PARYANYA DEV GOSWAMI BCE III ; YEAR : 2012

ROLL NO: 000910401053

JADAVPUR UNIVERSITY

SEMINAR

REPORT

DESICCATED SOIL

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DESICCATED SOIL

[SEMINAR REPORT]

UNDER THE GUIDANCE OF

PROF. R. B. SAHU

PREPARED BY

PARYANYA DEV GOSWAMI

JADAVPUR UNIVERSITY

FACULTY OF ENGINEERING AND TECHNOLOGY

DEPARTMENT OF CIVIL ENGINEERING

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CERTIFICATE OF APPROVAL

I hereby approve that this seminar paper prepared

under my supervision by PARYANYA DEV GOSWAMI

(000910401053) entitled by myself as creditable study

of an engineering subject carried and prepared in a

manner satisfactory to warrant its acceptance in

partial fulfilment of the requirement for the degree of

bachelor of Civil Engineering.

Prof. R. B. SAHU

Department of Civil Engineering

Jadavpur University

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ACKNOWLEDGEMENT

I am extremely grateful to Prof. R.B.SAHU for his kind

and valuable guidance rendered to me for the

preparation of the seminar report and for his active

encouragement and support all along the seminar

session.

I am also thankful to my classmates for

their valuable suggestions which helped me to complete

this seminar report with satisfaction.

PARYANYA DEV GOSWAMI

BCE –III

Roll NO: 000910401053

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CONTENTS :- 1. Introduction .

2. What is desiccation ?

3. Driscoll’s formula (Clay Desiccation) .

4. Causes of Desiccation .

5. Swelling behaviour of desiccated clay .

6. Some Discussions .

7. Damages by Soil Desiccation .

8. Remedial Measures .

9. Conclusion .

10. Bibliography .

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DESICCATED SOIL

INTRODUCTION :-

When a homeowner discovers cracks in his property it is a cause for

great concern . Public knowledge of subsidence is now widespread and

confirmation that what is probably your most valuable asset is

suffering subsidence damage results in considerable anxiety.

Subsidence did not always provoke such emotion. Prior to 1971 UK

household insurance policies did not include this peril . This is not to

say that buildings did not suffer from ground movement and

consequential fracturing but these seldom resulted in structural instability

and the risk of collapse.

Fractures would generally open and close seasonally and filling them in

preparation for redecoration was regarded as a normal maintenance task .

Insurers added subsidence cover to their domestic policies largely at the

behest of mortgage lenders, believing it exposed them to little additional

risk . It was not until the 1975 drought that widespread damage resulted

in the submission of a large number of claims. Understandably insurers

then became wary of offering cover to new policy holders if their

properties exhibited any indications of movement whatsoever, i.e. if they

perceived there could be a greater than normal risk.

Valuation surveyors, mindful of their exposure to professional indemnity

claims, reported all minor fractures regardless of whether they considered

them to be serious. The unintended consequence of this was that a house

with any cracks was effectively uninsurable and therefore unmarketable.

To enable such properties to be sold without suffering a diminution in

market value ( to reflect the cost of possible underpinning) insurers came

under pressure to ensure either the cause was removed ( this generally

applied to trees whose roots were encroaching beneath the affected

foundations) or the foundations were improved by underpinning.

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Policyholders would often remove their own trees but to get third party’s

tree removed to avoid underpinning , often requiring the consent of the

Local Planning Authority for protected trees or cooperation of the

Highways Department, proved very problematic.

If the third party’s tree remained and insurers agreed to meet the cost of

underpinning , adjusters were invariably required to seek recoveries from

the owners whose trees they considered responsible for the damage .

Liability was rarely conceded and recoveries were often based upon

superficial technical considerations.

This was the situation that adjusters faced in the late 1970s and early

1980s when Richard Driscoll published a paper titled “ The influence of vegetation on the swelling and shrinking of clay soils in Britain ” . This

provided an objective measure for “ significant desiccation ” and was

utilised by adjusters to demonstrate the involvement of trees.

What is Desiccation ? :-

Desiccation is the state of extreme dryness, or the process of extreme

drying.

A desiccant is a hygroscopic (attracts and holds water) substance

that induces or sustains such a state in its local vicinity in a moderately

sealed container.

Clay soil comprises clay mineral particles that are molecularly

bonded with water to form a cohesive material. London clay, by way of

example, typically contains 50% water by volume. A clay subsoil can be

considered to behave similarly to jelly. The overall volume of the subsoil

(or jelly) is largely dependent upon the amount of water that is bonded at

any given time to the clay minerals ( or gelatine ). The soil ( or jelly ) will

shrink if water is removed and then subsequently expand if water is

reabsorbed. This expansion ceases when the clay achieves its former or a

new state of equilibrium, i.e. when there is no longer a negative pore

pressure to draw additional moisture into the soil. It’s important to

appreciate that the solid particles do not shrink. Volume change totally

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depends upon variation in moisture content. For a clay subsoil the natural

( or equilibrium) moisture content depends upon its geological history.

If any water is withdrawn the clay by definition becomes

“desiccated”- that is drier than in its natural state. It follows that a

reduction in moisture content will be accompanied by clay shrinkage

and the soil does not have to attain a state of ‘significant desiccation’

before clay shrinkage commences.

Desiccation cracks

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Driscoll’s Formula (Clay Desiccation ):-

In June 1983 Richard Driscoll’s paper, “The influence of vegetation on the swelling and shrinking of clay soils in Britain” , was published in

Geotechnique , a research journal for geotechnical specialists. It offered

“a tentative means of determining the likelihood of significant desiccation

by relating in-situ moisture contents to liquid limits”.

It is important to note that the objective of this paper was to help

geotechnical engineers decide if desiccation existed and thus ascertain

whether costly foundations were required for new buildings erected on

sites from which trees were to be removed or if underpinning was

required to stabilise buildings that had already suffered some heavy

damage.

Driscoll addressed whether significant desiccation existed at the time of

sampling and thus “ it could be taken to have occurred (developed) when

the soil suction was sufficiently large that on its release a low rise

building could be lifted”.

In other words , the suction by tree roots overcomes the inherent internal

negative pore pressure within the clay and increases this suction beyond

equilibrium levels . When the tree is removed these increased suctions

draw in any available water and the clay rehydrates and swells to its

original volume.

Driscoll’s tentative assessment of desiccation compared the actual

moisture contents of high plasticity clays with their liquid and plastic

limits.

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He suggested---

i. If the moisture content exceeds 50% of its liquid limit

there is no desiccation. ii. At 50% the onset of desiccation occurs and below 40%

the soil sample attains significant desiccation. iii. Significant desiccation can also be identified if the

actual moisture content is less than the plastic limit plus 2 % , although this is a less reliable indicator with increasing unreliability for soils with higher plastic limits .

Driscoll’s formula, as it is sometimes referred to, therefore provides a

method (but not the only method) of assessing desiccation. In situ shear

vanes were available prior to 1983 and various methods of assessing soil

suctions have been developed since Driscoll’s paper was published in

Geotechnique .

Causes of desiccation :-

i. Effect of vegetation :-

One of the objectives of the Clarens investigation was to establish the

distance to which a line of trees, or a single tree, is likely to affect the

water content of the soil, and therefore potentially affect adjacent

structures built on shallow foundations. It was seen that in the case of a

row of bushes or trees, the distance is 5–6 m, regardless of the fact that

the poplar trees are more than twice the height of the ouhout bushes, but

with a similar lateral spread of their canopies.

Figure 2 shows the position

of the single tree, a corkscrew willow, relative to a house on the site, and

gives the dimensions of the tree’s leaf canopy in 2004. Below the location

diagram, three typical soil water content profiles are shown for 15 April

2004, 5 August 2004 and 30 November 2004. The corresponding vertical

sections showing the water stored in the upper 500 mm of the soil profile

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appear in Figure 1. The water content contours in Figure 2 are difficult to

interpret unambiguously, but clearly show the persistent desiccation of

the soil around the tree, even in April, the end of the wet season.

The radius to which the tree

exerts an influence on the soil water is shown most clearly in Figure 1.

This shows that the main influence of the tree’s root system extends to a

radius of 4–5 m from the tree, whereas the tree’s leaf canopy had a radius

of 3-7 m. Tree roots may have had some influence beyond a 4–5 m

radius, but considering the variation through the year of the water stored

in the soil close to the house , the influence must have been minor beyond

5 m. (The ground surface was not irrigated at all during the experiment.)

Applying the water balance equation to Figure 1, assuming that the radial

storage profiles are uniform round the tree, and considering the time

period 15 April to 30 November 2004, the calculated recharge in the area

of influence would be 34 mm, and the water usage of the tree together

with the grass growing around it would be 320 mm in a period when the

increment in rainfall was 286 m...

Figure:1 (Radial profile of water stored in the soil between tree and house)

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Figure:2 (Contours of soil water content between lone tree and house at three times in 2004 ) :-

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ii. Natural Moisture Content :-

Desiccation and the associated clay shrinkage process occurs due to

any reduction in the clay’s natural moisture content and is not

restricted nor limited to the predefined limits of Driscoll’s

‘formula’ . Shear vane results or soil suctions provide indications of

desiccation but failure to satisfy such criteria at the time of testing

does not preclude the possibility (in many cases the probability) of

clay shrinkage subsidence.

Swelling behaviour of desiccated

clay :-

i. Vertical and Volumetric Swell Potentials of As-Compacted and

Desiccated Specimens :-

The vertical swell potential of an as-compacted specimen was calculated

as ∆h/hi , where ∆h is the increase in thickness of a compacted specimen

on first wetting under a surcharge of 6.25 kPa and hi is the compacted

thickness (13.5 mm) of the specimen. The as-compacted specimen

expanded only in the vertical direction on first wetting. Consequently, the

vertical swell potential of an as-compacted specimen equalled its

volumetric swell potential. A desiccated specimen expands in both the

vertical and lateral directions on wetting. The vertical swell potential of a

desiccated specimen was calculated as ∆h/hshrunken, where ∆h is the

increase in thickness of a desiccated specimen on wetting under a

surcharge of 6.25 kPa, and hshrunken is the thickness of the desiccated

specimen prior to wetting. The volumetric swell potential of a desiccated

specimen was calculated as ∆e/(1+eshrunken), where ∆e is the increase in

void ratio of a desiccated specimen on wetting under a surcharge of 6.25

kPa and eshrunken is the void ratio of the same specimen prior to wetting.

The volumetric swell potential of a desiccated specimen differs from its

vertical swell potential as it swells both in vertical and lateral directions.

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ii. Swelling Behaviour of As-Compacted and Desiccated

Specimens :

Figure:3(Pre-inundation dry density-swell potential relations for compacted and

desiccated. specimens: Compaction water content = hygroscopic water content )

Figure: 4( Pre-inundation

dry density-swell potential relations for as-compacted and desiccated. specimens:

Compaction water content = 8% dry of OMC.)

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Figure: 5 (Pre-inundation dry density-swell potential

relations for as-compacted and desiccated specimens: Compaction water

content = 3% dry of OMC

Figure : 6( Pre-

inundation dry density-swell potential relations for as compacted and

desiccated specimens: Compaction water content = 9% wet of OMC.)

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Expansive soils that occur in the arid and semi-arid parts of the world are

subjected to periodic ground movements from seasonal moisture content

changes. These ground movements manifest as heave during wet season,

and shrinkage during dry season. Such cyclic swell-shrink movements of

the ground cause considerable damage to the structures founded on them.

Several researchers have performed laboratory investigations to

understand the cyclic swell-shrink behaviour of soils .

.Figures 3 to 6 plot the volumetric and vertical swell potentials of as-

compacted and desiccated specimens belonging to different compaction

water content series. In each figure, point A represents the vertical (or

volumetric) swell potential of an as-compacted specimen and points A!

and A!! represent the vertical and volumetric swell potentials of the same

specimen in the desiccated state .Similar explanation holds good for

points B, B!, B!!, C, C!, C!! And D, D!, D!!.

It is interesting to note from Figs. 3 to 6 that the dry densities of all

desiccated specimens vary within a narrow range of 1.6 to 1.8 Mg/m3.

The dry density of a desiccated specimen was obtained by averaging the

dry density of its fragments, thus avoiding contribution from shrinkage

cracks. The water contents of all the desiccated specimens were also

nearly constant and ranged between 2 and 3%. For any compaction water

content series, the volumetric swell potentials of the as-compacted

specimens expectedly increased with dry density. However, the

desiccated specimens of any compaction water content series showed

marked variations in their volumetric swell potentials despite possessing

near-similar dry densities and moisture contents prior to inundation.

Desiccated specimens of a given compaction water content series

exhibited different volumetric swell potentials, presumably owing to

differences in their lateral swell magnitudes. Fig. 7 plots vertical and

volumetric swell potential versus percent reduction in the cross-sectional

area for desiccated specimens belonging to various compaction water

content series.

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The percent reduction in the cross-sectional area of a desiccated

specimen is defined as the ratio, (A0-Ad)/A0 x 100, where A0 is the cross-

sectional area of an as compacted specimen and Ad is the cross-sectional

area of the same specimen in the desiccated condition.

The plots in Fig. 7 illustrate that irrespective of its compaction water

content, a desiccated specimen that has experienced a greater reduction in

the cross-sectional area exhibits a larger volumetric swell potential. The

desiccated specimens of all four-compaction water content series,

however, exhibit near-identical vertical swell potentials of 15%. The

trend of results in Fig. 7 implies that swelling in the lateral direction is

preferred to swelling in the vertical direction.

Figure:7 (Correlations between percent reduction in cross-sectional area and swell

potentials of desiccated specimens.)

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iii. Void Ratio Changes from Cyclic Wetting-Drying Effects :-

Figure 8 plots eshrinkage versus initial (as-compacted) void ratio for

expansive soil specimens belonging to different compaction water content

series. The eshrinkage values in Figure 8 represent the difference between

the void ratio of an as-compact specimen and its desiccated counterpart.

A positive eshrinkage value indicates that an as-compacted specimen

experiences a reduction in void ratio from wetting-drying effects.

Comparatively, negative eshrinkage value indicates that an as-compacted

specimen experiences an increase in void ratio from cyclic wetting-drying

effects. The plot in Fig. 8 reveals that the eshrinkage values become less

positive in magnitude with reduction in initial void ratio. Specimens

compacted to an initial void ratio of 0.64 (point A in Fig. 8) at water

contents permissible with respect to the zero air voids line do not

experience any change in void ratio at the end of four cycles of wetting

and drying under the nominal surcharge of 6.25 kPa. In addition,

volumetric swell potentials of specimens compacted to the optimum void

ratio of 0.64 (dry density = 1.64 Mg/m3) are unaffected by the cyclic

wetting-drying process as illustrated by points x in Figs. 3 and 4. The

results of Figs. 3, 4, 5, 6, 7 and 8 were collated in Fig. 9 that plots the

relationships between swell potentials of the desiccated specimens and

their eshrinkage values. It is shown, from this figure, that irrespective of the

compaction water content, specimens that experience a larger reduction in

void ratio from four cycles of wetting and drying (i.e. specimens with

more positive eshrinkage values) also exhibit a larger volumetric swell

potential in the desiccated condition. The fairly linear relationship in Fig.

8 also suggests that the volumetric swell potentials of the desiccated

specimens are independent of their compaction water contents, but are

strongly influenced by their initial void ratios. This inference is supported

by the near-linear relationship between volumetric swell potentials and

initial void ratios of desiccated specimens belonging to different

compaction water content series in Fig. 10. The near constant vertical

swell potentials in Figs. 9 and 10 suggest that the vertical swell potentials

of the desiccated specimens are independent of their compaction water

contents, as well as their initial void ratios.

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Figure: 8 ( Influence of initial void ratio on Figure: 9 (Correlations between

eshrinkage values of soil specimens ) eshrinkage values and swell potentials of

desiccated specimens )

Figure: 10 ( Correlations between initial void ratios and swell potentials of

desiccated specimens.)

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SOME DISCUSSIONS :-

i. Although , usually desiccation refers to complete removal of

moisture, e.g. desiccated coconut, when used to describe a clay

soil it means the subsoil has attained a lower moisture content

than the soil’s natural / equilibrium moisture content (

Institution of Structural Engineers , Subsidence of low rise

Buildings 2000 and BRE Digest 412, 1996 ). BRE Digest 412

states: ‘Clearly , w < 0.4wL should be used only as a rough guide and it is unwise to base an assessment of desiccation solely on this criterion , particularly if desiccation is slight.’ The evidence

should be considered in the round and Driscoll’s formula should

not be regarded as ‘The Holy Grail!’

ii. Data shows that re-saturation of the dried soil by prolonged

exposure to an atmosphere of high relative humidity causes

negligible changes in void ratio with very high apparent pre

consolidation pressure.

iii. Inundation in water , which results in considerable swelling, is

found to greatly diminish the effects of the desiccation stresses

so that the soils exhibit low apparent pre consolidation pressures.

iv. Permeability measurements and Scanning Electron Microscopy

indicates that the method of saturation adopted can have

significant influence on soil fabric, which in turn affect the

compressibility characteristics of soils.

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DAMAGES BY SOIL DESICCATION :-

i. Damage will usually be very noticeable in a domestic property

that has suffered differential foundation movement where the clay

has attained ‘significant desiccation’ but the onset of property

damage will be dependent upon the type of construction and thus

varies from property to property. Sometimes a 1 % moisture

reduction (or less) in a clay subsoil will induce sufficient

differential foundation movement to cause damage . It is not

always possible for a laboratory to identify such small

reductions and in some instances the soil may have recovered

from its earlier deficiency prior to sampling. The apparent lack of

desiccation by reference to Driscoll’s formula does not dismiss

the possibility of clay shrinkage subsidence damage.

ii. During desiccation , soils are subjected to very high

shrinkage stresses , often of the order of 1000 kPa . However ,

studies on re-saturated desiccated soils do not always show

evidence of these soils being subjected to very high stresses. This

is an important issue and the reason can be attributed to the type of

saturation , either by raising the relative humidity or by inundation,

which has been found to influence the volume change behaviour of

these soils .

iii. Subsidence as it affects buildings is usually the result of soil

desiccation or drying out. This is normally only a problem where the

soil type is shrinkable clay.

Serious subsidence ‘event years’ as they

are known arise when there is a shortage of ground water and a

subsequent permanent decrease in soil moisture which is not

replaced. Settlement of the soil leads to movement and occasionally

damage to structures which are subjected to the stress of differential

movement under their foundations.

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Subsidence Crack

Various elements contribute to loss of soil moisture, one of which is

vegetation. Trees have a variable demand for water depending on

their species. The National House Builders Council has published

data dividing some common species into categories based upon their

water demand. Oak, poplar, willow, eucalyptus, elm and Leyland

cypress are all classified as having high water demand, but the

majority of other species have moderate demand.

iv. Desiccation is a process rather than a state with only empirically

defined parameters as to when it becomes significant. It is over

simplistic to suggest a clay with a moisture content of 39% of its

liquid limit has the ability to cause damage whereas moisture losses

between 50% and 40% have no effect. If a reduction in moisture

content results in a degree of clay shrinkage, as confirmed by BRE

Digest 240, then any moisture loss has the potential to cause

subsidence damage. At its simplest, subsidence damage occurs when

a building is “bent” by differential foundation movement and the

stresses generated during that process exceed the building’s ability to

withstand them. Masonry is very good in compression but cracks

under tension. Provided the crack damage indicates subsidence

damage.

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Remedial MEASURES :-

i. If you think that your property is suffering from subsidence,

you should first discuss this with your household insurer,

who may be able to give or provide specialist advice.

ii. Remote boreholes may provide some assistance in

identifying the pre-damage moisture content but in practice

these are often of limited value due to variations in plasticity

indices across the site and the moisture deficiency being too

small to identify by comparison. In addition, it is often

difficult to site a sufficiently close remote borehole that is

completely unaffected by vegetation.

iii. Key factors in identifying clay shrinkage subsidence damage

are distortion, the associated pattern of damage in the

building and the season in which this occurred. Such factors

should be considered in conjunction with the subsoil

(shrinkable or not), roots (present or not) and any

information provided by monitoring. Significant closing of

fractures and increases in level throughout the winter are

uniquely peculiar to clay re-hydration following earlier

desiccation but the absence of such movement ‘after the

event’ does not dismiss the possibility of clay shrinkage

subsidence.

iv. If trees which are within a Conservation Area or are

protected by a Tree Preservation Order are felt to be

implicated in a subsidence incident, an application to fell or

prune the trees is still necessary. Clearly, there must be both

evidence of subsidence and a demonstrable link with the

tree(s) before consent will be granted.

The following information must accompany any application for tree

work :

• Species of tree;

• Its approximate age range;

• Condition survey and past pruning record;

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• Tree root survey;

• Dimensions (height, crown spread, diameter);

• Distance to property and the section damaged if this is

further away;

• Recommendations for management.

In addition, the following technical information is required in order

to demonstrate a causal link with the tree:

• Age of property;

• Depth of foundation;

• Extension history;

• Drainage construction and condition;

• Spatial arrangement and amplitude of damage, i.e. the

location of tree in relation to property and magnitude of

damage;

• Monitoring results to determine movement cycles and

patterns in relation to tree growth and time of year.

In relation to the last two points, monitoring should be undertaken to

establish movement patterns sufficient to link it to the tree.

v. Gromko (1974) stated that the heave of expansive soils can

be effectively controlled by the compaction control method,

wherein the expansive soils are compacted at moisture

contents in excess of their Proctor Optimum Water Content.

Day (1994) had, however, shown that the beneficial effects

of compaction control and aging in reducing the swell

potentials of expansive soils are lost on repeated wetting and

drying of the compacted specimens. The results of this study

indicated the existence of an optimum void ratio for the

examined expansive soil. Volumetric swell potential of this

expansive soil is expected to show the same long-term

potential as at the beginning, if compacted at the optimum

void ratio. This optimum void ratio is independent of the

compaction water content but is expected to be dependent

on:---

(1) the surcharge pressure at which the unsaturated soil

is wetted and dried.

(2) the amount of shrinkage permitted during each

drying cycle.

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Hence, if an earth structure were to be compacted at the

optimum void ratio determined by the field wetting-drying

pattern and the overburden pressure, the benefits of

compaction control on the volumetric swell potential of the

compacted earth structure can be sustained despite the

detrimental influence of wetting-drying.

Conclusion :-

The following conclusions emerged from this study:

i. The volumetric swell potentials of the desiccated soil specimens

were independent of their compaction water contents but were

strongly influenced by their initial void ratios.

ii. The vertical swell potentials of the desiccated specimens were

independent of their compaction water contents, as well as their

initial void ratios.

iii. A loosely compacted specimen experienced a larger reduction in

void ratio from four cycles of wetting and drying and exhibited a

higher volumetric swell potential in the desiccated state.

iv. The compacted expansive soil examined in this study exhibited an

optimum void ratio. Specimens compacted to this void ratio do not

experience any change in void ratio at the end of four cycles of

wetting and drying. In addition, volumetric swell potentials of

specimens compacted to the optimum void ratio are unaffected by

the cyclic wetting-drying process.

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BIBLIOGRAPHY :- 1. Desiccation of soil by vegetation and potential interaction with

buildings– a field study.- by G. E. Blight(JOURNAL OF THE SOUTH

AFRICAN INSTITUTION OF CIVIL ENGINEERING ,Vol -51, No 2,

2009, Pages 20–29, Paper 709).

2. LABORATORY AND FIELD DETERMINATION OF

PRECONSOLIDATION PRESSURES AT GLOUCESTER –

by S. LEROUEILL , SAMSONA , N. D. M. BOZOZUK.

3. Prediction of Consolidation Properties of Partially Saturated Clays –

by M. D Sarma (Materials and Research Division, Roads

Department, Ministry of Works and Transport, Republic of

Botswana,Formerly, Senior Research Fellow, Council of Scientific

and Industrial Research, India.) & D. Sarma (Bergstan Africa,

Gaborone, Republic of Botswana.)

4. Swelling Behavior of a Desiccated Clay - by K. S. Subba Rao,

Sudhakar M. Rao and S. Gangadhar.

5. TREES and SUBSIDENCE –COURESY (Countryside Advisory

Note .)

6. Structural Development in Surficial Heavy Clay Soils : A Synthesis

of Mechanics – by J . K. Kodikara , S.L.Barbour ,

D.G.Fredlund .

7. Clay Desiccation Demystified -COURTESY (CILA).

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