Proceedings of International Conference on Problematic Soils, 25-27 May 2005,

Eastern Mediterranean University, Famagusta, N. Cyprus

1

Suction Controlled Wetting-Drying Cycles on a

Compacted Scaly Clay

C. Airò Farulla1, C. Jommi

2

1Dept. of Structural and Geotechnical Engineering, Università degli Studi di Palermo, Italy2Dept. of Structural Engineering, Politecnico di Milano, Italy

Abstract

Due to their structural features and natural water content, scaly clays are being used in Sicily

in the construction of earth dams and waste isolation fills. A reliable design of the earth

constructions requires the characterisation of the behaviour of the compacted scaly clay in

unsaturated conditions. Experimental data from oedometer tests on compacted samples, in

suction controlled wetting-drying cycles are presented. The compacted specimens swell

moderately upon wetting, but they experience a relevant irreversible volume reduction upon

the first drying. The overall behaviour appears to result from the structural arrangement of the

compacted material. The interpretation of the oedometer data is supported by a microstructure

study, performed by both SEM and MIP investigations. The study provides a first insight in

the role of the natural microstructure features of the clay on its overall response.

Keywords: scaly clays, compacted soils, cyclic drying-wetting, suction controlled oedometer,

microstructure.

1 Introduction

Tectonised clays are over-consolidated and highly fissured, due to their complex geological

history. A thick network of discontinuities subdivides the clays in tightly interlocked hard

clayey fragments or scales. The size of the scales ranges most frequently from 1 to 20 mm.

The natural water content of the clays varies, usually, from 0.10 to 0.20 down to a notable

depth from the ground surface, and in their upper part they are frequently unsaturated (Airò

Farulla and Valore, 1993).

Independently from their geological origin and age, and from their physical and mineralogical

properties, the mechanical behaviour of the different natural formations is similar, as it is

governed mainly by the structural pattern. Hence, tectonised clays usually share the same

difficulties in being characterised for the analysis of geotechnical problems. Extensive

experimental studies have already demonstrated that the structural arrangement is responsible

of a significant variability of the geotechnical characteristics of the scaly clays determined

from standard laboratory investigations, as for both the classification parameters and the

hydraulic and mechanical properties (Airò Farulla and Nocilla, 1984).

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Nevertheless, owing to the same structural features and natural water content, these clays may

be placed and compacted as excavated, to obtain a material with low permeability and

appreciable shear strength and stiffness, suitable as core material in earth dams and as barrier

in waste isolation fills. For this reason, in the early 60’s these scaly clays were adopted in the

construction of the cores of the Scanzano and Rossella dams near to Palermo. The soil for the

construction was retrieved from borrow pits close to the dams, and the foundation soil itself

belongs to the same scaly clay formation (Airò Farulla and Valore, 1993).

Owing to the widespread extension of these formations in the south of Italy and in Sicily, and

due to their technical interest as construction materials, extensive field and laboratory

geotechnical investigations have been carried out at the Università di Palermo in the last forty

years, to characterise the mechanical behaviour of the scaly clays.

Previous investigations had been carried out on samples compacted in the laboratory or

collected from the core dams, but saturated before testing (Airò Farulla and Valore, 1993). A

more reliable determination of the material properties would require taking into account that

unsaturated conditions prevail in the service life of the earth constructions. In fact, the soil is

compacted in unsaturated conditions and, after compaction, it will experience a number of

drying and wetting cycles, due to the impounding and draw-down operations, and due to the

climatic changes.

For this reasons, an experimental study on the hydro-mechanical behaviour of the compacted

scaly clays in unsaturated conditions was initiated recently at the Università di Palermo,

which laboratory is now equipped with two suction-controlled oedometers and a suction

controlled triaxial apparatus (Di Mariano et al., 2000; Airò Farulla, 2004).

In this paper the attention is focused on the evolution of the behaviour in wetting-drying

cycles, performed at constant vertical stress in a suction controlled oedometer, which simulate

the wetting-drying cycles that the compacted soil will experience during the earth

constructions service life. The oedometer investigation is supported by an analysis of the

microstructure of the as-compacted material, in order to provide an insight in the possible

microstructure mechanisms governing the overall hydro-mechanical response. Both scanning

electron microscopy (SEM) and mercury intrusion porosimetry (MIP) were adopted for the

microstructure investigation. A first interpretation is proposed on the microstructure

mechanisms, which tend to govern the overall hydro-mechanical behaviour in the wetting and

drying cycles.

2 Testing Equipment and Experimental Programme

The soil used in the investigation was prepared from the same scaly clay used in the

construction of the Scanzano and Rossella dams. The material is a kaolinitic-illitic clay with a

small quantity of smectite. The liquid limit is wl = 0.58, and the plasticity index Ip = 30. The

specific gravity is Gs = 2.78.

The air-dried material, with a hygroscopic water content, wh = 0.05, was ground with a rubber

pestle, and the material passing at n° 4 ASTM sieve was selected. The moulding water

content, w0, was chosen in the range 0.15 ÷ 0.17. Distilled water was sprayed over the soil,

and carefully mixed. A curing time of 24 hours was adopted for homogenisation of the water

content. The soil was dynamically compacted with modified Proctor energy to the target dry

density γd = 16.7 ÷ 17.6 kN/m3. Both the mixing water content and the target dry density were

chosen in order to reproduce the corresponding mean values of the material compacted in situ,

determined from the control measurements performed during the construction. After

C. Airò Farulla, C. Jommi

3

compaction, specimens were trimmed for the oedometer tests and for the SEM and MIP

investigation.

A suction controlled oedometer apparatus, designed at the UPC Barcelona (Di Mariano et al.,

2000; Airò Farulla, 2004) was used in the investigation. Displacements were measured with a

resolution of 1µm. The resolution of the water volume variation device was 0.02 cm3. The

tests were performed at a constant temperature of T=22° ± 1° C. Suction was imposed by

means of the axis translation technique, keeping a constant air pressure, ua = 1000 kPa, and

varying the applied water pressure, uw, according to the air overpressure technique. The

oedometer is equipped with a porous ceramic disk with an air entry value AEV = 1500 kPa.

Due to the high values of the control air pressure and of the degree of saturation of the tested

samples, the axis translation technique must be carefully applied in order to minimise the

volumetric deformation of the soil skeleton and the variation of the initial matric suction of

the specimen (Romero, 2001; Airò Farulla, 2004). The displacements measured during the

axis translation step were in the order of few µm and the variations in the degree of saturation

were negligible.

The suction of the as-compacted samples was measured by means of the filter paper

technique. The measurements showed a linear dependence of the logarithm of suction on the

moulding water content (Airò Farulla, 2004). The data gave a mean value of 800 kPa, which

was chosen as a standard reference value.

Two series of tests were programmed. In the first series, denoted by SL, the specimens,

prepared with a moulding water content of w0 = 0.17, were loaded up to the target vertical net

stress, (σv - ua) = 200 or 400 kPa, at constant water content. A suction s = ua - uw = 800 kPa

was then imposed, and a sufficient time was left for equalisation. In the second test series, LP,

the speciemens were prapered at a slightly lower water content of w0 = 0.15. The loading step

was performed nearly in undrained conditions for the water phase, but after a first equalisation

step at a suction s = 800 kPa. By reducing and increasing the water pressure in steps, one or

more wetting and drying cycles were then performed for all the specimen, at constant net

stress. In the following, the data of two specimens, SL24 and LP6, belonging to these series of

tests are presented and compared, with reference to the first wetting-drying-wetting cycle

performed at a constant vertical net stress of (σv - ua) = 200 kPa. The relevant state parameters

of the as-compacted samples are summarised in Table 1.

Table 1. Sample properties.

Sample ID w0

(-)γγd0

(kN/m3)

e0

(-)

Sr0

(-)

LP6 0.15 18.1 0.502 0.85

SL24 0.17 17.8 0.538 0.89

3 Microstructure Study

Previous studies had already highlighted that wetting induces significant changes in the

mechanical properties of the investigated tectonised clay (Airò Farulla and Valore, 1993). In

order to provide an insight in the microstructure features, which may be responsible of these

irreversible changes observed upon wetting, a combined SEM and MIP investigation was

initiated, starting from the as-compacted material.

SEM investigation was performed by progressively increasing the magnifications from 64x

up to 2000x. Due to the limited space availability, only the micrographs relative to the sample

GEOPROB 2005

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SL24 will be presented and discussed in the following. As already pointed out in previous

investigations, compaction of a de-structured natural soil does not eliminate some of the

natural features of the original soil. Hence, after compaction, the soil may present different

structure levels, which in turn are typical of natural or compacted materials (Jommi and

Sciotti, 2003). The frameworks outlined by Al-Rawas and McGown (1999) for expansive

soils and by Romero (1999) for compacted soils will be adopted to describe briefly the

qualitative microstructure features observed in the analysis of the compacted scaly clay.

Different structure levels appear from the micrographs presented in Figure 1, and different

types of assemblages are evident. Compaction of the de-structured clay does not eliminate the

scales, which still may appear as distinct units (Figure 1a). Inter-assemblage macro-pores,

with a characteristic dimension of the order of 100 µm, may be observed. Some of the largest

pores seem to have a fissure-like geometry. By increasing the magnification up to 1000x or

more, it can be observed that the scales are composed by very dense aggregates of oriented

elementary particle arrangements, which units are not distinguishable at the magnification

adopted (Figure 1b). The pores inside the scales are hardly discernible.

At the same magnification (1000x), a completely different structural arrangement may be

observed in other portions of the same sample (Figure 1c). Aggregates composed of different

particle arrangements and scale fragments, leaving some inter-aggregate pores of

characteristic dimensions of the order of 10µm, may be observed. An intra-aggregate pore

system, with characteristic dimensions of the order of 1µm or less, is clearly evident from the

micrographs in Figures 1c and 1d. The microstructure arrangement of these portions of the

sample presents typical features of clays compacted on the dry side of the optimum moisture

content.

Figure 1. SEM micrographs of SL24: (a) 64x; (b) 1000x; (c) 1000x; (d) 2000x.

A better insight in the pore structure levels can be provided by the MIP data. Mercury

intrusion porosimetry is based on the principle that mercury will not intrude the pore space of

a porous medium unless a pressure pm is provided. The pressure required for intrusion

C. Airò Farulla, C. Jommi

5

depends on the surface tension of mercury, tm, the contact angle between the fluid and the

particles, θm, and the pore size. Pores that are intruded under a given pressure pm are usually

referred to as having an equivalent entrance diameter D = - 4 tm cos θm / pm. During the test

the voids of the soil sample are intruded by gradually increasing the mercury pressure, while

the cumulative volume of intruded mercury is recorded. By assimilating the soil pore system

to a bundle of capillary tubes of different diameters an inferred pore size distribution is

provided.

The MIP data of the two samples SL24 and LP6 are shown in Figure 2. The cumulative

intruded volume is shown in Figure 2a. It is worth noting that MIP detects only a part of the

overall porosity of the as-compacted samples. The cumulative intrusion curves still show a

positive slope when the maximum allowable mercury pressure (200 MPa) is reached, showing

that the smallest pores could not be intruded. Other possible limitations in the intrusion of the

pore system are widely discussed elsewhere (see e.g. Romero, 1999), and will not be recalled

here, as they do not invalidate the following discussion. The pore size density function is

shown in Figure 2b. Four distinct classes may be identified. The first one (I), representing

almost 20% of the intruded volume, may be identified with the inter-assemblage pores,

observed in Figure 1a. The field (II) is thought to represent the pores between the aggregates

composed of scale fragments, hence the inter-aggregate pores in Figure 1c. In spite of the

difference of the moulding water content, the amount of porosity pertaining to these two

classes is almost the same for the two samples. A clear difference may be observed for the

third class (III), which may be identified with the intra-aggregate porous system. The mean

equivalent pore entrance diameter is different for the two samples, as both the cumulative

intrusion curve and the pore size density function show. For the specimen SL24, compacted at

w0 = 0.17 a mono-modal distribution with a peak at 0.025 µm is detected. For the sample

compacted at a lower initial water content of w0 = 0.15, a multi-modal distribution is observed

in the range associated to the intra-aggregates pore system (0.02 ÷ 0.7 µm). Bigger pores are

detected and more than one peak may be observed in Figure 2b. These observations are

consistent with the typical features of clayey soils compacted dry of optimum. The intra-

particle arrangements pores (IV) are intruded only partially. No significant difference could

be observed for the two samples, consistently with the idea that the elementary particle

arrangements are not modified by the methodology adopted in the preparation of the samples.

0 .0 01 0.01 0.1 1 10 1 00

E quiva lent Pore E n tran ce D ia m eter , D (µm )

0

20

40

60

80

1 00

1 20

Cu

mu

lati

ve

In

tru

de

d V

olu

me

(m

m3

/ g

)

LP 6_ a

LP 6_ b

SL 24

( I )( II )( III )( IV )

(a )

0 .0 01 0 .01 0 .1 1 10 1 00

E q u iv a le n t Po re E n tra nc e D iam et e r, D (µm )

0

0 .2

0 .4

0 .6

0 .8

1

Po

re S

ize

De

nsi

ty F

un

cti

on

, ∆

(n/n

ma

x)/

∆(l

og

D)

SL24

LP6

( II ) ( I )

( III )( IV )

(b )

Figure 2. MIP data for samples SL24 and LP6: (a) cumulative intruded volume versus

equivalent pore entrance diameter; (b) pore size density function.

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A further insight in the pore structure may be provided following fractal interpretations of the

MIP data (Korvin, 1992; De Las Cuevas, 1996; Romero, 1999; Jommi and Sciotti, 2003). The

interpretation is based on the concept that a portion of the pore space, which presented similar

geometrical characteristics irrespective of the scale of observation, might be identified with a

single fractal dimension, describing a repeatable, self-similar, geometrical structure. Hence, a

constant fractal dimension should be observed in a certain range of intrusion pressures if

pores of decreasing size but of the same geometrical type are being intruded.

According to Korvin (1992), the porous surface fractal dimension, Ds, of the pores can be

usefully adopted to classify the different possible pore structures on the basis of MIP data, as

it is related to the derivative of the cumulative intruded volume with respect to the intrusion

pressure:

( )*

*s

-dVlog D log p

dp*

4

≈ − (1)

where the non-dimensional variables V* = V(pm)/Vmax and p* = pm/pmax (where V(pm) is the

cumulative volume intruded for a given pressure pm, and Vmax is the total volume intruded for

the maximum applied pressure pmax) have been introduced.

0.00 01 0 .0 01 0.01 0.1 1A pplied P re ssure / M axim um A pplied P res sure , pm / pm ax

0.01

0.1

1

10

1 00

10 00

d(V

(pm

) /

Vm

ax

) /

d(p

m /

pm

ax

)

L P 6 _ a

L P 6 _ b

S L 2 4

Ds - 4 = - 1 .8 8

Ds - 4 = -1 .1 0

Ds - 4 = - 1 .9 0

( I ) ( II ) ( III ) ( IV )

Figure 3. MIP data for samples SL24 and LP6: fractal description of the pore structures.

The intervals of self-similarity or scale invariance of the different pore types characterising

the compacted soils can be inferred from the log-log plot presented in Figure 3. The linear

parts of the plots identify self-similar pore structures (Romero, 1999). According to De Las

Cuevas (1996), a surface fractal dimension Ds = 2 characterises a fissure-like porosity, while

Ds = 3 describes a more space-filling volumetric pore structure.

The data presented in Figure 3 show that except for the range (III), a fractal structure may be

inferred for the different pore structures already delimited on the basis of the pore size density

function. The biggest pores (I), corresponding to the inter-assemblage (inter-scales) pores,

have a fractal dimension of Ds = 2.12, tending to a fissure-like type of porosity. In the range

(II), the surface fractal dimension, Ds = 2.90, seems to describe well the increasing roughness

of the surface of the inter-aggregate pores shown in Figure 1c. The intra-aggregate pores,

C. Airò Farulla, C. Jommi

7

associated to the range (III), representing nearly half of the total detected porosity, do not

have a fractal character. The micrograph shown in Figure 1c highlights that this porosity

system is composed of pores of different dimensions and shapes, depending on the different

form of aggregations of the scale fragments. This observation seems to be consistent with the

different modal distribution observed in the pore size-density function. A fractal, fissure-like,

type of porosity is recovered for the intra-particle arrangement pores (IV), where Ds = 2.10.

4 Oedometer Data

In Figure 4 the data of the first steps of the oedometer tests performed on the two samples at

(σv - ua) = 200 kPa, are presented. In Figure 4a the stress path followed by the samples is

described in the net stress-suction plane. The data presented in the following plots refer to the

suction controlled wetting-drying-wetting cycle performed after the equalisation stage. The

data are represented in terms of the state variables void ratio, e, and water ratio,

ew = w Gs = Sr e.

1 0 10 0 1 00 0

N et S tre s s , σv - u a (k P a)

1 0

10 0

1 00 0

Su

ctio

n,

s (k

Pa

)

(a )

firs

t w

ett

ing

se

co

nd

we

ttin

g80 0 kPa

LP6

SL24

0 .3 6 0 .4 0 0 .4 4 0 .4 8 0 .5 2

W a te r R a ti o , e w

1 0

10 0

1 00 0

Su

cti

on

, s

(k

Pa

)

LP6

SL24

(c)

1 0 10 0 1 0 0 0

S u c ti o n , s ( k P a )

0 .4 4

0 .4 6

0 .4 8

0 .5 0

0 .5 2

Vo

id R

atio

, e

LP6

SL24

(b )

0 .4 0 0 .4 4 0 .4 8 0 .5 2

W a te r R a ti o , e w

0 .4 4

0 .4 6

0 .4 8

0 .5 0

0 .5 2

Vo

id R

ati

o,

e

LP 6

SL24

Sr = 0.8 S r = 0.9 Sr = 1(d )

Figure 4. Oedometer data: (a) σv-ua – s; (b) s – e; (c) ew – s; (d) ew – e.

In the first wetting path, despite the compaction had been performed on the dry side or near to

the optimum water content (Airò Farulla, 2004), the samples swell, although moderately

(Fig. 4b). It is worth noting that swelling upon first wetting has been observed for the scaly

clay in a wider range of applied vertical net-stress, (σv - ua) = 200 ÷ 800 kPa (Di Mariano et

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al., 2000; Airo’ Farulla, 2004). The samples undergo a relevant increase in the water ratio

(Fig. 4c), and they apparently reach full saturation for a suction of 10 kPa.

At the beginning of the first drying path, the response of the material is very stiff, both in

terms of void ratio and water ratio, until a suction of 200 kPa is reached. A yielding point may

be observed at this stage, where the stiffness decreases and a significant reduction in the water

ratio is observed. At the end of the first wetting-drying cycle, an appreciable net reduction of

the volume may be recorded. On the contrary, the exchanged volumes of water are mostly

reversible over the first cycle.

On the onset of the second wetting path, the samples swell again, although only a limited

portion of the accumulated volume reduction is recovered, highlighting the irreversible nature

of the volume reduction experienced during the first drying step. On the contrary, the second

wetting path follows the first wetting branch of the retention curve, until the suction attains a

value of 200 kPa. The last portion of the wetting curve is different, due to the volume

reduction previously experienced. Comparing the two wetting paths, it may be inferred that

shrinkage affected the retention characteristics only of those pores, which can exchange water

in the suction range between 10 and 200 kPa. The minimum dimension of these pores may be

deduced by means of the Laplace equation D = 4 tw / (ua - uw ), where tw is the surface tension

of water. A value of the order of 1 µm is obtained, indicating that the irrecoverable void ratio

reduction might be associated to the inter-aggregate pore system.

In Figure 4d the evolution of the state variables e and ew is represented. The slope of the

interpolating curves represents the ratio between the change in void ratio and the change in

water ratio for the same suction variation. The plot clearly shows that a suction decrease

mainly affects the water ratio along the wetting paths. The slope of the e – ew lines describing

the wetting paths of both samples is almost constant (de/dew = 0.24), highlighting that

reversible wetting mechanisms take place (Di Mariano et. al, 2002), except for the first

wetting of the sample LP6, which undergoes saturation at nearly constant void ratio.

5 Discussion

The data presented compare well with previous experimental data on similar soils (e.g.

Romero, 1999) and suggest that the overall response of the compacted scaly clay be governed

by the interplay at the micro-scale of the different structure levels resulting from compaction.

During the first wetting path, the swelling nature of the aggregates might compensate the

tendency to collapse of the inter-aggregate pore system, typical of clayey soils compacted dry

of optimum. The two effects result in an overall moderate volume increase, which is almost

inhibited for the sample compacted at the lowest water content. The first wetting path

probably induces a sort of softening of the aggregates, which experience a significant

irreversible volume decrease upon the following drying. The expansive nature of the scaly

clay seems to be due to the aggregates structure, more than to the clay minerals themselves.

Scanning electron microscopy and mercury intrusion porosimetry highlighted the multiple

microstructure levels resulting from the compaction of the natural scaly clay, and may help in

the interpretation of the oedometer test data. The pore structure geometry may be useful

investigated by means of a fractal interpretation of the MIP data. The description presented in

this work is limited to the as-compacted material, but the effects of the wetting-drying cycles

on the microstructure of the compacted soil are being investigated. The ongoing experimental

activity is now aiming at studying how further drying-wetting cycles will affect both the

compacted soil microstructure and its overall hydro-mechanical response.

C. Airò Farulla, C. Jommi

9

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