thepercent clay in the soil is determined from a particle size analysis. Forexample, if the clay content in the whole soil is 35.6 percent, and thissoil would be classified as having a very high expansion potential perTable 1.

Plasticity Index :- The plasticity index (PI) is defined as the liquidlimit (LL) minus the plastic limit (PL). Per ASTM, the LL and PL

are performed on soil that is finer than the No. 40 sieve. Thus whencorrelating the PI and expansion potential, the PI to be used in Table2 should be the PI of the whole sample. The PI of the whole sampleis equal the PI from the Atterberg limits times the fraction of soilpassing the No. 40 sieve.


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Expansive Soil Classification Chart :- The propertiesof clays and their behaviour is influenced by the presence ofcertain clay minerals even in small quantities. The thickness of

the oriented water around a clay particle is dependent on typeof clay mineral. Thus, The plasticity of a clay depends upon:i) The nature of clay mineral present.ii) Amount of clay mineral present. On the basis of lab. Tests, Skempton(1953) observed that for

a given soil the plasticity index is directly proportional to the

percent of clay size fraction (i.e percent by weight finer than0.002mm in size). He introduced the concept of Activity, byrelating the plasticity to the quantity of clay-size particles,and defined the activity (Ac) as the ratio of plasticity diametersmaller than two microns present in the soil.


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Kaolinite 0.4---0.5Illite 0.51.0Montmorillonite 1.07.0


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Swell-shrink testsSwelling tests may be broadly divided into two types

are

swelling strain.Swelling strain tests may be linear i.e. one dimensional(1D) or volumetric, i.e. three dimensional (3-D).

Swelling pressure test.

Swelling pressure tests are almost always onedimensional and traditionally used oedometer typeof testing arrangements. However, shrinkage testsdeal solely with the measurement of shrinkagestrain in either 1-D or 3-D.


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Standard swelling pressure test Free-swell test


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Treatment of expansive soils Essentially treatment of expansive soils can be

grouped under two categories:

1. Soil Stabilisation removal/replacement;remould and compact; pre-wetting, andchemical/cement stabilisation.

2. Water content control methods horizontal barriers (membranes, asphalt andrigid barriers); Vertical barriers; electrochemicalsoil treatment, and heat treatment.


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The various stabilisers can be grouped intothree categories

Traditional stabilisers lime and cement By-product stabilisers cement/lime kiln

dust and fly ash

Non-traditional stabilisers e.g.sulfonated oils, potassium compounds,ammonium compounds and polymers.


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Stabilization of Expansive soil with

CNS soil layer Cohesive Non Swellinglayer (CNS layer) Recently

a new method developed on India

It has been observed that canals could beconstructed in expansive soils or can be madeof expansive soil but only a limited thickness ofaround 1m of CNS layer liningwould be needed toeliminate determental effects of expansive soils.

If roads are to be constructed on expansive soil aprovision of a layer of non expansive soil below theusual sub base/base course would be very effectivein eliminating determental effects of expansive soil.


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IIT, Bombay- CNS soil Certain studies carried out at IIT, Bombay have revealed the following about

CNS soil. (Refer Katti, 1979). Cohesion remaining constant of CNS layer, the heave of underlying

expansive soil reduced rapidly with increase in thickness of CNS soil layer,

and attained a volume of no heave with limited thickness of CNS layer,generally around 1m. Decrease in heave with increase in thickness of CNSlayer is not linear but exponential in nature.

Thickness of CNS layer needed to resist a prevalent swelling pressure ofunderlying expansive soil varies with cohesion. Normally increase in cohesionreduces thickness but the relation may not be linear.

Shear strength of the underlying expansive soil with the interface with CNS

layer and below, increase with thickness of CNS layer and attains a shearstrength value nearly equal to that of no volume change conditions, when thethickness of CNS layer reaches a value of which prevents a whole system fromupward heave. The increase in shear strength is not linear.


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Remedial optionsFew Examples of remedial measures employed for

foundations include:

Repair and replace structural elements or correctimproper design features. Underpinning. Provide structural adjustments of addition

structural support e.g. post tensioning.

Stiffen foundations. Provide drainage control. Stabilise water contents of foundation soils. Install moisture barriers to control water content

fluctuations.


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Foundation options in expansive soils

Foundation alternatives when dealing withpotentially expansive soils follow three options:

(i) Use of structural alternatives, e.g. stiffened raft(ii) Use of ground improvement techniques

(iii) A combination of (i) and (ii)


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Unit 8 Expansive Soils

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