Scaringi Gianvito - PhD Thesis

Università degli Studi della Basilicata
Dottorato di Ricerca in
Rischio Sismico, Ingegneria Strutturale e Geotecnica
INFLUENCE OF PORE FLUID COMPOSITION
ON CLAY BEHAVIOUR AND CHEMO-MECHANICAL
STUDY OF A CLAYEY LANDSLIDE
Settore Scientifico-Disciplinare
ICAR/07
Coordinatrice del Dottorato
Prof.ssa Caterina Di Maio
Tutor
Prof.ssa Caterina Di Maio
Dottorando
Dott. Gianvito Scaringi
A.A. 2014/2015, Ciclo XXVIII

ACKNOWLEDGEMENTS
I would like to thank first and foremost my advisor, Prof. Caterina Di Maio. Her
advice, guidance, support and inspiration were fundamental throughout my
undergraduate and graduate studies and in each achievement of this research.
Thanks are also due to Prof. Roberto Vassallo for his precious advices, his critical
point of view and his constant support.
I would also like to thank Dr. Angela Perrone and Dr. Enzo Rizzo of the CNR-IMAA
Institute for kindly lending their testing equipment and for helping me in the
interpretation of the test results. Thanks are also due to Prof. Paolo Simonini and Prof.
Simonetta Cola of the University of Padova for the X-ray tomography on laboratory
specimens, to Prof. Salvatore Masi and Mr. Domenico Molfese for the ICP-AES
analyses of fluid samples and to Mr. Alessandro Laurita for the ESEM micrographs.
Special thanks are due to the technical staff, to the undergraduate and graduate
temporary members of the geotechnical research unit and to my doctoral colleagues,
with whom I had the pleasure to collaborate and with whom I shared a piece of my
scientific and personal growth. Last, but not least, I wish to thank my better half, my
family and my friends for their continuous support and encouragement.

SUMMARY
Abstract..................................................................................................................................1
1 Introduction....................................................................................................................2
2 Influence of pore fluid composition on clay behaviour.................................................5
2.1 State of the Art ........................................................................................................6
2.2 Experimental results relative to the Costa della Gaveta soil...............................23
2.2.1 Residual shear strength..................................................................................23
2.2.2 Observation of the shear surface....................................................................36
3 Influence of pore fluid composition on creep behaviour.............................................42
3.1 Shear creep: a brief overview of the phenomenon...............................................43
3.2 Experimental results relative to the Costa della Gaveta soil...............................49
3.2.1 Stress-controlled shear tests on the Costa della Gaveta soil .........................49
3.3 Experimental results relative to other clays .........................................................58
3.3.1 Stress-controlled shear tests on bentonite......................................................58
3.3.2 Water content and pore ion concentration at the end of the tests ..................77
3.4 Modelization of ion diffusion and strength reduction..........................................80
3.5 Discussion.............................................................................................................89
4 Pore fluid composition in clays of marine origin ........................................................91
4.1 Data from Literature..............................................................................................92
4.2 Pore fluid composition at Costa della Gaveta ...................................................103
4.3 Electrical resistivity of the system solid skeleton – pore fluid ...........................113
5 Conclusion.................................................................................................................120
References..........................................................................................................................122

1
ABSTRACT
This work reports on experimental results aimed at evaluating the influence of pore
solution composition on some aspects of clay behaviour. Besides some pure clays, the soil
of Costa della Gaveta hill (Potenza, Italy) has been analysed trying to understand the
implications of test results on the behaviour of the landslides there occurring.
Several shear tests have been carried out, both under controlled rate of displacement, to
evaluate the influence of pore fluid composition on the residual shear strength, and under
constant shear stresses, to evaluate the rheological behaviour of the soil along a slip surface
in residual condition when subjected to changes in pore fluid composition. The
composition of the pore fluid is shown to affect the residual shear strength of the tested soil
noticeably. The tests carried out under constant shear stresses showed that a pore solution
concentration decrease can produce an increase in displacement rate on a pre-existing slip
surface with a pattern typical of tertiary creep.
The natural pore fluid composition of the Costa della Gaveta soil was evaluated on a large
number of samples, both by chemical and by electrical analyses. Some preliminary
evaluations of the electrical resistivity of the system solid skeleton – pore fluid were made
as well. The natural pore fluid is shown to be a composite ion solution, in which Na+
is the
most abundant cation. Its concentration decreases noticeably from the depth towards the
ground surface, from values close to that of seawater to negligible values. The
concentration range evaluated in situ corresponds to the range in which the greatest
gradients in the residual friction angle have been evaluated.

2
1 INTRODUCTION
The composition of the pore fluid affects the mechanical behaviour of clays significantly
(e.g. Bolt, 1956; Kenney, 1967; Mesri and Olson, 1971; Mitchell et al., 1973; Sridharan
and Ventakappa Rao, 1973; Di Maio, 1996a, 1998). Several studies, in particular, showed
the great influence that the pore fluid composition exerts on the residual shear strength
(among others: Kenney, 1967; Chattopadhyay, 1972; Sridharan and Ventakappa Rao,
1979; Sridharan, 1991; Di Maio and Fenelli, 1994; Di Maio, 1996b; Anson and Hawkins,
1998).
The residual shear strength is the minimum strength that a soil can exhibit, under a definite
normal stress, after large displacements along a regular slip surface (e.g. Skempton, 1985).
Its evaluation is thus very important in engineering problems concerning slope stability and
in predicting landslide movements. Changes in the available strength due to pore pressure
variations induced by changing hydraulic boundary conditions are generally accounted for
in such problems, while the influence of pore fluid composition is often neglected,
although its effects can be dramatic.
The composition of the pore fluid of clays in nature can vary, in space and in time, due to
different natural and anthropic processes (e.g. Bjerrum, 1954; Rosenqvist, 1955; Quigley et
al., 1983; Pearson et al., 2003; Torres et al., 2011). The mechanical properties can thus
change and, consequently, affect soil stability and landslide movements, as shown, for
instance, by Gregersen (1981), Moore and Brundsen (1996), Geertsema and Torrance
(2005), Zhang et al. (2009) and Zhang et al. (2013).
This work reports on experimental results aimed at characterising the natural pore fluid
composition in a clayey slope affected by landslides, and at evaluating the influence of
pore fluid composition on the residual shear strength and on the rheological behaviour of

1. Introduction
3
the soil along the slip surface. To this aim, the case study of the Costa della Gaveta slope
(Di Maio et al., 2010, 2011, 2012, 2013), located in the Southern Italian Apennines, was
considered. Costa della Gaveta hill is formed by a marine origin clay formation, locally
known as the Varicoloured Clays. The hill is affected by several different landslides.
The homonymous Costa della Gaveta landslide, a very slow earthflow in steady state
motion (Hungr et al., 2014) involves a volume of 6 million cubic metres soil, with
displacements concentrated in a narrow shear zone in the residual condition, which reaches
a depth of about 40 m (Di Maio et al., 2010). Several aspects of the landslide behaviour
have been studied, such as: the response of pore pressures to rainfall and their effects on
landslide displacements, the time trend of displacements on the shear surface and of
deformations in the landslide body, and the possible triggering factors (Di Maio et al.
2010; Vassallo et al. 2012; Di Maio et al. 2013; Vassallo et al., 2015a). More recently, the
research has also been focused on the characterisation of the natural pore fluid composition
and on its role in the mechanical behaviour of the soil (Di Maio et al., 2015a, 2015b; Di
Maio and Scaringi, 2015).
The Varco d’Izzo landslide, located a few hundred metres East of the Costa della Gaveta
landslide, is a wider – more than 1 km large – and complex landslide system whose
movements cause severe damage to houses and infrastructures, with very different rates of
displacement from site to site (Di Maio et al., 2012). An earthflow within the landslide
system also affects a 200 m long railway tunnel. The interaction between this latter and the
landslide body is currently under study (Vassallo et al., 2015b). The area is being
monitored through several inclinometers, GPS stations and piezometers (Di Maio et al.,
2011, 2012; Calcaterra et al., 2012).
The results of laboratory tests, for the evaluation of the residual shear strength of the Costa
della Gaveta material with different pore solutions, are reported in Chapter 2. Several
direct and ring shear tests were carried out on reconstituted specimens in absence of
chemical gradients between the pore fluid and the cell fluid. Some other tests were carried
out in order to evaluate the behaviour of the soil when subjected to a decrease or to an
increase in pore fluid ion concentration. Some first observations by means of X-ray
tomography and ESEM microscopy have been performed after the shear tests to
characterise the soil along the slip surface.

1. Introduction
4
In Chapter 3 the influence of pore fluid composition on creep behaviour is studied by
means of stress-controlled tests on pre-sheared specimens of the Costa della Gaveta soil.
Tests results relative to a sodium bentonite are also reported in order to attempt a
generalisation of the results. During the course of the tests, the specimens were exposed to
distilled water in order to simulate a process of pore ion concentration decrease. Often, the
specimens were analysed after the tests in order to determine water content and ion
concentration profiles along the specimen’s height.
In Chapter 4, the results relative to the experimental evaluation of the natural pore fluid
composition of the Costa della Gaveta soil are reported. Both chemical and electrical
analyses have been carried out on the pore fluid and some first electrical resistivity
measurements were performed on many undisturbed specimens, reconstituted specimens
and slurries. In situ electrical resistivity tomographies were also carried out.

5
2 INFLUENCE OF PORE FLUID
COMPOSITION ON CLAY BEHAVIOUR
The residual shear strength is the minimum strength that a soil can exhibit under a given
normal stress. It is generally the available shear strength on the slip surface of active
landslides which have experienced large displacements along a regular slip surface
(Skempton, 1985). A reliable evaluation of the residual shear strength is thus essential in
stability analyses and for predicting landslide displacements.
The first part of this Chapter is a review of some of the main studies on the influence of
pore fluid composition on the residual shear strength. Then, the Chapter reports on the
results of a number of laboratory tests carried out in this work to investigate the influence
of pore fluid composition on the residual shear strength of the Costa della Gaveta soil. To
this aim, several direct and ring shear tests were performed.
The residual shear strength of a soil is greatly influenced by the mineralogy of its clay
components. Such influence is here analysed by comparing the results of tests carried out
on different clays.
The Chapter also reports on the results of ESEM and X-ray observations of sheared
specimens, carried out in order to observe the soil fabric in the shear zone and on the slip
surface.

2. Influence of pore fluid composition on clay behaviour
6
2.1 STATE OF THE ART
The chemical composition of the pore fluid influences several aspects of the mechanical
behaviour of clays, such as volume change, hydraulic conductivity, swelling pressure,
osmotic efficiency and shear strength.
Experimental results regarding, in particular, the residual shear strength were reported,
among others, by Kenney (1967), Ventakappa Rao (1972), Balasubramonian (1972),
Chattopadhyay (1972). In the following years, different Authors (e.g. Sridharan and
Ventakappa Rao, 1979; Moore, 1991; Di Maio and Fenelli, 1994; Di Maio, 1996a,b;
Anson and Hawkins, 1998; Tiwari et al., 2005) pointed out the influence of pore fluid
composition and ion concentration on the residual shear strength of different clays. The
Authors also gave interpretations of their results, attempting to consider them in a unique
framework which could be suitable for different clays and/or be able to explain also the
influence on other mechanical aspects comprehensively.
Sridharan and Ventakappa Rao (1979) investigated the drained shear strength of kaolinitic
and montmorillonitic clays prepared with different pore fluids (i.e. distilled water and
various organic fluids). Figure 2.1, for example, shows the results relative to specimens of
compacted kaolinite: the influence of the used fluid is evident. The Authors interpreted the
results as a function of the dielectric constant of the pore fluid and observed that both for
kaolinitic and for montmorillonitic clays the shear strength seemed to decrease when the
dielectric constant increased, as shown by Figure 2.2. Furthermore, the results were found
consistent with a modified effective stress concept accounting for electrical attractive and
repulsive interparticle forces (among others: Bolt, 1956; Lambe, 1960; Sridharan, 1968;
Sridharan and Ventakappa Rao, 1973).
In order to evaluate the influence of pore fluid composition on the residual shear strength,
Chatterji and Morgestern (1989) performed shear tests on specimens of Na-
montmorillonite prepared with a concentrated (33.6 g/l) NaCl solution and subsequently
leached with distilled water. Similarly to Sridharan and Ventakappa Rao (1979), they
interpreted the results in terms of a modified effective stress concept accounting, in

7
particular, for the repulsion force in the diffuse double layer (DDL; Gouy, 1910; Chapman,
1913). The Authors showed that, by this concept, it is possible to find a unique value of
residual friction angle which is independent of pore fluid salinity, as shown by Figure 2.3.
The Authors also reported that, for clays such as kaolinite, being the DDL repulsion forces
lower, the residual shear strength does not appear to be influenced by pore fluid
composition significantly.
Figure 2.1 Drained shear strength of statically compacted kaolinite prepared with different
fluids (Sridharan and Ventakappa Rao, 1979).

8
Figure 2.2 Shear strength normalised with respect to the normal pressure against the
dielectric constant of the used pore fluid for specimens of kaolinite (left) and
montmorillonite (right) (Sridharan and Ventakappa Rao, 1979).
Figure 2.3 Residual shear strength against true effective stress in the formulation by
Chatterji and Morgernstern (1989) for specimens of sodium montmorillonite prepared with
a concentrated NaCl solution, before and after leaching with distilled water.
Some decades earlier, the DDL concept had been used by Bolt (1956) to predict the
volume change behaviour of clays. The Author interpreted the compression behaviour of
montmorillonite and illite in salt solutions at different concentrations and provided a
relation between the void ratio, e, and the swelling pressure, p. The two quantities were
related to the specific surface of the clay, the interparticle distance, the ion concentration at
mid-plane between two particles and the ion concentration in the bulk solution.
Subsequently, Mitchell (1960) investigated the volume change behaviour of Na-kaolinite,
Na-illite and Na-montmorillonite. He concluded that the DDL theory is not applicable to

9
all clays, but only to those containing clay particles of diameter smaller than 0.2-1.0 µm. A
detailed study on the applicability of the DDL theory was also conducted by Sridharan and
Jayadeva (1982), who showed that the e – log p relation is primarily controlled by the
specific surface of the clay. Furthermore, they evaluated that the contribution of the Van
der Waals attractive forces is negligible if compared to the repulsion forces caused by the
interacting diffuse double layers in the range of pressures in engineering practice. The
DDL concept was also used by Olson and Mesri (1970) and Mesri and Olson (1971), and
proved to work satisfactorily in interpreting the consolidation curves of artificially
sedimented Na-montmorillonite, consolidated in water or in solutions of NaCl at different
concentrations (Figure 2.4). They showed the remarkable difference of void ratio against
the normal effective stress for specimens saturated with different fluids and also noticed
that the clay, when prepared with some organic fluids, exhibited much lower void ratios
and much higher hydraulic conductivities (4-6 orders of magnitude!) than when prepared
with water.
Figure 2.4 Void ratio against normal applied stress for specimens of Na-montmorillonite
saturated with NaCl solutions at different concentrations (Mesri and Olson, 1971).

10
The link between the DDL concept and the dielectric constant of the pore fluid in
explaining the mechanical behaviour of clays was shown with respect to the volume
change behaviour by Sridharan and Ventakappa Rao (1973). The Authors recognised two
mechanisms related to the clays’ microstructure, i.e. the shearing resistance at the contact
points, on which shear displacements and/or sliding between particles depend, and the
long-range electrical repulsive forces, on which the DDL behaviour depends. The former
mechanism was found to prevail in kaolinite, while the latter in montmorillonite. Chen et
al. (2000) observed that the compression index of kaolinite changes with the dielectric
constant of the organic fluids in a way similar to the Hamaker constant, on which the
attractive van der Waals forces depend and shows a minimum at D = 24. Similar results
were found by Moore and Mitchell (1974). Calvello et al. (2005) reported evidence of the
dependence of the compression index, coefficient of consolidation and hydraulic
conductivity on the pore fluid dielectric constant also for smectitic clays (Figure 2.5).
However, the relations between clay properties and dielectric constant appeared different
than those found for kaolinite, thus possibly highlighting the different mechanisms
controlling the compressibility of the two clays.
Di Maio (2004a) and Calvello et al. (2005) analysed the residual shear strength of different
smectitic soils prepared with water, salt solutions or organic fluid in terms of the dielectric
constant of the pore fluid. They found that residual strength decreases with the dielectric
constant increasing up to D = 80 (Figure 2.6). It is worth noting that a non-polar organic
fluids, such as cyclohexane, with very low dielectric constant, produced the same
behaviour as that of dry specimens.

11
Figure 2.5 Compression index, Cc, normalized with respect to that of materials
reconstituted with distilled water, against pore fluid static dielectric constant, D, for Na-
montmorillonite (Calvello et al., 2005).
Figure 2.6 Residual friction coefficient τr/σ’n against the pore fluid static dielectric
constant D for different smectitic soils (Calvello et al., 2005).

12
Furthermore, Di Maio et al. (2004) performed a large number of oedometer tests on
different natural soils containing smectite, illite and kaolinite and on some of their
mixtures. The materials were reconstituted with – and submerged in – water, salt solutions
or organic fluids. The Authors found a good agreement between the intrinsic compression
index against the void ratio at the liquid limit and the regression line found by Burland
(1990), both for soils prepared with water and for soils prepared with salt solutions (Figure
2.7). According to the Authors, this suggests that the liquid limit (which is a measure of the
soil strength under standardised conditions) can be a reference state to predict the
compression behaviour, in the range of validity of the relation, also with pore fluids
different from water.
Figure 2.7 Intrinsic compression index Cc* against void ratio eL at liquid limit. For each
materials the values of Cc* obtained with different pore solutions are reported (Di Maio et
al., 2004).
Di Maio and Fenelli (1994) published the result of direct shear tests carried out on a
sodium bentonite reconstituted with distilled water and sheared to the residual condition
while in a bath of distilled water. The specimen was subsequently exposed to a
concentrated NaCl solution. This caused a progressive and noticeable increase in the shear
strength (Figure 2.8). Subsequent re-exposure to water produced a progressive shear
strength decrease down to the value attained before exposure to the salt solution. The
effects on the residual sear strength of the exposure to NaCl solutions of sodium bentonite

13
are thus reversible. The test was repeated on a specimen of kaolin, which did not exhibit
any strength variations. Tests conducted on mixtures of bentonite and kaolin showed that
the strength variation due to the exposure to salt solution is remarkable for bentonite
contents as low as 25% in dry weight, under the investigated normal stress, meaning that
such a percentage is able to control the residual shear strength of the mixture.
Di Maio and Fenelli (1997), performing several compression tests with exposure to
different fluids on specimens of natural soils containing different clay minerals, showed
that the influence of pore fluid composition is very significant for soils containing smectite.
The Authors thus stressed the importance of using the appropriate pore fluid when
evaluating the possible mechanical behaviour in situ. In fact, if a specimen of a soil whose
natural pore fluid is a salt solution is tested in a bath of distilled water, it can exhibit a
behaviour which can differ significantly from that in situ, due to possible transient
phenomena (e.g. ion diffusion, osmotic water flow) occurring in the course of the test.
Di Maio (1996a) showed the remarkable effects of the exposure of a sodium bentonite to a
fluid different from its pore fluid and Di Maio (1996b) showed similar effects for several
natural soils containing montmorillonite. Among the results of the direct shear tests, Di
Maio (1996a) reported those relative to two specimens (see Figure 2.9), one reconstituted
with a concentrated NaCl solution and sheared to the residual condition while submerged
in the same solution (specimen 1a) and another reconstituted with water and sheared to the
residual condition while submerged in water (specimen 1b). Their residual shear strength
resulted very different: τr/σ’n ≈ 0.1 in water and τr/σ’n ≈ 0.3 in salt solution. Specimen 1b,
initially in water, was then exposed to the salt solution, showing a progressive strength
increase. Conversely, specimen 1a, initially in salt solution, was exposed to water, showing
a progressive strength decrease. At the end of the process, the specimen exposed to water
had reached the same strength as that reconstituted with – and submerged in – water, while
the specimen exposed to the salt solution had reached the same strength as that
reconstituted with – and submerged in – the salt solution. This was considered a further
confirmation of the reversibility of the effects of NaCl solutions on sodium bentonite, this
time proved also on a specimen reconstituted with the salt solution.

14
Figure 2.8 Shear trends of bentonite, sheared in water and then exposed to NaCl solution
and finally to water again (Di Maio and Fenelli, 1994).

15
τ/σa
sheardisplacements(mm)
Figure 2.9 Shear trends of bentonite specimens first mixed or exposed to saturated NaCl
solution, and then to water (Di Maio, 1996a).

16
A different behaviour was observed with the exposure of water saturated Na-bentonite to
CaCl2 and KCl solutions. Both solutions produced a progressive residual shear strength
increase, but the subsequent re-exposure to water did not cause but a negligible shear
strength decrease. Di Maio (1996a) showed that the irreversibility is exhibited also in terms
of volume changes. During the course of oedometer tests, the Author showed in fact that if
a specimen of sodium bentonite reconstituted with water is exposed to a NaCl solution, it
exhibits a volume decrease under constant Terzaghi’s effective stresses. If, afterwards, the
specimen is re-exposed to water, it undergoes a volume increase (Figure 2.10), the
magnitude of volume changes depending on Terzaghi’s effective stresses. On the contrary,
the effect of the exposure to CaCl2 solutions were non-reversible upon re-exposure to
distilled water (Figure 2.11). Similarly, irreversibility was observed after exposure to KCl
solutions. This was attributed to ion-exchange which probably transformed the Na-
montmorillonite into K-montmorillonite or Ca-montmorillonite, which are characterised by
smaller double layers. Di Maio (1998) showed the possibility of reversing the exchange
reaction by re-exposing the specimens to concentrated NaCl solutions and then to water
(Figure 2.11), but discussed that such process is unlikely to occur in nature, thus
introducing a possible long lasting chemical treatment to improve the mechanical
characteristics of the clay.
Regarding the influence of pore fluid ion concentration, Di Maio (1996a) showed that most
of variations in the residual shear strength of sodium bentonite with respect to NaCl
solutions occur in the range 0-1 mol/l, while the residual shear strength does not change
significantly for concentrations from 1 mol/l to saturation. The same trend was observed on
the liquid limit against NaCl concentration, i.e. wL decreases noticeably from water to 1
mol/l NaCl solution, while does not vary much for higher concentrations. Such dependence
of the residual shear strength on the solution concentration was confirmed by Di Maio
(2004a) on several natural soils containing montmorillonite (Figure 2.12).

17
Figure 2.10 Consolidation produced by exposure to NaCl solution and swelling caused by
exposure to water under two different normal stresses (Di Maio, 1996a).
Figure 2.11 Volume change due to mechanical consolidation and exposure to NaCl
solution, CaCl2 solution and water (Di Maio, 1998).

18
Figure 2.12 Residual shear strength against NaCl solution molarity for different clay soils
under σ’n = 200 kPa (Di Maio, 2004a).
Xu et al. (2014) have recently proposed a new definition for the effective stress which, in
particular, was used to interpret the volume change behaviour of smectitic clays. They
assumed that the clay surface has a fractal dimension, D. A modified effective stress pe was
defined, which takes into account this fractal dimension. By means of this concept, they
found a unique relation between the void ratio e and pe which is insensitive to pore fluid
composition and applied such relation to different smectitic soils. Figure 2.13 shows the
void ratio against such modified effective stress for two soils: the Bisaccia clay (data from
Calvello et al., 2005) and the Ponza bentonite (data from Di Maio et al., 2004) with water
and different NaCl solutions. The e-pe relation predicted by the model, represented by the
solid lines in the figure, seems to agree with the experimental data for concentrations up to
saturation. However, this relation does not prove satisfactory in predicting the residual
shear strength at NaCl concentrations higher than 1 mol/l, as shown by Figure 2.14. This
suggests that, at high concentrations, the shear resistance is limited by other phenomena
rather than electrostatic forces of the DDL.

19
Figure 2.13 Void ratio against modified effective stress for two smectite rich clays with
different pore fluids (Xu et al., 2014).
0 1000 2000 3000
pe (kPa)
water
0.1 M NaCl
0.6 M NaCl
saturatedNaCl
Bisaccia clay
0
20
40
60
80
100
120
0 1000 2000 3000
τr(kPa)
pe (kPa)
water
0.2 M NaCl
0.5 M NaCl
1M NaCl
saturatedNaCl
Ponza
bentonite
Figure 2.14 Residual shear strength against the modified effective stress defined by Xu et
al. (2014) for the Ponza bentonite and the Bisaccia clay (data from: Di Maio, 2004a; Di
Maio et al., 2004; Calvello et al., 2005).
Di Maio and Onorati (2000) showed that the pore fluid composition has a remarkable
influence also on the shear strength determined by means of triaxial tests. The Authors
performed CiU triaxial tests on normally consolidated (see Figure 2.15) and
overconsolidated specimens of the montmorillonitic Bisaccia clay. Important effects of
pore fluid composition were noticed, more recently, by Zhang et al. (2013) on the
undrained shear strength, by Siddiqua et al. (2014) on the stress-strain behaviour during
triaxial tests and by Gratchev and Sassa (2013) on the cyclic shear behaviour. The latter
Authors performed also some tests by using pore fluids characterised by different values of
pH.

20
0
100
200
300
400
0 200 400 600
p' (kPa)
q(kPa)
distilled
water
1 M NaCl
solution
0 200 400 600
0
σ' (kPa)
τ(kPa)
70
140
210
350
280 1 M NaCl
solution
distilled
water
420
Figure 2.15 CU triaxial tests on normally consolidated specimens of the Bisaccia clay (Di
Maio and Onorati, 2000).
As for the influence of pH on the mechanical behaviour of clays, Suarez et al. (1984)
showed the effects on hydraulic conductivity and clay structure. The effect of pH is
particularly important in practice when contaminated soils, e.g. by acid leachate, are
considered. Also Palomino and Santamarina (2005) investigated the effect of pH on clay
structure. They produced a fabric map for kaolinite as a function of pore solution
concentration and pH, highlighting the changes in particle arrangement and surface charge.
Gajo and Maines (2007) showed that acid solutions influence both the volume change
behaviour and the residual shear strength of sodium bentonite. In particular, the residual
shear strength evaluated in acid solutions is higher than that in water (Figure 2.16). The
effects of exposure to an acid solution (i.e. to H+
cation) are similar to those of other
cations different from Na+
. They do not appear reversible by re-exposing the specimens to
water, like those of calcium and potassium chloride, but can be reversed by exposing the
clay to a basic solution. The results of the shear tests, as well as those relative to
compression tests, were interpreted by the Authors with the concepts of cation exchange on
permanently charged surface sites and of acid-base reactions on variably charged sites.
According to the Authors, some aspects of the chemo-mechanical interaction of active
clays subjected to pH variations of the pore fluid can actually be roughly described without
considering the acid–base reactions, whereas the effects of exposure first to an inorganic
acid and then to bases or salts cannot be understood without taking the role of acid–base
reactions at the clay edges into account.

21
Figure 2.16 Residual shear strength as a function of normal effective stress on shear plane
raised to power of -1/3 (Gajo and Maines, 2007).
Wahid et al. (2011a,b) showed that the mechanical behaviour of kaolin is influenced by pH
much more than by pore fluid salinity. This was attributed to the major role played by the
variably charged sites, which affects edge-to-face particle interaction and can thus produce
irreversible strains. Additional examples of the influence of pH, with respect to the
compressibility of natural clays are reported, for example, by Gratchev and Towhata
(2011, 2015) for different clay formations in Japan containing different amounts of
smectite, illite, chlorite and kaolinite. Finally Zhao et al. (2011) reported that, in addition,
acid solutions could influence the residual shear strength of clays by changing the clay type
(from illite to smectite to kaolinite).
The influence of pore fluid composition on the residual shear strength has a practical
importance in slope stability, since can play a major role in the reactivation and
movements of landslides in clay soils. Furthermore, as pointed out by Di Maio et al.
(2015a) and similarly to what already suggested by Di Maio and Fenelli (1997), the
evaluation of the available residual shear strength along slip surfaces in clay soils should
be done taking into account also the natural pore fluid composition, i.e. by considering the

22
soil as a solid skeleton – pore fluid system governed by a chemo-mechanical coupling. As
a matter of fact, the Authors showed that the use of distilled water as pore fluid and cell
fluid during the tests can lead to an estimation of a value of residual shear strength which is
different from that available in situ. Furthermore, the use of a unique value of residual
friction angle in stability analyses may be misleading even in soils which are
“homogeneous”, if the pore fluid composition is not homogeneous.

23
2.2 EXPERIMENTAL RESULTS RELATIVE
TO THE COSTA DELLA GAVETA SOIL
2.2.1 Residual shear strength
The residual shear strength was evaluated in the course of displacement-controlled shear
tests by means of different apparatuses: the Casagrande and the reversal direct shear, and
the Bishop and the Bromhead ring shear. The tests were usually performed at v = 0.005
mm/min in the Casagrande, reversal and Bishop apparatuses and at v = 0.018 mm/min in
the Bromhead apparatus, which is the lowest displacement rate that the machine in use
allows.
Since the object of the study is the residual state, which is independent of initial conditions
and stress history, the specimens were prepared by hydrating the powdered, oven-dried,
material (fraction finer than 0.425 mm) at water contents generally lower than the liquid
limit relative to the material hydrated with the used fluid. This was done in order to reduce
the volume decrease due to consolidation and the consolidation time as well.
In some cases, the specimens tested in the Casagrande, reversal and Bishop devices were
cut manually, both before and during the course of the tests, to ensure the flatness of the
shear surface and to reduce the time required to achieve the residual state.
In order to investigate the effect of the pore fluid composition, two groups of tests were
conducted: 1. some specimens were reconstituted with salt solutions at different
concentration and tested in a bath of the same solution, that is, in absence of chemical
gradients; 2. some specimens, pre-sheared to the residual condition, were exposed to a
fluid different from the pore fluid by replacing the cell fluid, i.e. the tests were carried out
in presence of chemical gradients.
The tests were performed on several specimens of the Costa della Gaveta soil. The
material was extracted from different boreholes, whose locations are indicated in Figure
2.17. For comparison, some tests were conducted also on specimens of a sodium bentonite
and of a kaolin.

24
N
Potenza
Costa della Gaveta
landslide
Varco d’Izzo
landslide
Ii: inclinometer
casings
Pi, Si, TP, CP:
boreholes with
piezometers
TM, TV: boreholes
with tensiometers
Ki: boreholes
centre of ERT2
0 250 500 m
I11
S11
S9
S5
I5
S4
I4
I3
S3
I2
S2
S1
I1
S8
I8 I7
S7
I10
I9
I12
P12 S10
I6
S6
Figure 2.17 Portion of the Costa della Gaveta slope with location of the boreholes.

25
Some properties of the tested materials are reported in Table 2.1. The Costa della Gaveta
soil is characterised, in general, by high clay fraction. The clay minerals are abundant and,
among them, illite-muscovite, kaolinite and smectite were found (Summa, 2006). The
chosen bentonite, provided by Laviosa Minerals SpA, Livorno, Italy, is mainly composed
of sodium montmorillonite and exhibits characteristics very similar to those of the Ponza
bentonite, which was used in past experimentations extensively (e.g. Di Maio, 1996a;
Calvello et al., 2005) and was the reference soil for constitutive modelling (e.g. Gajo and
Loret, 2003). The used kaolin is mainly composed of kaolinite and is sold by Imerys Ltd,
UK, under the trademark Speswhite.
Material
Borehole-
Sample Depth (m)
c.f.
(%)
γγγγs
(g/cm3
)
wL
(%)
wP
(%)
IP
(%)
A
Costa della
Gaveta soil
S7-CD2 28.0 - 29.6 52 2.67 65.2 26.2 39.2 0.75
S9-MIX 23.5 – 24.8 45 - 55.9 - - -
S9-A 24.0 – 24.8 48 2.66 64.3 - - -
S9-CD18 24.8 – 25.0 46 - 51.8 - - 0.52
S9-B 25.2 - 27.2 36 2.65 53.9 - - -
I9b-CD9bis 8.3 - 8.6 35 2.58 55.6 - - -
I9b-CD12 11.5- 11.7 - - 61.0 - - -
I9b-A 11.7 - 12.4 - - 64.9 - - -
I9b-CD12 11.5 - 11.7 - - 60.9 - - -
I9c-CD18 4.00 - 4.35 33 2.67 77.8 28.6 49.2 1.49
S10-CD20 9.3 – 9.5 47 - 65.4 - - 0.52
I15-CD6 18.3 60 2.52 123 46.9 76.1 1.27
Bentonite - - 82 2.75 324 44.8 279.2 3.4
Kaolin - - 75 2.60 66.8 32.9 33.9 0.45
Table 2.1 Physical properties and Atterberg limits of the tested soils.
In order to get some preliminary information on the influence of pore fluid composition on
the behaviour of the tested soils, their liquid and plastic limits were evaluated by hydrating
the materials both with distilled water and with various salt solutions at different
concentrations. The results are shown in Figure 2.18 against the molarity of the used
solution. It can be seen that the liquid limit of the Costa della Gaveta soil does not vary
with the pore solution concentration significantly. Only one sample (I15-CD6),

26
characterised by a liquid limit in water sensibly higher than that of the others, shows to be
significantly influenced by the used fluid, probably because of a different clay mineralogy.
The liquid limits in NaCl and in KCl solutions seem consistent to one another. The liquid
limit of the tested bentonite is influenced by the used fluid noticeably. The values decrease
noticeably in the range 0-1 mol/l, independently of the used solution, while much smaller
variations are seen at higher concentrations. Only small effects of pore solution
concentration are evaluated for the tested kaolin.
0
50
100
150
200
250
300
350
0 1 2 3 4 5 6
wL(%)
solutionmolarity(M)
NaCl
KCl
CaCl2.6H2O
MgCl2.6H2O
NaCl
KCl
CaCl2⋅6H2O
MgCl2⋅6H2O
wP NaCl
Bentonite
0
50
100
150
200
250
300
350
0 1 2 3 4 5 6
wL(%)
solutionmolarity(M)
S9-A
I9b-A
I9b-CD12
I15-CD6
Costa della Gaveta
NaCl solutions
0 1 2 3 4 5 6
solutionmolarity(M)
S9-A
I9b-A
I9b-CD9bis
I9c-CD18
S7-CD2
Costa della Gaveta
KCl solutions
0 1 2 3 4 5 6
solutionmolarity(M)
NaCl
KCl
wP
Kaolin
Figure 2.18 Liquid limit, wL, of the tested materials in water and salt solutions at different
concentrations. Some determinations of the plastic limit, wP are indicated as well.
While the influence of pore solution concentration on the liquid limit seems small, the
influence on the residual shear strength is noticeable. Figure 2.19, for instance, shows the
results of shear tests carried out, in the Bromhead apparatus, on the same material prepared
with water, with 0.2 M NaCl solution and with 2 M NaCl solution. The residual friction
coefficient τr/σ’n of the material varies between less than 0.2 in water and about 0.3 in the

27
concentrated salt solution, which corresponds to a variation in the residual friction angle
ϕ’r from about 10° to about 16°. The use of a relatively less concentrated solution (0.2 M
NaCl) produces a strength increase, with respect to the strength obtained in water, which is
already significant.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 20 40 60 80
τ/σ'n
horizontaldisplacement (mm)
Costa della Gaveta (S9, 24-25 m, σ'n = 150-200 kPa)
Bromhead apparatus (v = 0.018 mm/min)
2 M NaCl
0.2 M NaCl
water
Figure 2.19 Friction coefficient against the horizontal displacement for specimens of the
Costa della Gaveta soil tested in water and in NaCl solutions.
Several other shear tests were carried out on many specimens of the Costa della Gaveta
soil. Figure 2.20 shows the results comprehensively in terms of the residual shear strength
against the normal applied stress. In particular, Figure 2.20a refers to specimens
reconstituted with distilled water and tested in a bath of distilled water as well. The results
are compared to those previously obtained on other specimens of the Costa della Gaveta
soil (Di Maio et al., 2010, 2013). The results, which seem consistent to one another, lie
between two lines through the origin corresponding to ϕ’r = 8° and ϕ’r = 10°. The effect of
the testing apparatus seems negligible, as well as that of the normal stress for σ’n > 100
kPa.
In Figure 2.20b the results relative to specimens reconstituted with – and submerged in –
solutions of NaCl at different concentrations (tests without chemical gradients) are added
to those shown in Figure 2.20a. It can be seen that the residual shear strength of all
specimens in salt solutions is significantly higher than that of specimens in water. The

28
experimental points can be interpreted in terms of residual friction angles ranging between
13° and 20°, that is up to twice those evaluated in water.
other specimens
S9-A (Bishop)
S9-A (Bromhead)
S9-B (Bromhead)
S9-B (Casagrande)
S9-MIX (Bromhead)
S9-MIX (Casagrande)
S7-CD2 (Casagrande)
0
20
40
60
80
100
0 100 200 300 400 500
τr(kPa)
σ'n (kPa)
Tests in distilled water
ϕ'r = 8°
ϕ'r = 10°
0
20
40
60
80
100
0 100 200 300 400 500
τr(kPa)
σ'n (kPa)
ϕ'r = 8.5°
ϕ'r = 13°
ϕ'r = 20°Tests in salt
solutions
a)
b)tests in water
0.2 M NaCl
0.5 M NaCl
1 M NaCl
2 M NaCl
5 M NaCl
Saturated Solution NaCl
Figure 2.20 Residual shear strength against normal effective stress of specimens of Costa
della Gaveta soil: a) specimens tested in distilled water; the results are compared to those
of other specimens from different samples (data from Di Maio et al., 2010; 2013); b) tests
in NaCl solutions at various concentrations, compared to those obtained in water.
In Figure 2.21 the values of residual shear strength relative to specimens of kaolin (a) and
bentonite (b) reconstituted with – and submerged in – water or 1 M NaCl solution are
plotted against the normal applied stress. These specimens were tested in different
apparatuses, without observing significant influence of the testing device on the results. A
noticeable difference between the residual shear strength in water and in solution can be

29
seen for the used bentonite: a residual friction angle ϕ’r = 5° can be evaluated in water,
while ϕ’r = 17° can be evaluated in the 1 M NaCl solution. For the tested kaolin, the same
value of residual friction angle, ϕ’r = 13°, was evaluated in different apparatuses, under
different normal stresses, both on specimens in water and in 1 M NaCl solution.
0
20
40
60
80
100
0 100 200 300 400 500
τr(kPa)
σ'n (kPa)
Casagrande Reversal Bromhead
Kaolin
1M NaCl
Casagrande distilled water
ϕ'r ≈ 13°
0
20
40
60
80
100
0 100 200 300 400 500
τr(kPa)
σ'n (kPa)
Casagrande Bishop Bromhead
ϕ'r = 17°
ϕ'r = 5°
1M NaCl
distilled water
Bentonite
a)
b)
Figure 2.21 Residual shear strength of kaolin (a) and bentonite (b) in water and 1M NaCl
solution evaluated by means of different apparatuses.
Figure 2.22 shows the residual friction angle ϕ’r against the NaCl concentration in the pore
fluid of several specimens of the Costa della Gaveta soil, tested under similar normal
stresses. The residual shear strength of two undisturbed specimens taken close to the shear
surface in borehole K1bis (8.3 and 8.4 m) is also shown. A residual friction angle of 12°
was evaluated on both specimens. The pore ion concentration was evaluated on the

30
material from the same undisturbed sample. Subsequently, the specimens were sheared
further and exposed to distilled water, allowing ion diffusion outward from the pores. This
caused a decrease in the residual friction angle from 12° to 9.8°, suggesting that the
available residual strength on the slip surface of the landslide can decrease further as an
effect of ion concentration decrease.
The figure shows that the Costa della Gaveta soil exhibits a noticeable shear strength
increase with increasing NaCl concentration. The experimental points relative to the
undisturbed specimens lie on the same curve as that of the reconstituted specimens. The
relation between ϕ’r and pore solution molarity is not linear, with higher gradients at lower
concentrations. In particular, most of the strength variations are achieved within the range
0 – 1 mol/l.
5
10
15
20
0 1 2 3 4 5 6
residualfrictionangle,ϕ'r
NaCl molarity, M
K1bis undisturbed100 kPa
S9-Areconstituted150-175kPa
S9B reconstituted150-225kPa
S9-MIX reconstituted204 kPa
undisturbed K1bis specimens
close to the slip surface
K1bis after exposureto water
Figure 2.22 Residual friction angle against NaCl concentration in the pore solution for
reconstituted and undisturbed specimens of the Costa della Gaveta soil (mod. from Di
Maio et al., 2015c).
The results relative to the Costa della Gaveta soil, those relative to bentonite and those
obtained by Di Maio (2004a) on several soils are compared in Figure 2.23 in terms of
residual friction angle against NaCl concentration. The experimentation carried out by Di
Maio (2004a) was conducted under σ’n = 200 kPa, a value comparable to the normal
stresses applied during the tests shown in this section. The trend of residual shear strength

31
increase with concentration has practically the same shape for all materials, although the
magnitude of the effect of pore solution molarity is different. The highest dependence on
NaCl concentration is shown by the bentonite, whose ϕ’r ranges from 5° in water to more
than 20° in the 3 mol/l NaCl solution. The Ponza bentonite is mainly smectitic, Bisaccia
and Gela clays also contain relevant percentages of smectite (Di Maio, 2004a), which
probably control their behaviour.
0
5
10
15
20
0 1 2 3 4 5 6
ϕ'r(°)
NaCl concentration (mol/l)
Costa della Gaveta soil
Bisaccia clay (Di Maio, 2004a)
Gela clay (Di Maio, 2004a)
Ponza bentonite (Di Maio, 2004a)
Commercial bentonite
Figure 2.23 Residual friction angle against NaCl concentration in the pore fluid of
specimens of different clays.
Some of the specimens pre-sheared to the residual condition were subsequently exposed to
a different fluid and sheared further. In particular, some specimens initially in distilled
water were exposed to a concentrated salt solution.
Figure 2.24 shows the case of a specimen of Costa della Gaveta material which was
exposed to 1 mol/l solution of KCl. The exposure produced a gradual but noticeable shear
strength increase up to a value of residual shear strength triple than that attained in water.
On the subsequent re-exposure to distilled water, the shear strength exhibited only a
negligible decrease, thus suggesting that ion exchange had taken place. No effects were
seen on the volume change of the specimen.

32
0
20
40
60
80
100
τr(kPa)
S7CD2 -σ'n = 155 kPa
exposure to 1M KCl exposure to distilled water
manual cut
manual cut
-0.05
0.00
0.05
0 50 100 150 200 250
heightvariation(mm)
horizontaldisplacement(mm)
Figure 2.24 Shear strength and height variation of a specimen, reconstituted with – and
submerged in – distilled water, pre-sheared to the residual condition and then exposed to 1
M KCl solution and, subsequently, to distilled water.
Some other specimens were exposed to 1 M NaCl solution, which caused a significant
shear strength increase, although of lower magnitude than with KCl, to values consistent
with those obtained on specimens reconstituted with – and submerged in – 1 M NaCl
solution.
One specimen was prepared with the soil extracted from borehole S9 at a depth of about 26
m (close to the slip surface), reconstituted with distilled water and pre-sheared to the
residual condition in a bath of distilled water. During the course of the test, the specimen
was exposed to a composite “natural” solution, i.e. a solution prepared using NaCl, KCl,
MgCl2 and CaCl2 in proportions such that the cations Na+
, K+
, Mg2+
and Ca2+
would have
the same concentrations as those evaluated in the natural pore solution of the same sample:
0.372 M Na+
, 0.017 M K+
, 0.092 M Ca2+
, 0.045 M Mg2+
. The exposure caused a gradual
but significant shear strength increase (Figure 2.25), corresponding to a residual friction

33
angle increase from 7° to 13°, without significant volume changes. Figure 2.26 shows the
residual shear strength evaluated on the specimen, against the normal stress, during
different phases of the test. Since the beginning of the test, the cell water was frequently
replaced with distilled water. The values of the residual shear strength in this phase are
indicated in the figure by points 1-4. It can be seen that, probably as an effect of the
continuous exposure to water, the ions already in the pores diffused away, thus the residual
friction angle decreased. At point 4 the specimen was exposed to the “natural” solution
which caused the strength increase (to point 5) shown in Figure 2.25. The specimen was
then loaded (point 6), confirming the same value of the residual friction angle..
0
10
20
30
40
50
τr(kPa)
S9B - σ'n = 151 kPa
exposure to "natural solution"
-0.05
0.00
0.05
0 10 20 30 40 50 60 70 80 90 100
heightvariation(mm)
Figure 2.25 Shear strength of a specimen, reconstituted with – and exposed to – distilled
water, pre-sheared to the residual state and then exposed to the “natural solution”.

34
0
10
20
30
40
50
60
0 50 100 150 200 250
ττττr(kPa)
σσσσ'n (kPa)
S9B, Casagrande
apparatus
exposureto
natural solution
esposureto
water
1
2
3
4
5
6
Figure 2.26 Residual shear strength history against normal effective stress of the specimen
of S9B material.
The effects of exposure of pre-sheared specimens to fluids different from the pore fluid
were evaluated also on some specimens of bentonite for comparison.
A specimen was prepared by mixing the material with 1 mol/l NaCl solution. The
specimen was first sheared to the residual state while immersed in the same solution. The
residual shear strength was found consistent with the values reported in Figure 2.21b. The
cell fluid was then replaced by distilled water, which was renewed frequently to keep the
chemical gradient between the pore fluid and the cell fluid as high as possible, and the
specimen was sheared further. The shear strength, shown in Figure 2.27a against time,
gradually decreased and became finally equal to that of specimens prepared with water and
sheared while immersed in water (corresponding to ϕ’r ≈ 5°, as in Figure 2.21b). Figure
2.27b shows the height variations undergone by the specimen. Although the shear box is
not suitable to evaluate the volume change behaviour, it can be seen that significant
swelling started to occur after about 40 days of continuous exposure to water, that is when
the strength had already decreased noticeably.

35
0
10
20
30
40
50
60shearstrength,τ(kPa)
v = 0.0025 mm/min
manual cut
manual cut
manual cut
commercial bentonite
Casagrandeapparatus
σ'n = 150 kPa
τr in 1 M NaCl
τr in water
-1
0
1
2
3
4
5
6
7
heightvariations(mm)
0.0
0.2
0.4
0.6
0.8
1.0
0 10 20 30 40 50 60 70 80
NaClintheporefluid
(mol/l)
timesince exposureto water (days)
averageconcentration in
thespecimen evaluated
afterthe test
0.00
0.01
0.02
NaClinthecellfluid
(mol/l)
a)
b)
c)
d)
Figure 2.27 Exposure to distilled water of a specimen of bentonite reconstituted with 1 M
NaCl solution and sheared until the residual state while immersed in 1 M NaCl solution:
shear strength (a), height variations (b), NaCl concentration in the cell fluid before each
water renewal (c), and estimated average concentration in the pore fluid (d).

36
Before each water renewal, the Na+
concentration of the cell water was measured by means
of an ion-selective electrode to evaluate the possible ion diffusion. The values are plotted
in Figure 2.27c. Being the cell water and the pore water volumes known, it is possible to
estimate how the average NaCl concentration in the pores decreased during the process of
exposure to water (Figure 2.27d). In order to check whether the obtained curve of
concentration versus time was reliable, at the end of the test the specimen was oven-dried
to determine its water content and subsequently powdered and mixed with a known
amount of distilled water. Settlement of the suspension was allowed and the sodium
concentration of the supernatant fluid was measured. Under the hypothesis that all the ions
in the pore fluid were dispersed in the solution, the sodium concentration of the former
could be estimated. The result is represented by the red hollow marker in Figure 2.27d. The
value is consistent with the final concentration evaluated by means of measurements of
Na+
in the cell fluid.
2.2.2 Observation of the shear surface
In order to estimate soil parameters such as viscosity, it is important to evaluate the
thickness of the soil portion affected by shearing deformations. To this aim, and to
understand if the shear zone is characterised by different properties, some analyses have
been carried out by different techniques.
A specimen of Costa della Gaveta soil (S9B), reconstituted with distilled water, was
sheared in a bath of distilled water in the Bromhead apparatus. After the test, the specimen
was analysed by means of an environmental scanning electron microscope (ESEM) in
order to examine the material along the slip surface.
Figure 2.28 shows a ESEM micrograph of the investigated specimen. The figure refers to a
vertical cross section, in which the shear surface is located at the bottom. Close to the
surface, a zone in which the particle aggregates appear well aligned can be seen. The
thickness of this zone can be estimated in about 200 µm. However, a particle alignment in
the direction of shearing can be seen also on the top of the image, while on the left side a
band of particles with similar inclination can be seen. This suggests that all the area shown
in the micrograph, which has a thickness of about 1 mm, can be part of the shear band

37
whose thickness has been estimated to be about 1.5 mm for each half of a specimen tested
in the Casagrande apparatus (Di Maio et al., 2013).
Some additional micrographs, taken with different magnifications, are shown in Figure
2.29. It can be seen that the material is mostly constituted by platy particles arranged in
stacks with a preferential direction. The thickness of the stacks is in the order of several
microns, while the thickness of the single foils seems much lower than 1 µm.
aligned
aggregates
shearsurface
Figure 2.28 ESEM micrograph of the shear zone of a specimen of Costa della Gaveta soil
tested in the Bromhead apparatus.

38
Figure 2.29 ESEM micrographs with increasing magnification of the shear zone of a
specimen of Costa della Gaveta soil tested in the Bromhead apparatus
A second specimen of the same material, tested in the Casagrande apparatus, was
submitted to three dimensional X-ray tomography at the University of Padova, Italy.
The technique allows for the investigation of the whole specimen’s volume, overcoming
the limitation of the microscopy, by means of which only the surface can be studied. The
technique is similar to the X-ray analyses for medical purposes, it is non-invasive and does
not cause sample disturbance.
The instrument provides a 3D image made of “voxels” (i.e. 3D pixels) whose values can be
interpreted as a mean local density when the voxels are significantly larger than the grain
size. Alternatively, the single grains can be delineated when the voxels are significantly
smaller than them (Viggiani et al., 2015).

39
Some promising results regarding the use of this technique for geotechnical purposes have
been published, for instance, by Lenoir et al. (2007), Andò et al. (2011) and Viggiani et al.
(2015), who used the 3D X-ray tomography to reveal processes in soils such as strain
localisation, deformations due to volume removal, ice formation and desiccation cracks.
The tomography shown in this work was carried out by means of the Skyscan1172
instrument (Bruker microCT), equipped with a 11 Mp camera. The resulting voxel size
was 4.77 µm. The investigated specimen is a small portion of the shear specimen of about
6 mm side, sampled close to the slip surface. Since the observations were made some days
after the specimen was extracted, some drying of the material took place.
Figure 2.30 shows an example of 3D view of the shear surface and vertical cross sections
of the investigated specimen (the slip surface is located on the top). The shades of grey
show the different relative density of the material, which can possibly depend both on non-
homogeneity of the soil composition and of the water content. Lighter (i.e. relatively
denser) zones are possibly constituted by coarse grains or clay aggregates with relatively
lower water content. It can be seen that in the zone close to the slip surface the denser
zones are less abundant. About 1 mm below the slip surface, a zone characterised by lower
density, or even a void, can be seen. It is possible that this discontinuity was caused by
different shrinkage, due to drying, of the material close to the slip surface with respect to
the rest of the specimen, possibly because of different water contents resulting after
shearing.
Some statistical analyses have been carried out on the results of the X-ray tomography.
Figure 2.31a shows how the mean value of the relative density (in arbitrary units) varies in
the vertical direction. It can be seen that in most of the specimen’s volume the density
remains quite constant. However, it decreases towards the top, that is close to the shear
surface. Most of the decrease occurs in a zone about 1 mm thick., which corresponds to the
zone above the discontinuity seen in Figure 2.30. In Figure 2.31b the density distribution in
two horizontal sections of the specimen is plotted. The difference between the curves
relative to the shear zone and to the rest of the specimen is evident.

40
1 mm
1 mm
1 mm
Figure 2.30 3D view of the shear surface and vertical sections of the specimen of the Costa
della Gaveta soil seen by X-ray tomography.

41
0
20000
40000
60000
80000
100000
120000
20 30 40 50 60 70 80 90 100 110
frequency
class of density
lower
density
higher
density
h = 4.5 mm
h = 0.2 mm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
45 46 47 48 49 50 51
height(mm)
class of density
top
bottom
mean density
position of the
discontinuity
b)
a)
Figure 2.31 Variation of the mean relative density (arbitrary units) in the vertical direction
(a) and relative density distribution against frequency for two horizontal cross section of
the specimen.

42
3 INFLUENCE OF PORE FLUID
COMPOSITION ON CREEP BEHAVIOUR
This Chapter reports on the results of laboratory tests aimed at evaluating the mechanical
behaviour of the material along a pre-existing slip surface in the residual condition when
the specimen is subjected to changes in the pore fluid composition. The shear creep
behaviour and the chemically-induced displacement evolution were investigated by means
of shear tests under constant applied shear stresses in modified Casagrande and Bishop
apparatuses.
In the first paragraph, interpretations and modelization of creep phenomena reported in the
technical literature are reviewed and commented. Subsequently, the results of stress-
controlled shear tests on the Costa della Gaveta soil and on specimens of bentonite are
reported. Finally, the description of a simplified modelization of ion diffusion and shear
strength variation, which was helpful in the test interpretation, is presented. The main
results contained in this Chapter have been published by Di Maio and Scaringi (2015) and
Di Maio et al. (2015a).

3. Influence of pore fluid composition on creep behaviour
43
3.1 SHEAR CREEP: A BRIEF OVERVIEW OF THE
PHENOMENON
Creep is defined as the progressive, irrecoverable deformation of a soil element under a
state of constant effective stress (Kwok and Bolton, 2010). An increase in the deviatoric
stress level can result in a deformation response characterised by three successive phases
which are named primary, secondary and tertiary creep, characterised by decreasing,
constant and increasing strain rate respectively (Figure 3.1). The actual strain pattern is
hypothesised to depend on the type of soil, stress level and stress history (Singh and
Mitchell, 1968; Tavenas et al., 1978; Augustesen et al., 2004; Le et al., 2012).
Figure 3.1 Definition of creep stages according: strain versus time (a) and log(strain rate)
versus log(time) (b) (Augustesen et al., 2004).
Failure of cemented bonds or increase in the ratio of tangential to normal forces at the
interparticle contacts are among the processes which can lead to creep rupture for loss of
strength, in drained conditions and in the absence of chemical changes (Kuhn, 1987; Kuhn
and Mitchell, 1993; Mitchell and Soga, 2005; Kwok and Bolton, 2010).
The magnitude of creep strains increases with increasing plasticity, activity and water
content of the soil. The most active clays usually exhibit the greatest time-dependent
response because the smaller the particle size, the greater is the specific surface, and the
greater the water adsorption (Mitchell and Soga, 2005).

44
Most soils have a characteristic relationship between strain rate and time. This was shown,
for instance, by Bishop (1966) for drained triaxial compression creep of London clay and
by Murayama and Shibata (1958) for undrained triaxial compression creep of soft Osaka
clay.
Pore pressures may change during creep according to the volume change tendency of the
soil and to the possibility of drainage during the deformation process (Mitchell and Soga,
2005).
The theoretical shape of the curve of creep strain against time (Figure 3.1) may not exist at
all, as discussed by Ter-Stepanian (1992) who observed that a “jump-like structure
reorganization” may occur, reflecting a stochastic character for the deformation. This
behaviour was observed during a shear creep test on an undisturbed specimen of
overconsolidated clay.
Ter-Stepanian (1992) suggests the existence of four levels of deformation, two of them
concerning the deformation of matter and two of them the deformation of
particles/aggregates. In particular, regarding the matter, the Author focuses on (1) a
molecular level, which consists of displacement of particles by surmounting energy
barriers, and (2) on mutual displacement of particles as a result of bond failures, but
without rearrangement. With respect to the particle/aggregate deformation, the Author
points out (3) a structural level of soil deformation involving mutual rearrangement of
particles, and (4) deformations at the aggregate level.
Deformations at levels (3) and (4) should not be uniform due to the particulate nature of
soils and should proceed through a series of structural readjustments corresponding to the
relative movement of particles with respect to each other, thus leading to an irregular
sequence of deformations. Regarding the effects of particle rearrangement, Kuhn (1987)
developed a discrete element model that considers “visco-frictional” sliding at interparticle
contacts. Subsequently, Kuhn and Mitchell (1993) performed numerical analyses using a
discrete element model, obtaining a discontinuous creep behaviour comparable to that
observed on several soils.
In order to investigate deformations at levels (1) and (2), creep phenomena can be studied
as a rate process by means of the theory of absolute reaction rates (Eyring, 1936; Glasstone

45
et al., 1941), which is based on statistical mechanics. An adaptation of the theory to the
study of soil behaviour can be found, among others, in Feda (1989, 1992) and in Kuhn and
Mitchell (1993). The concept is that atoms, molecules and/or particles involved in a
deformation process (termed “flow units”) are constrained from relative movement by
energy barriers which separate adjacent equilibrium positions. In order to produce a
displacement, the flow unit must overcome the barrier by acquiring a surplus of potential
energy, termed the “activation energy”, ∆F. The potential energy of the flow unit after the
displacement may be lower than, equal to, or higher than the potential energy before the
displacement, thus defining conditions of increased stability, steady-state or decreased
stability respectively.
The activation energy may be provided by thermal energy or by an applied potential. If this
latter is not directional, flow units can surmount the energy barrier with equal probability
in all directions, therefore no macroscopic deformation is produced. On the contrary, if a
directional potential, such as gravity or a shear stress, is applied, than the barrier heights
are not equal in all directions, but lower in the direction of shearing and higher in the
opposite direction. Consequently, the barriers are most probably crossed in the direction of
shearing, thus producing a macroscopic deformation. A schematic representation of the
effect of a shear force on the activation energy required for deformation is shown by
Figure 3.2 (Mitchell and Soga, 2005).
Figure 3.2 Schematic representation of energy barriers in rate process theory in absence
and in presence of a directional potential (Mitchell and Soga, 2005).

46
Mitchell et al. (1968) showed that the rate of macroscopic deformation resulting from the
application of a directional potential, such as a shear force, can be expressed as a function
of the applied potential and of thermodynamic parameters, as in Figure 3.3. However, the
equation obtained by the Authors, since it is referred to deformations at levels (1) and (2)
only, does not account for structural changes. Therefore, if shear stress and thermodynamic
parameters (e.g. temperature) do not vary, than the strain rate remains constant, i.e. a
secondary creep is produced. In order to generalise their result, the Authors introduced a
parameter (termed X in Figure 3.3, and further defined by Ter-Stepanian, 1975) which can
be both structure and time dependent, so that primary and tertiary creep due to
deformations at level (3) and (4) could be included in the model.
Figure 3.3 Strain rate as a function of an applied directional potential according to the
rate process theory (Mitchell and Soga, 2005).
Notwithstanding this limitation, the equation was used by Kuhn and Mitchell (1993) as
part of the particle contact law in their discrete element modelling, and by Puzrin and
Houlsby (2003) as an internal function of a thermo-mechanically based model, deriving a
rate-dependent constitutive model for soil. Mitchell and Soga (2005) reported that the real
behaviour of many systems is substantially consistent with the statistical mechanics
formulation of the rate process theory. Different parts of the formulation have been tested
separately by Mitchell et al. (1968), giving results according to predictions.
Different Authors, among whom Mitchell et al. (1968), provided some ranges of activation
energy for soil creep. Mitchell and Soga (2005), following Andersland and Douglas
(1970), concluded that variations in water content (including complete drying), adsorbed
cation type, consolidation pressure, void ratio, and pore fluid have no significant effect on
the required activation energy. As a consequence, variations in strain rate in the absence of
structural rearrangements would not be due to changes in the activation energy but only to
changes in the number of bonds. However, this does not seem reasonable in phyllosilicates
with face-to-face orientation, which are kept together by electrostatic forces. In order to
preserve electroneutrality, the total charge of the adsorbed cations cannot change and,

47
therefore, the number of interparticle weak bonds will remain constant. On the contrary, it
must be considered that an increase in the double layer thickness, due to a decrease in ion
concentration or to an increase in the dielectric constant of the pore fluid, could weaken the
bonds and reduce the activation energy required to break them.
Additional considerations by Mitchell and Soga (2005) are the following: 1. the number of
bonds is directly proportional to effective consolidation pressure for normally consolidated
clays; 2. overconsolidation leads to more bonds than in normally consolidated clay at the
same effective consolidation pressure.
In fact, the validity of the conclusions drawn by Andersland and Douglas (1970) relies
upon the existence of solid-to-solid contacts between clay particles. Evidence of this have
been provided for some cases, for instance, by Matsui et al. (1977, 1980) by means of
photomicrographs, and by Koerner et al. (1977) by means of acoustic emissions. However,
this may not be valid in the case of smectites, especially in the residual condition. Normal
effective stresses and shear stresses can be transmitted only at interparticle contacts in most
soils. Pure sodium montmorillonite may be an exception (Mitchell and Soga, 2005) since a
relevant part of the normal stress can be carried by physicochemical forces of interaction.
Deformation at large strain can approach a steady-state condition in which there is little
further structural change with time (this is the case of residual state). This means that,
following Ter-Stepanian (1992), creep strains are due only to level 1 and level 2
deformations (rearrangement of matter). The governing equations of the rate process
theory may be rewritten in a form which is similar to the Coulomb equation for strength
(see Mitchell and Soga, 2005) which states that both cohesion and friction depend on the
number of bonds times the bond strength, and that the values of c and ϕ should depend on
the rate of deformation and the temperature. As a consequence, in the absence of structural
rearrangements, the shearing resistance should increase linearly with the logarithm of the
strain rate. Karlsson (1963) gave experimental evidence of this by means of vane tests on
different remoulded clays subjected to shear at different rates. The rate effect on the
residual shear strength may follow the same law provided that no changes in the shearing
mode occur (see Lupini et al., 1981, and Tika et al., 1996). Conversely, transition from
laminar to turbulent shearing mode, which involves particle rearrangement, should result in
a different strength – rate relationship.

48
A possible volumetric-deviatoric creep coupling may occur, as highlighted by Mitchell and
Soga (2005). This implies that a rapid application of a stress or a strain can result in rapid
change of pore water pressure in a saturated soil under undrained conditions. The rapid
application of a shear stress on clay specimens, i.e. characterised by very low hydraulic
conductivity, may result in pore fluid pressure excess. The dissipation of pore pressure
excess produces an increase in the effective normal stress, which may result in a creep
phase characterised by a decreasing strain rate, i.e. can appear as primary creep.
Furthermore, when a shear creep test is performed, the necessary time for primary
consolidation of the specimen is waited before applying the shear force but, for the entire
duration of the test, volumetric creep takes place. Consequently, the shear strength of the
material may increase due to the formation of additional bonds and/or to the strengthening
of existing bonds, as proved by Nakagawa et al. (1995).
Mitchell and Soga (2005) reported four possible causes of strength loss which lead to
failure under shear creep: (1) failure of cementation bonds, if a significant portion of the
strength of a soil is due to cementation; (2) in the absence of chemical or mineralogical
changes the strength depends on effective stresses: if creep causes changes in effective
stresses, then strength changes will also occur; (3) in almost all soils, shear causes changes
in pore pressure during undrained deformation and changes in water content during drained
deformation); (4) water content changes cause strength changes.
Besides these reasons, also chemical changes, such as pore fluid composition variation in
certain types of soil, can cause shear strength changes and, consequently, it can be
reasonable to expect that they can produce creep failure.

49
3.2 EXPERIMENTAL RESULTS
RELATIVE TO THE COSTA DELLA GAVETA SOIL
The chemical composition of the pore fluid affects the mechanical behaviour of clays
noticeably. The influence of pore fluid composition on the residual shear strength of the
Costa della Gaveta soil, determined by displacement-controlled tests, was shown in
section 2.2.1. The following paragraph shows the results relative to stress-controlled tests.
3.2.1 Stress-controlled shear tests on the Costa della Gaveta soil
In order to investigate the rheological behaviour of the soil along a pre-existing shear
surface, direct and ring shear tests were carried out under constant shear forces or stresses
(“force-controlled” or “stress-controlled” tests).
To perform such type of tests, the Casagrande and the Bishop apparatuses were modified
(Figure 3.4) in order to convert vertical forces, applied by means of dead loads, into
horizontal forces acting on the upper box or upper ring respectively (Di Maio et al., 2013,
2015a; Di Maio and Scaringi, 2015). During shearing in the ring shear device the contact
area does not change, thus constant forces correspond to constant average shear stresses,
i.e. the test is properly a “stress-controlled” test. On the contrary, small area variations
occur in the Casagrande direct shear and the test can be considered only “force-controlled”.
However, the small area variations during the test (< 2%) have been accounted for in the
interpretation of the results.

50
a) b)
load cellload cell
Figure 3.4 Schematic representation of the direct shear apparatus modified to perform
force-controlled tests (a). Picture of the Bishop ring shear modified to perform stress-
controlled tests (b).
The tests reported in this section were carried out on specimens of the Costa della Gaveta
soil. Subsequently, further tests were performed on specimens of sodium bentonite in order
to compare the obtained results to those relative to a pure clay and to see whether they
have more general validity. The results of these latter tests are reported in section 3.3.
The adopted test procedure was the following:
1. the specimens were prepared by mixing the powdered material with a concentrated salt
solution (1 M NaCl) and were sheared until the residual state was attained while immersed
in the same solution (displacement-controlled phase without chemical gradients);
2. the apparatuses were modified as in Figure 3.4 to perform the force/stress-controlled
tests (force/stress-controlled phase, or creep phase);
3. at the end of this phase, the original configuration of the apparatuses was restored to
perform additional displacement-controlled shearing to verify the available shear strength.
For sake of simplicity the test phases will be referred to as first, second, and third test
phases respectively. Table 3.1 summarises the test phases, the parameters which were
monitored and the used instruments. The table also reports the fluid in which the
specimens were submerged during each phase.

51
Phase Test mode Cell fluid Measured quantities and instruments
1 displacement-
controlled shear
test
water or salt solutions
at different
concentrations
horizontal displacements (LVDT),
shear strength (load cell), height
variations (LVDT)
2a force-controlled
or stress-
controlled shear
test
same as in phase 1 horizontal displacements (LVDT),
variations (LVDT)
2b force-controlled
or stress-
controlled shear
test
distilled water
(frequently renewed)
horizontal displacements, height
variations, cell fluid electrical
conductivity (4-electrode conductivity
probe) and/or Na+
concentration (ion-
selective electrode)
3 displacement-
controlled shear
test
distilled water
(frequently renewed)
horizontal displacements (LVDT),
variations (LVDT), cell fluid electrical
conductivity (4-electrode conductivity
probe) and/or Na+
concentration (ion-
selective electrode)
Table 3.1 Test phases, measured parameters and devices.
The first phase is similar to those described in Chapter 2. Each tested material was sheared
to the residual under displacement rate condition and the residual strength was determined
both with distilled water and 1 M NaCl solution as pore and cell fluid, in absence of
chemical gradients and in drained conditions.
In the second phase all the specimens prepared with and immersed in 1 M NaCl solution,
were subjected to an average horizontal shear stress lower than the residual strength
obtained, under the same normal stress, with the salt solution and higher than the residual
strength obtained for the same material with distilled water (Figure 3.5, Table 3.2).
The application of the horizontal force caused very small horizontal displacements with
decreasing rate (Figure 3.6a). This process occurred under constant effective stresses, i.e. it

52
is a primary creep (Augustesen et al., 2004). Subsequently (time = 0 in Figure 3.6) the cell
solution was replaced by distilled water, which was frequently renewed (usually twice a
day) to keep the chemical gradient between the pore fluid and the cell fluid as high as
possible.
0
10
20
30
40
50
60
70
τr(kPa)
τr in 1M NaCl
τr in dist. water
τ applied
S9A
Costa della Gaveta
0
10
20
30
40
50
60
70
0 100 200 300 400
τr(kPa)
σ'n (kPa)
τr in 1M NaCl
τr in dist. water
τ applied
B2
P1
Costa della Gaveta
a)
b)
Figure 3.5 Test conditions of the specimens of the Costa della Gaveta soil submitted to
stress/force-controlled shear tests.
Spec.
Borehole
- Sample
Shear
apparatus
σσσσ’n
(kPa)
ττττr in 1 M NaCl
solution (kPa)
Applied ττττ
(kPa)
ττττr in water
(kPa)
P1 S9-MIX Casagrande 204 55 45.0 35
S9A S9-A Casagrande 253 50 44.3 36
B2 S9-MIX Bishop 205 55 49.8 35
Table 3.2 Test conditions of the specimens of Costa della Gaveta soil submitted to
stress/force-controlled shear tests.

53
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-20 0 20 40 60 80
1M NaCl
solution
distilled
water
S9A
B2
P1
0
50
100
150
-20 0 20 40 60 80
displacementrate(µm/day)
S9A
B2P1
-0.1
0.0
0.1
-20 0 20 40 60 80
heightvariation(mm)
time (days)
S9A
B2
P1
a)
b)
c)
exposure to water
exposure to water
Figure 3.6 Effect of exposure to distilled water of specimens Costa della Gaveta soil
reconstituted with 1 M NaCl solution, previously immersed in the same solution and then
(time=0) exposed to distilled water: horizontal displacement, displacement rate and height
variation against time.

54
As a consequence of exposure to distilled water, the displacement rate increased (Figure
3.6b) with a non linear trend, with a pattern similar to that of secondary and then tertiary
creep, until “failure”. This term seems inaccurate since the specimens are subjected to
shearing along a pre-existing shear surface in residual condition. The term “failure” is used
here to indicate a dramatic increase in the displacement rate.
More in detail, the displacement rate of the specimens of the Costa della Gaveta soil
remained in the order of 1-10 µm/day for some weeks (Figure 3.6b) Afterwards, specimens
S9A and P1 experienced a sudden rate increase, while B2, tested in the Bishop apparatus,
underwent a progressive and more regular displacement rate increase. The causes of such
different patterns probably depend on the different machines as well as sub-experimental
differences. Figure 3.6c shows that all the specimens, when exposed to water, exhibited
some tendency to swell. Specimen B2 underwent a noticeable height decrease due to soil
loss from the gap between the box halves, possibly due to the loss of strength of the
material in contact with water.
In order to understand better the volume change behaviour of the Costa della Gaveta soil
with different pore fluids, several specimens were submitted to oedometer tests. For
instance, Figure 3.7 shows that a specimen reconstituted with 1 M NaCl solution - and
exposed to water in the course of the test - exhibits a noticeable tendency to swell.
0.0
0.1
0.2
0.3
0.4
0.1 1 10 100
heightvariation(mm)
time (days)
S9B (σ'n = 150 kPa)
Figure 3.7 Effect of the exposure to distilled water during the course of an oedometer test,
on a specimen of Costa della Gaveta soil, initially in equilibrium with 1 M NaCl solution.

55
Soon after failure, in order to evaluate the available shear strength at the end of the stress-
controlled phase, the apparatuses were turned to the displacement-controlled mode and the
specimens were sheared further (third test phase).
Figure 3.8, Figure 3.9 and Figure 3.10 plot the shear strength available after failure for
specimens S9A, P1 and B2 respectively. The curves are compared to the applied shear
stress during the second phase and to the shear strength exhibited by the specimens during
the first test phase, while immersed in 1 M NaCl solution. It can be seen that the available
shear strength in the third phase is much lower than that of the material in the NaCl
solution and close to the shear stress applied during the second test phase.
The height variations undergone by the specimens are shown as well. It can be seen that
S9A and P1, after the creep phase, continued to swell, while the height of specimen B2
continued to decrease due to soil extrusion. On the contrary, before the creep phase, that is
while the specimens were submerged in the NaCl solution, the height variations had
become practically negligible.
0
20
40
60
80
τ(kPa)
τ in 1M NaCl
applied shearstress
τafter failure
S9A
-0.10
-0.05
0.00
0.05
0.10
0 2 4 6 8 10
heightvariation(mm)
Figure 3.8 Shear strength and height variation against the horizontal displacement in 1M
NaCl solution (before the creep test), applied shear stress during the creep phase with
exposure to distilled water, shear strength and height variation after creep failure for
specimen S9A.

56
0
20
40
60
80
τ(kPa)
τ in 1M NaCl
applied shearstress
τ after failure
P1
-0.02
-0.01
0.00
0.01
0.02
0 5 10 15 20 25 30 35 40
heightvariation(mm)
specimen P1.
0
20
40
60
80
τ(kPa)
τ in 1M NaCl
applied shearstress
τ after failure
B2
-1.5
-1.0
-0.5
0.0
0 5 10 15 20 25 30 35 40
heightvariation(mm)
specimen B2.

57
During exposure to distilled water of specimens P1 and B2, both in the second and in the
third test phases, the electrical conductivity of the cell fluid was often measured before
water renewal because its values allow an estimation of the amount of salt diffused
outward from the specimen’s pores in the time period between consecutive fluid renewals.
The values of conductivity are plotted in Figure 3.11 against the time since the beginning
of exposure to water. Unfortunately, the conductivity was not measured during the first
days, therefore an estimation of the cumulative amount of salt diffused, and thus of the
average NaCl concentration in the pore fluid could not be made. However, it can be
noticed that the values of electrical conductivity generally decreased with time for both
specimens. Significantly higher values were recorded for P1 when the water renewal were
not performed twice a day but less frequently.
0
500
1000
1500
2000
0 10 20 30 40 50 60 70 80 90
electricalconductivity(µS/cm)
time(days)
P1
B2
Figure 3.11 Electrical conductivity - since first exposure to distilled water - of the cell fluid
of specimens P1 and B2 measured before water renewal

58
3.3 EXPERIMENTAL RESULTS
RELATIVE TO OTHER CLAYS
The experimentation carried out on the Costa della Gaveta soil was repeated for other
materials. The following paragraph reports on the results relative to a sodium bentonite.
The testing procedure was the same as that used for the Costa della Gaveta soil.
3.3.1 Stress-controlled shear tests on bentonite
The specimens were reconstituted with 1 M NaCl solution and first sheared to the residual
state under constant rate of displacement (v = 0.005 mm/min) and under different normal
stresses in the range 75 kPa < σ’n < 300 kPa. The attained values of residual shear strength,
as well as those evaluated on the same material reconstituted with – and submerged in –
distilled water, are plotted in Figure 3.12 and reported in Table 3.3 together with the
indication of the used apparatus and the test conditions. Specimens B1, C1 and L13, and
specimens C2 and C4, were submitted to the same stress conditions in different
apparatuses in order to check data reproducibility. Specimens L8 and L9, sheared to the
residual condition under the same vertical stress (σ’n = 150 kPa) as specimen C1, were
submitted to different shear stresses during the creep phase. Specimen L11 was prepared
with a 0.6 M NaCl solution instead of a 1 M NaCl solution.

59
0
20
40
60
80
100
0 100 200 300 400
τr(kPa)
σ'n (kPa)
τr in 1M NaCl
τr in dist. water
τ applied
C3
C2, C4
L9
C1, B1, L11, L13
L8
bentonite
L5
Figure 3.12 Test conditions of the specimens of bentonite submitted to stress/force-
controlled shear tests.
Spec. Material
Shear
apparatus
σσσσ’n
(kPa)
ττττr in 1 M NaCl
solution (kPa)
Applied ττττ
(kPa)
ττττr in water
(kPa)
B1 bentonite Bishop 150 46 29.3 12.4
C1 bentonite Casagrande 150 46 29.3 12.4
L5 bentonite Casagrande 75 23 14.7 6.2
L11 bentonite Bishop 150 40 (0.6 M NaCl) 29.3 12.4
Table 3.3 Test conditions of the specimens of bentonite submitted to stress/force-controlled
shear tests.

60
Figure 3.13a shows the results, in terms of horizontal displacements against time, relative
to the specimens which underwent all the three test phases without technical problems.
These curves, therefore, are of easier interpretation and will be discussed in more detail.
The results relative to all the performed tests are shown in Figure 3.14.
Similar to the specimens of the Costa della Gaveta soil, in the second phase (see Figure
3.6) the application of a shear stress lower than the residual shear strength attained in the
solution cause only small displacements with decreasing rate, which became negligible or
even null within a couple of weeks. Subsequently, in the third phase (time t=0 in Figure
3.13), the cell fluid was replaced by distilled water, which was renewed frequently –
usually twice a day – to remove the ions diffusing outward from the specimens’ pores and
to keep the concentration gradient between the pore fluid and the cell fluid as high as
possible.
The displacement rate (Figure 3.13b) increased significantly soon after the exposure to
water. Within a few days the displacement rate reached values of about 50 µm/day.
Subsequently, it remained roughly constant for some time, resembling secondary creep.
This phase had a longer duration for specimen C3, sheared under a normal stress higher
than that of specimens C2 and C1. Furthermore, the higher the displacement rate in this
phase, the lower the normal stress. Finally, after 15-25 days of continuous exposure to
water, the displacement rate increased more rapidly to values typical of failure.
The specimens of the Costa della Gaveta soil tested in the Casagrande apparatus
experienced sudden failure, while the displacement rate increased progressively in the
Bishop apparatus. On the contrary, all specimens of bentonite experienced a progressive
increase of the displacement rate, until failure, independently of the used apparatus.
Figure 3.13a shows that the two specimens submitted to the same stress conditions in
different apparatuses (B1 and C1) exhibited a very similar behaviour in terms of
displacements against time. This can be considered a validation of the tests carried out by
means of the Casagrande apparatus, which are inevitably less accurate than those in the
Bishop apparatus.

61
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-20 -10 0 10 20 30
1M NaCl
solution
distilled
water
C3
C2
B1C1
0
100
200
300
400
500
-20 -10 0 10 20 30
C3B1
C2
C1
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
-20 -10 0 10 20 30
heightvariation(mm)
time (days)
C3
B1
C2
C1
a)
b)
c)
exposure to water
exposure to water
exposure to water
Figure 3.13 Effects of exposure to distilled water of bentonite reconstituted with 1 M NaCl
solution and subjected to stress-controlled tests: horizontal displacement, displacement
rate and height variations against time.

62
Due to the exposure to water, all specimens exhibited swelling (Figure 3.13c), of
increasing magnitude with normal stress decreasing. Furthermore, the specimen tested in
the Bishop apparatus (B1) underwent more significant swelling than the specimen tested in
the Casagrande shear box (C1) under the same normal stress.
Besides the tests shown in Figure 3.13, additional tests were performed, during which
different technical difficulties arose. Typically, significant oxidation of the metallic
components in contact with the salt solution for long periods of time. Although the cell and
the components, where possible, were periodically cleaned, in some cases these
phenomena were found anyway responsible of additional friction between the two half-
boxes or within them, thus slowing down the displacements, impeding the correct
application of the normal loads and preventing free volume changes. The results of such
tests, however, are reported in Figure 3.14 in terms of horizontal displacements,
displacement rate and height variation against time during the force/stress-controlled
phase.
Notwithstanding the technical difficulties, all specimens reached failure, although with
displacement patterns that do not seem easily correlated to the stress state. For example,
specimens C2 and C4, which were tested under the same conditions, did not exhibit the
same displacement pattern. Specimens L8, sheared by a lower force than that applied on
C1, reached failure after a longer time than that needed for C1. This is consistent with the
fact that more time was needed to produce a larger decrease in the available strength.
However, specimen L9, sheared by a force which was higher than that on C1, did not reach
failure in a time shorter than that needed for this latter. The test on specimen L5 was not
considered for further interpretation because at the normal applied stress σ’n = 75 kPa
significant swelling took place before creep failure (Figure 3.14c), increasing the gap
between the box halves noticeably and thus possibly modifying the stress state of the
material along the slip surface.

63
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-20 -10 0 10 20 30 40 50 60
1M NaCl
solution
distilled
water
C3
C2
B1C1
C4
L8L9
L13
L11
L5
pore pressure
transducer removed,
load piston changed
0
100
200
300
400
500
-20 -10 0 10 20 30 40 50 60
time (days)
C3B1
C2
C1 C4
L8
L9
L13
L11
L5
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
-20 -10 0 10 20 30 40 50 60
heightvariation(mm)
time (days)
C2
B1
L5
C1
L13
C3
L8
C4
L11
L9
a)
b)
c)
Figure 3.14 Horizontal displacements (a), displacement rate (b) and height variation (c)
against time for specimens of bentonite in the course of force/stress-controlled tests (all
tests).

64
The displacement pattern of specimen L13 is significantly different from that of the other
specimen. In this case, a miniature pore pressure transducer which was installed inside the
specimen, close to the shear surface, may have slowed down the displacements noticeably.
In fact, after the removal of the transducer, the displacement rate increased noticeably, the
specimen exhibited swelling and it eventually reached failure within a few days.
After failure, the shear apparatuses were turned back to the displacement-controlled mode
(third test phase) and the specimens were sheared further in order to evaluate the available
shear strength. Figure 3.15, Figure 3.16, Figure 3.17 and Figure 3.18 show the results in
terms of shear strength and height variations undergone by specimens B1, C1, C2 and C3
respectively, against the cumulative horizontal displacement. The shear strength measured
during the first and the third test phases was plotted. For the second phase, the applied
shear stress is indicated.
Similarly to the specimens of the Costa della Gaveta soil, also the specimens of bentonite
exhibited, after failure, a shear strength not higher than the applied shear stress during the
creep phase, and much smaller than that exhibited while they were immersed in the 1 M
NaCl solution.
To observe the effect on shear strength of exposure to distilled water directly, the
specimens were sheared further, renewing the cell water frequently. All specimens
exhibited a continuous decrease in strength, until a minimum value, very close to that
obtained for the water-saturated specimens tested in a bath of distilled water. Furthermore,
all specimens continued to swell and, often, significant soil loss was seen from the gap
between the box halves. The test on specimen C2 (Figure 3.17) was interrupted before
reaching the minimum value of shear strength because the load piston was significantly
tilted, thus preventing the correct application of the normal load and the height variation.

65
0
10
20
30
40
50
60
τ(kPa)
τr in dist. water
B1 (σ'n = 150 kPa)
τr in 1M NaCl
applied shear stress
phase 1 phase 2 phase 3
exposure to distilled water
-0.5
0.0
0.5
1.0
1.5
2.0
0 20 40 60 80 100 120 140 160 180
heightvariation(mm)
horizontal displacement (mm)
soil extrusion
Figure 3.15 Shear strength and height variation against shear displacement in the three
different test phases for specimen B1.
0
10
20
30
40
50
60
τ(kPa)
τr in dist. water
C1 (σ'n = 150 kPa)
τr in 1M NaCl
applied shear stress
phase 1 2 phase 3
shearing under different
normal stresses
exposure to distilled water
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
0 20 40 60 80 100 120 140 160 180
heightvariation(mm)
horizontal displacement (mm)
shearing under different
normal stresses
Figure 3.16 Shear strength and height variation against shear displacement in the three
different test phases for specimen C1.

Scaringi Gianvito - PhD Thesis

Scaringi Gianvito - PhD Thesis

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