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Abstract

The letter describes an investigation of the microstructural features of a high-plasticity clay, in both its natural conditions and reconstituted in the laboratory. Scanning electron microscopy is used here to characterise the fabric at different magnification, while image processing of the micrographs delivers a quantitative assessment of the fabric orientation. Results of Energy dispersive X-ray spectroscopy and swelling tests, as reported in previous work by the authors, are used to characterise the bonding nature and strength, as well as mercury intrusion porosimetry to investigate the clay porosimetry. Despite their identical composition, the natural and the reconstituted clay have experienced different deposition and loading history, generating different microstructural features that are shown to underlie their differences in state. For both clays, 1D compression to medium-high pressures is seen to determine a well oriented medium magnification fabric. However, larger scale observations and the corresponding image processing results reveal non-uniform local fabric features, hence making fabric characterisation dependent on the scale of analysis and bringing about the issue of identifying the clay micro-scale representative element volume relating to the clay macro-behaviour. The micro-REV is identified for the clays under study and its connection with the macro-behaviour characterized. The microstructural evolution induced by 1D compression to very high pressures is shown to concern mainly the clay porosity and porosimetry, the fabric orientation being steady, thus explaining the isotropic hardening observed in laboratory tests.
Accepted manuscript doi:
10.1680/jgele.18.00230
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Accepted manuscript doi:
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Submitted: 07 December 2018
Published online in ‘accepted manuscript’ format: 27 August 2019
Manuscript title: Characterisation of the multi-scale fabric features of high plasticity clays
Authors: F. Cotecchia*, S. Guglielmi*, F. Cafaro* and A. Gens
Affiliations: *Department of Civil, Environmental, Land, Building Engineering and
Chemistry, Politecnico di Bari, Bari, Italy and Departamento de Ingeniería del Terreno,
Universitat Politécnica de Catalunya, Barcelona, Spain
Corresponding author: S. Guglielmi, Department of Civil, Environmental, Land, Building
Engineering and Chemistry, Politecnico di Bari, Via Edoardo Orabona, 4, 70126 Bari (BA),
Italy. Tel.: +39 080 5963363.
E-mail: simona.guglielmi@poliba.it
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Accepted manuscript doi:
10.1680/jgele.18.00230
Abstract
The letter describes an investigation of the microstructural features of a high-plasticity clay, in both its natural
conditions and reconstituted in the laboratory. Scanning electron microscopy is used here to characterise the
fabric at different magnification, while image processing of the micrographs delivers a quantitative assessment
of the fabric orientation. Results of Energy dispersive X-ray spectroscopy and swelling tests, as reported in
previous work by the authors, are used to characterise the bonding nature and strength, as well as mercury
intrusion porosimetry to investigate the clay porosimetry. Despite their identical composition, the natural and
the reconstituted clay have experienced different deposition and loading history, generating different
microstructural features that are shown to underlie their differences in state. For both clays, 1D compression to
medium-high pressures is seen to determine a well oriented medium magnification fabric. However, larger scale
observations and the corresponding image processing results reveal non-uniform local fabric features, hence
making fabric characterisation dependent on the scale of analysis and bringing about the issue of identifying the
clay micro-scale representative element volume relating to the clay macro-behaviour. The micro-REV is
identified for the clays under study and its connection with the macro-behaviour characterized. The
microstructural evolution induced by 1D compression to very high pressures is shown to concern mainly the
clay porosity and porosimetry, the fabric orientation being steady, thus explaining the isotropic hardening
observed in laboratory tests.
Keywords: clays; fabric/structure of soils; microscopy
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Accepted manuscript doi:
10.1680/jgele.18.00230
1. Introduction
The present letter discusses the microstructural features that a high plasticity natural clay
achieves through its geological history, after one-dimensional (1D) compression in the
laboratory (lab), and when reconstituted and subjected to 1D compression (Burland, 1990).
The microstructure at the different macro-states: void ratio, e - vertical effective stress, v, is
herein investigated making use of the scanning electron microscope (SEM) and the image
processing of the micrographs at different magnifications. Results of Energy Dispersive X-
ray Spectroscopy (EDS) in the SEM, mercury intrusion porosimetry (MIP) and, for the
bonding strength, swelling tests (Schmertmann, 1969), reported in previous work, are also
recalled.
The research results are aimed at characterizing the micro-scale sources of clay macro-
behavioural features (e.g. Delage and Lefebvre, 1984; Hattab et al., 2013; Lima et al, 2008),
traditionally modelled through the calibration of macro-mechanics models. Furthermore, they
provide evidence of the complex processes taking place at colloidal scale, which should be
modelled within micro-mechanical models of clays (e.g. Ebrahimi et al., 2012 and 2014,
Anandarajah, 2000; Yao and Anandarajah, 2003, Ebrahimi et al., 2016; Liu et al., 2015;
Sjoblom, 2016).
2. Composition, history and macro-behaviour of the clay
The investigated natural clay is the stiff Pappadai clay, deposited in the early Pleistocene in a
quiet marine environment, within reducing conditions. The mineralogy and the index
properties of the clay are reported in Table 1. It is mainly illitic, but includes a significant
amount of smectite, interstratified illite-smectite and carbonatic silt. The Total Dissolved
Solids (TDS) content and electrical conductivity of the squeezed pore fluid in the natural clay
at Montemesola, in the same geological basin as Pappadai clay, allow to recognize a low
salinity pore water (Fidelibus et al., 2018).
The history and the macro-behaviour of both the natural and the reconstituted Pappadai clay
are discussed by Cotecchia & Chandler (1995 and 1997). Few macro-behaviour aspects,
relating to the clay micro-scale processes, are recalled here.
The state of the natural clay, A in the compression plane in Figure 1, results from
overconsolidation due to unloading (OCR= p/v0 = 3; Cotecchia & Chandler 1995). When
subjected to 1D compression in the lab (Figure 1), the clay exhibits gross yield at y about
twice p (yield stress ratio, YSR=y/v0 2OCR) as result of diagenesis under burial,
which has increased the strength of the clay bonding.
Following Burland (1990), the reconstituted Pappadai clay (parameters*), i.e. the clay
achieving its microstructure through a lab history common for all reconstituted clays, was
prepared as clay slurry of water content 1.25∙LL. Given the low salt concentration in the pore
fluid (Fidelibus et al., 2018) of the natural clay, distilled water was used to prepare the
corresponding reconstituted clay slurry. Distilled water was also used to preserve saturation
of both the natural and the reconstituted clay specimens during the mechanical tests. Its 1D
compression curve plots to the left of the gross yield states of the natural clay (Figure 1).
The natural clay swell sensitivity, i.e. Cs*/ Csi = 2.5 (Schmertmann, 1969), confirms the
higher strength of the natural clay bonding with respect to that of the reconstituted, partly due
to diagenesis. On the whole, the natural microstructure provides the clay with a stress
sensitivity Sσ, y/*e ( p‟yis/p*yis in isotropic compression; Cotecchia and Chandler, 2000)
equal to 3.5 (Figure 1). Upon compression, the swell sensitivity of the natural clay drops to 1
soon after gross yield (Cs*/Cs,py, Figure 1), but Sσ decreases gradually, as function of the
plastic volumetric strain, vp, keeping a value above 1 up to high pressures.
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Accepted manuscript doi:
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The clay microstructure is an internal variable of the hardening function in several
constitutive laws (e.g. Rouainia & Wood, 2000; Baudet & Stallebrass 2004). Cotecchia &
Chandler (2000) showed that Sσ(vp) is suited to represent its effects on the clay gross yield
hardening, since the state boundary surface of several clays is normalized by the function
Sσ(vp)∙pe*(e) (pe* on the normal-consolidation line of the reconstituted clay; Schofield &
Wroth 1968). Accordingly, an isotropic volumetric gross yield hardening function matches
the large strain behaviour of several clays, either natural, or reconstituted, as shown for
Pappadai clay in Figure 2.
3. Microstructure of the natural and the reconstituted clay
3.1 Testing procedures
Micrographs were achieved for vertical fractures of freeze-dried clay specimens, by means of
SEM (gold coated) and Field Emission SEM (FESEM, carbon coated), as discussed by
Cotecchia & Chandler (1998), Cotecchia et al. (2016) and Guglielmi et al. (2018). Digital
image processing of the micrographs was carried out through an operator-independent
technique, discussed for clays by Martinez-Nistal et al. (1999) and Mitaritonna et al. (2014).
It is based on the thinning of the elongated bright regions across the micrograph, which
represent either the edges of single particles, or the contour of oriented particle aggregates; an
example of the thinning procedure is shown in Figure 3a. The thinning results in a field of
vectors of varying orientation, processed to derive both a histogram of the detected
orientations and a scalar “index of fabric orientation”, L (e.g. Figure 3b). For a medium
oriented fabric, L is about 0.21; L rises with increasing orientation, reaching 1 for a complete
preferred orientation, c.p.o., fabric (Smart, 1969; Sides and Barden, 1970), whereas it is
lower than 0.15 for “randomly oriented fabric” (Martinez-Nistal et al., 1999). The SEM
micrographs and image processing data are discussed first for specimens pre-consolidated to
medium-high pressures, i.e. states A* and A in Figure 1, and afterwards for those compressed
to higher pressures, i.e. states B, C and C* in the figure.
EDS was used in the A state investigation (Cotecchia and Chandler, 1997). MIP tests were
carried out on the clay (Guglielmi et al., 2018) at states A, B and C (Figure 1).
3.2 Microstructural features of the clay compressed to medium-high pressures
The reconstituted clay reaches state A* (e=1.28 - v=20 kPa; Figure 1) through swelling
from p*=200 kPa. Figure 4a shows a micrograph exemplifying the A* clay fabric on
vertical fractures of about 104 m2 size area (cubic volume 10-3 mm3), investigated at 103
magnification, defined as medium scale‟ (Collins and McGown, 1974; O'Brien and Slatt,
1990). At such scale, the fabric is found to match a repetitive pattern, formed of densely
packed domains (Figure 4c) in face to face contact, forming stacks (Sides and Barden, 1970;
Figures 4b and c), locally confining either macro-pores (i.e. diameter above 1 m; Matsuo
and Kamon, 1977; Guglielmi et al., 2018), or aggregates of randomly oriented
particles/domains, in edge to face contact (Figures 4b and c). The image processing of several
medium magnification micrographs of the A* clay has delivered L values varying in a narrow
range, 0.23-0.27 (e.g. Figure 4d). Hence, the fabric pattern repeatedly detected at medium
scale, is generally characterized by a medium-good orientation (Martinez-Nistal et al. 1999).
A repetitive porosimetry is expected to correspond to such fabric pattern.
Also for the overconsolidated natural diagenized clay, at state A (e=0.88 - v=414 kPa;
Figure 1), both the qualitative analysis and the image processing of several medium scale
micrographs has resulted in the recognition of a repetitive fabric. Also this is formed by a
dense packing of stacks, locally burying either randomly oriented domains (e.g. bookhouse,
Figures 4c and 5b), or macropores, or micro-fossils (Cotecchia and Chandler, 1998). The
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Accepted manuscript doi:
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direction histograms resulting from the image processing (e.g. Figure 5c) deliver L in the
range 0.24-0.37, indicative of a good orientation fabric. The MIP test results for state A show
that the clay pore size distribution is mainly monomodal, with dominant pore size in the
range of micro-porosity and a well distributed macro-porosity (Guglielmi et al., 2018).
Therefore, the micro-investigation results show that, irrespective of the differences in history,
both the natural and the reconstituted clay have achieved, through 1D pre-consolidation to
medium-high pressures, a medium-scale fabric characterized by a medium to good
orientation.
At higher magnifications (i.e. 104-105; clay portions of about 10-6 mm3 volume; „large‟ scale
hereafter), in either the A* reconstituted, or the A natural clay, the local fabric is highly
variable, going from a c.p.o. (Figures 6a and b) to a randomly oriented fabric (Figures 6c and
d). For both the clays, the image processing at the large scale results in variable indices of
orientation, 0.15<L<0.28, accordingly (Figure 6). Such variability in local fabric is likely to
apply to other natural and reconstituted clays 1D pre-consolidated to medium-high pressures;
as such it gives evidence to the general need of characterizing the micro-scale representative
element volume of the clay, micro-REV, in order to assess the micro-scale features of the
clay. By definition, the micro-REV fabric must include, in a repetitive way, the different local
fabric features, according to their frequency, so as to fulfil the role of internal variable
controlling the clay macro-behaviour.
For the clays under study, the micro-REV size is necessarily higher than 10-6 mm3, which is
investigated at large-scale (Figure 6). Rather, the clay micro-REV corresponds to the clay
portion, of size about 10-3 mm3, investigated at the medium-scale and found to be
characterized by a repetitive fabric and L value. Given so, the data show that by medium
preconsolidation pressures the micro-REV of the reconstituted Pappadai clay achieves a
degree of orientation only slightly lower than that of the natural clay preconsolidated to a p
about 5 times higher. At the same time, the micro-REV structures of the clays at A and A*
differ for: i) the stronger bonding of the A natural clay (Cs*/Csi = 2.5), which is partly effect
of an amorphous calcite film binding the natural clay particles, effect of diagenesis and
detected through EDS (Cotecchia & Chandler 1997); ii) the higher size and quantity of the
macro-pores in the A* clay state; iii) a rather more chaotic fabric within the randomly
oriented fabric portions locally occurring in the natural clay.
3.3 Microstructural features of the clay compressed to higher pressures
Figure 7a shows a medium-magnification micrograph of the reconstituted clay 1D
compressed to v=22 MPa (state C*, Figure 1). With respect to the micro-REV fabric at A*,
at C* the fabric packing is far denser, the macro-pores are much fewer and the layers of piled
stacks thicker. Nonetheless, the average degree of orientation at C* is about that at A*, since
L values about 0.24 are recorded. Mitaritonna et al. (2014) reported a similar finding for the
illitic reconstituted Lucera clay, shown to have a medium-scale L (=0.28) constant in 1D
(=q/p‟=0.6) compression from v=140 kPa to 1900 kPa. It follows that the orientation of
the micro-REV fabric does not increase in 1D compression from medium to very high
pressures, differently from what assumed in the literature (e.g. Morgenstern and Tchalenko,
1967; Tchalenko, 1967; Delage and Lefebvre, 1984; Lapierre et al., 1990; Hicher et al., 2000;
Hattab et al., 2013). Furthermore, at C*, the same as at A*, the fabric within the micro-REV
is still not uniform, as evident at large scale (example of local randomly oriented fabric in
Figure 7b).
Mitaritonna et al. (2014) showed also that, in constant compression, a constancy in elastic
stiffness anisotropy corresponds to the constant medium-scale degree of fabric orientation.
Conversely, such anisotropy varies when the medium-scale fabric achieves a different
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Accepted manuscript doi:
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orientation index, L, after a significant compression at a different . This finding suggests
that the clay macro-behaviour at very small strains relates with the medium-scale clay fabric
features, here recognized to be the clay micro-REV fabric.
The micro-investigation data discussed above for reconstituted Pappadai clay show that also
the clay hardening law relates with the clay micro-REV fabric features. The finding that, in
1D compression to very high pressures, the changes in micro-REV fabric concern mainly the
clay porosity and porosimetry, but not the index of orientation, is consistent with the isotropic
volumetric hardening function fitting the large-strain macro-behaviour of reconstituted clays
(e.g. Schofield and Wroth 1968; Roscoe and Burland, 1968; Figure 2). Hence, the clay micro-
REV fabric, characterized at the medium scale, is confirmed to be an appropriate internal
variable of the clay macro-behaviour.
For the natural clay, gross yielding is observed to cause major weakening of the natural
bonding, given the drop in Cs*/Cs recorded between states A and B (soon after gross yield).
Figure 8a shows one of several medium scale micrographs for state B, which suggest that
gross yielding causes also some fabric rearrangement, e.g. the chaotic filling of macro-pores
with particle aggregates. However, the micro-REV fabric does not attain a higher orientation
degree, as confirmed by the L value (Figure 8a).
With post-gross yield compression to state C (v=25 MPa), the natural micro-REV fabric
(Figure 9a) is repeatedly formed by thickened layers of c.p.o. fabric, interbedding randomly
oriented fabric portions, as recognized through large scale micro-investigation (Figures 9c
and d). Therefore, the micro-REV fabric is not uniform even in the natural clay at very high
pressures and 1D compression determines either the piling up of stacks in extremely low
porosity layers, or the shift of randomly oriented aggregates (e.g. Figs. 8b and 9c), which do
not necessarily collapse under very high pressures. MIP testing shows that, while the clay
macro-porosity becomes negligible by state B, a progressive reduction of the dominant pore
size is attained all way through compression (Guglielmi et al., 2018). In the meanwhile, the
micro-REV fabric does not achieve a much higher degree of orientation, since values of L in
the range 0.24<L<0.345, are recorded.
Such constancy in micro-REV fabric orientation is consistent with the isotropic volumetric
gross yield hardening law fitting the natural clay macro-scale behaviour (Figure 2),
Sσ(vp)∙pe*(e) (Cotecchia & Chandler 2000). This accounts for a positive hardening conferred
by the positive volumetric straining, and, through Sσ(vp), for the negative hardening
conferred by the weakening of the clay structure.
4. Conclusions
The paper highlights that the fabric is not uniform in clays 1D compressed from medium to
very high pressures. Hence, it is necessary to identify the micro-REV fabric that, for the clays
under study, has 10-3 mm3 volume. This is characterized through the image processing of 103
magnification micrographs and has been shown to relate with the clay macro-response, at
both small and large strains.
Furthermore, the letter reveals that, for both the natural and the reconstituted clay, the micro-
REV fabric orientation is medium to good by medium pressures and does not increase much
during constant compression to very high pressures. The isotropic volumetric gross yield
hardening of the clay is an effect of such micro-REV structure evolution with compression.
At large scale, compression determines either the coalescence of stacks, or the shift of
randomly oriented fabric portions, which do not necessarily undergo collapse.
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Accepted manuscript doi:
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Acknowledgements
The authors thank Dr. Vito Summa and Dr. Antonio Lettino of the Institute of Methodologies
for Environmental Analysis (IMAA) of the National Research Council (CNR) at Tito Scalo
(PZ, Italy) for the use of the FESEM. They are also grateful to Dr. Angel Martinez-Nistal for
the image processing of the micrographs.
The authors express their gratitude to Fondazione Puglia for supporting the research.
List of notation
A clay activity
CF clay fraction
CRS constant rate of strain oedometer test
Cs swelling index
Cs*/Cs swell sensitivity
e void ratio
L index of fabric orientation
OCR overconsolidation ratio: p/v
OED conventional oedometer test
PI plasticity index
S Stress Sensitivity
w water content
YSR yield stress ratio: y/v
e equivalent vertical effective stress on the ICL
'p vertical (geological) preconsolidation pressure
'v vertical effective stress
'y vertical effective stress at yield
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Table captions
Table 1. Index properties, mineralogy and initial state of natural Pappadai clay (after
Cotecchia and Chandler, 1997).
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Table 1. Index properties, mineralogy and initial state of natural Pappadai clay (after
Cotecchia and Chandler, 1997).
Composition and physical
properties
Specific gravity, Gs
2.75
Clay fraction, CF
58%
Silt fraction, MF
41%
Sand fraction, SF
1%
Liquid limit. LL
65%
Plasticity index, PI
35%
Activity, A
0.6
Natural water content, w0
≈31%
In situ void ratio, e0
0.88
Carbonate content
28%
Mineralogy
Quartz
3%
Feldspar
1%
Carbonate
22%
Dolomite
6%
Kaolinite
12%
Chlorite
14%
Illite
20%
Smectite
12%
Interstratified
10%
Total
100%
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Figure captions
Figure 1. One-dimensional compression and swelling tests on both natural and reconstituted
(*) Pappadai clay (adapted from Cotecchia and Chandler, 1997).
Figure 2. Pappadai clay: gross yield data of the natural clay and stress paths of the
reconstituted clay, behaviour normalized for both volume and structure (after
Cotecchia and Chandler, 2000).
Figure 3. Natural Pappadai clay, A (Fig.1): a) FESEM with processed overlay and b)
corresponding direction histogram and index of fabric orientation.
Figure 4. Reconstituted Pappadai clay, A* (Fig.1): a) medium magnification micrograph
(after Cotecchia et al., 2016) with b) examples of different local fabric arrangements;
c) classification of fabrics (after Sides and Barden, 1970, modified); d) direction
histogram and index of fabric orientation of micrograph 4a (after Cotecchia et al.,
2016).
Figure 5. Natural Pappadai clay, A (Fig.1): a) medium magnification micrograph with b)
examples of different local fabric arrangements; c) corresponding direction histogram
and index of fabric orientation (after Cotecchia et al., 2016).
Figure 6. High magnification micrographs of Pappadai clay, with examples of different local
fabric arrangements (i.e. complete preferred orientation, c.p.o., and randomly oriented
fabric), and corresponding indices of fabric orientation. a) and c), reconstituted clay
(state A*, Fig.1); b) and d), natural clay (state A, Fig.1).
Figure 7. Compressed reconstituted Pappadai clay, C* (Fig.1): a) medium magnification
micrograph (with examples of different local fabric arrangements) and index of fabric
orientation; b) high magnification micrograph and index of fabric orientation.
Figure 8. Compressed natural Pappadai clay, B (Fig.1): a) medium magnification micrograph
(with examples of different local fabric arrangements; after Cotecchia et al., 2016
modified) and index of fabric orientation; b) high magnification micrograph and index
of fabric orientation.
Figure 9. Compressed natural Pappadai clay, C (Fig.1): a) medium magnification micrograph
(with examples of different local fabric arrangements) and b) corresponding direction
histogram and index of fabric orientation; c) and d) higher magnification micrographs
and indices of fabric orientation, indicative of the variability in fabric arrangement.
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
Accepted manuscript doi:
10.1680/jgele.18.00230
Downloaded by [ Federica Cotecchia] on [10/10/19]. Copyright © ICE Publishing, all rights reserved.
... The clay subjected to study is the mid-Pleistocene Pappadai clay from Southern Italy, whose geological history is known in some detail [9]. This study is one aspect of a wider research [2,3,[10][11][12] aimed at identifying the main physical factors and internal features which control, at the micro-scale, the material response, causing given behavioural facets. The research final aim is at assessing the influence of the different aspects of behaviour on model parameter values, hence supporting constitutive modelling and finding a relation between classes of behaviour and corresponding models and classes of clays. ...
... The image processing technique proposed by Martinez-Nistal et al. (1999) has been used to quantify the orientation of the fabric of natural Pappadai clay at medium magnification [12]. The "index of fabric orientation", L (see [15] for the methodology used in the image processing), varies between 0.21 and 1 for "medium to very oriented fabric", and is in the range 0,24-0,37 for Pappadai clay [12]. ...
... The image processing technique proposed by Martinez-Nistal et al. (1999) has been used to quantify the orientation of the fabric of natural Pappadai clay at medium magnification [12]. The "index of fabric orientation", L (see [15] for the methodology used in the image processing), varies between 0.21 and 1 for "medium to very oriented fabric", and is in the range 0,24-0,37 for Pappadai clay [12]. This value confirms the above qualitative interpretation of the SEM micrographs. ...
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... Methods from materials science, such as scanning and transmission electron microscopy (SEM/TEM), mercury intrusion porosimetry (MIP), and wide or small angle X-ray or neutron scattering (W/SAXS, SANS) (e.g., Glatter and Kratky 1982;Toer and Reimer 1998;Giesche 2006) have been introduced in geomechanics to further probe clays at microscale despite their inherent limitations for use in fine-grained materials (Yao and Liu 2012;Deirieh et al. 2018). The ultimate aim is to link the micro and macro response (e.g., Pusch 1970;Delage and Lefebvre 1984;Djéran-Maigre et al. 1998;Hicher et al. 2000;Ringdal et al. 2010;Delage 2010;Hattab and Fleureau 2011;Hattab et al. 2013;Suuronen et al. 2014;Wensrich et al. 2018;Birmpilis et al. 2019;Cotecchia et al. 2019;Delage and Tessier 2021;Abed and Sołowski 2020;Schuck et al. 2020;Dor et al. 2020;Zhao et al. 2020). Regardless of the experimental method, either a bulk response of the complete sample volume (MIP, W/SAXS, SANS), a twodimensional (2D) map of the integrated response along the transmitted X-ray/electron beam (scanning W/SAXS and TEM), or a 2D surface profile (with a certain depth of view) is obtained (SEM). ...
... In coarse grained materials, the orientation of contact normal vectors, void vectors, and branch vectors or simply the orientation of the primary axis of the particle (e.g., Bathurst and Rothenburg 1990;Fonseca et al. 2013;Kuhn et al. 2015) are all used as input for the fabric tensor in contemporary continuum models for coarse grained materials (Wang et al. 2020). In contrast, for clay samples, the particle orientations are most often extracted as a measure for fabric from microscopy data (e.g., Cotecchia et al. 2019;Zhao et al. 2020), W/SAXS (Birmpilis et al. 2019), or SANS (Wensrich et al. 2018). However, the continuum models starting from the fabric measured experimentally at the particle scale are still in their infancy. ...
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... Delage and Lefebvre (1984) employed Scanning Electron Microscopy (SEM) and Mercury Intrusion Porosimetry (MIP) to study the effect of consolidation on intact, remoulded and reconstituted Canadian sensitive clays. Since the seminal work of Delage and Lefebvre (1984), Electron Microscopy and MIP have greatly contributed to revealing the evolving microstructure of artificial and natural clays before and after mechanical perturbation (Lapierre et al., 1990;Tanaka and Locat, 1999;Mitaritonna et al., 2014;Cotecchia et al., 2019). Hattab and her co-workers (Hattab and Fleureau, 2010;Hattab et al., 2013) studied the evolution of the fabric orientation in relation to stress history for, respectively, reconstituted kaolin and reconstituted natural clays. ...
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... Gasparre et al. (2007) characterised the distinct London Clay units according to their fabric using SEM. Cotecchia et al. (2019) demonstrated the clear link between the mechanical behaviour of the soil and the variations in the microstructure of an Apennine stiff natural clay. ...
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... With the development of image processing technology, it makes it possible to study soil microstructure. Microstructure parameters are usually obtained by means of scanning SEM (Cotecchia et al. 2019;Shi et al. 2018) and microscopic CT scanning technologies (Kikkawa et al. 2013;Matsushima et al. 2004). SEM, as a convenient and effective technology, has been widely used in the study of soil microstructure (Liu et al. 2005). ...
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Chapter
The paper presents a research approach in which the investigation of the macro-behaviour of a natural stiff clay through element testing is systematically combined with the analysis of the microstructural features of the clay and of the changes taking place at the micro-scale. The objective is that of recognizing internal features and processes causing specific behavioural facets and assess their influence on algorithms and parameter values adopted by models, with the final purpose of connecting classes of behaviour and corresponding models to classes of clays. The microstructural features of the undisturbed natural Pappadai clay are analysed first and then, by comparison, the microstructure evolution is checked under different loading paths. In the present paper, the microstructural assessment of the natural clay after one-dimensional compression to medium and large pressures is discussed. The clay fabric is qualitatively investigated by means of scanning electron microscopy (SEM); a statistical analysis of the orientation of particles is carried out by means of image processing, allowing to quantify the fabric orientation. The bonding strength is assessed by means of chemical micro-probing in the SEM and indirectly through the effects of on purpose strain paths affecting it. The pore size distribution of the clay is investigated by means of mercury intrusion porosimetry (MIP).
Book
Fabric analysis techniques x-radiography, petrography and scanning electron microscopy descriptions miscellaneous features in argillaceous rocks case studies of specific distinctive features case study of fabric analysis in evaluating sedimentary processes and environments formation of shale by compaction of flocculated clay-A model fabrics of some hydrocarbon source rocks and oil shales fabric of geopressured shale composition of argillaceous rocks.
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