Global Journal of Engineering Sciences (GJES)
Iris Publishers
Authored by Simona Guglielmi
The paper presents part
of a research work in the field of the interpretation of the mechanical
response of clays to support their constitutive modelling, according to an
approach which combines the investigation of the soil element macro-behaviour,
through laboratory experimental testing, with the observation of the soil
features and processes at the micro-scale, through scanning electron microscopy,
SEM.
The effect of compression on
the mechanical behaviour of clays, once reconstituted in the laboratory, has
been the subject of experimental studies for many years, and has been extended
to the behaviour of natural clays, which develop, in their geological history,
different structure from that forming in the reconstituted during normal
consolidation in the laboratory [1-3]. To date, several studies in the
literature have shown that processes other than simple compression can result
in an increase in the strength of clays [1-6], one example being diagenesis,
which is one of those geological processes that cause an increase in stiffness
and strength of the natural clay above that provided solely by the reduction in
volume with compression.
Diagenetic processes, which
are responsible for important changes in natural clays at the microscale,
typically occur at depth, under high effective stresses. They give rise to the
aggradation of swelling minerals, the increase in bonding (non-frictional
interparticle forces [7,8]) and changes in fabric (the arrangement of the soil
particles [7,8]). As a result, natural clays can develop a structure stronger
than that of the corresponding (same void ratio) reconstituted clay.
This paper examines the
microstructure of a natural stiff, diagenetically modified clay, by comparison
with the microstructure of the same clay when reconstituted in the laboratory.
This comparison is then extended to explore the evolution of microstructure
after 1D compression to states preand post- gross-yield, up to large pre eological
history is known in some detail [9]. This study is one aspect of a wider
research [2,3,10-12] 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.
Materials and Methods
Pappadai clay is a Pleistocene
marine clay which was deposited in a quiet sea in the Montemesola Basin, near
Taranto. The engineering geology of the clay is discussed by Cotecchia &
Chandler (1995), who present paleontological and microstructural data
demonstrating both the stillness of the water and the reducing conditions of
the depositional environment. The main properties of the clay and its
mineralogical composition are shown in Table 1.
Table
1:Index Properties and mineralogy of Pappadai clay.
The clay is of high
plasticity and possesses a high carbonate content. Cotecchia & Chandler
(1995) have shown that the carbonate content is largely concentrated in the
sand and the silt fractions of the soil, since the carbonates increase with
reducing clay fraction and plasticity index [13]. Large part of the sand and
silt fractions are formed of shells of foraminifera and nannofossils. However,
as will be seen, at least part of the carbonates present contribute to bonding
in the clay.
Block samples were taken from
a depth of 25 m during construction of a reservoir draw-off shaft and were
subjected to mechanical testing [2]. The clay forming the block samples is
massive and regularly laminated.
At the sampling location the
clay is overconsolidated due to the erosion of some 120 m of overlying sediments.
Prior to erosion the clay had undergone diagenesis resulting in a decrease in
the portion of smectites, an increase in the portion of the nonswelling
minerals (intergrades, Table 1), and a decrease in activity, all with respect
to depth [9]. Thus, it is likely that there will have been changes in
microstructure of the clay under the stress levels required to generate the
mineralogical changes, that is towards the end of normal consolidation.
After unloading due to
erosion, the clay in the top strata of the deposit underwent deep drying and
subsequently swelled. Cotecchia e Chandler (1995) confirmed this reconstruction
through the modelling of the state history of the deposit [9], the results of
which are shown in Figure 1. The current profile of liquidity index against
vertical effective stress at Pappadai and the modelled one are compared in
Figure 1. The history of normal consolidation (stage (i)-O→P), erosion (stage
(ii)-P→N), drying of the sole top strata due to lowering of the water table (stage
(iii)-N→M) and water table rise (stage (iv)-M→B) was modelled and resulted in
the state paths represented in the figure as continuous lines.
The last profile is “S” shaped
and, as such, it is consistent with the profile of liquidity index on the
undisturbed samples taken at different depth along a borehole and a shaft
nearby; this latter one is the block sample B in the figure. Such undisturbed
block sample is that which the experimental data presented in the following
refer to. Its preconsolidation state corresponds to point P in Figure 1, as for
an overconsolidation ratio (OCR) of 3.1. Such OCR resulted solely from the
erosion cycle (ii) cited before, since the sample was not affected by the deep
drying, which instead affected the clay layers above. For the full explanation
of the reconstruction of the geological history of the clay and of the state
history modelling, whose results are sketched in Figure 1, refer to [2,9].
The natural clay was
reconstituted in the laboratory at a water content 1,6x liquid limit, and was
one- dimensionally consolidated in a consolidometer to a vertical effective
stress of 200 kPa, point A* in Figure 1, at which point it was removed from the
consolidometer and placed in an oedometer where the one dimensional loading was
continued. As seen in Figure 1, natural Pappadai clay followed a sedimentation
compression curve (SCC; see [14] for the definition, or [1,2]) to the right of
the normal consolidation line of the reconstituted clay (ICL; [1]). Thus, after
a normal consolidation to point P, the natural clay already had a structure
different from that of the reconstituted clay, and later underwent further
changes through diagenesis.
The qualitative analysis of
the clay fabric is carried out on both the natural and the reconstituted clay
by means of scanning electron microscopy (SEM), using freeze-dried gold-coated
clay specimens trimmed from the undisturbed block sample and from the specimens
subjected to 1D compression after completion of the test and rapid undrained
unloading. The microstructure of the natural clay is examined at different
stages of 1D loading, i.e. at undisturbed state, soon after gross yield and to
large pressures, and compared to the microstructure of the reconstituted clay
after consolidation in the consolidometer and compressed to large pressures.
SEM micrographs taken on
vertical fractures are shown, and sketches are reported in which fabric
features are identified and local fabric arrangements are recognized. The
orientation of the fabric is then quantified by means of digital image
processing [15] on micrographs of size corresponding to the micro-scale
representative element volume (micro-REV, [12]), recognised for the clay under
study as the clay portion of size about 10-3 mm3, investigated at the medium magnification
[12]. The microstructural data are related to the observed macro- response of
the clay and to the constitutive hydro-mechanical parameters, highlighting what
the constitutive laws deviced to represent the material macroresponse in the
frame of porous media hydro-mechanics are reflecting at the micro-scale.
The bonding of
Pappadai clay is such that a small undisturbed sample softens only a little
after being submerged in water for several months. Drying at 120°C caused the
clay to open up along the contact of the clay with silt bedding planes, and the
softening of the clay on submergence was then more significant. Hence, the
diagenetic bonding of Pappadai clay is significantly reduced by drying.
The fabric on a
vertical fracture of reconstituted Pappadai clay, A* in Figure 1, is shown at
medium-high magnification in Figure 4. As with the natural clay, a non-uniform
orientation of the clay particles took place during one-dimensional normal
consolidation. Both stacks and randomly oriented flocculated fabric areas can
be recognized (Figure 4b). The image processing of several medium magnification
micrographs allows to identify values of L in the range 0.23-0.27, indicative
of a well oriented fabric.
So, although the
reconstituted fabric (see for example Figure 4c) is found to be more open than
that of the natural clay, as expected given the difference in void ratio
between points A* and B in Figure 1, the natural fabric is not more oriented.
Rather, the natural fabric appears to have less regular alternations of
oriented and flocculated fabrics than the reconstituted, despite the much
higher preconsolidation pressure, hence appearing far more complex, probably
the consequence of diagenesis.
The average gross
yield stress (i.e. the stress beyond which a transient major stiffness decay
occurs, as reported by [3]) for the reconstituted clay is 200 kPa (Figure 5),
i.e. the maximum stress attained in the initial consolidation in the
consolidometer. Beyond gross yield, the CRS oedometer test on the reconstituted
clay defines the intrinsic compression line (ICL; [1]) of Pappadai clay, for
the stress range 0.2-22 MPa.
The current state of
the natural clay in situ is indicated as C in the figure with a vertical stress
at the sampling depth, σ’v0, of 415 kPa. Representative compression and
swelling curves are shown, some compression stages prior to swelling being
omitted for clarity.
The gross yield state
for the natural clay lies far to the right of both the current clay state C and
the ICL, confirming that the natural clay is both overconsolidated and also has
a different structure from the reconstituted clay. If overconsolidated only by
simple geological unloading, the clay gross yield should lie at about the
preconsolidation state (Figure 5), which also lies to the right of the ICL
since the structure of the natural clay was already stronger than the
reconstituted clay after normal consolidation (Figure 1). However, the natural
clay does not gross yield until well beyond the preconsolidation stress, since
the gross yield stress is σ’y≈2600 kPa (Yield, Figure 5). This observation
suggests that a strengthening of the clay structure occurred as a result of
additional bonding acquired during diagenesis, increasing the gross yield
stress of the clay. Consequently, the current yield stress ratio YSR= σ’y/ σ’v
of the natural clay, that is the ratio of the yield stress σ’y to the current
vertical stress is σ’v (YSR=σ’y/ σ’v), is twice the value of the clay’s
geological OCR.
It follows that
Pappadai clay is an example of a stiff clay which owes its high strength not
only to its considerable compression during normal consolidation, but also to a
strengthening of the clay structure due to diagenetic bonding. The effects of
such diagenetic bonding are evident in the mechanical response of the clay to
loading.
In test OED 7 (Figure
5), a natural clay sample was unloaded from its in situ state, then reloaded
(reloading is not shown in Figure 5). In the laboratory the undisturbed state
corresponds to a suction of about 700 kPa, measured by the filter paper method
[20]. In the initial swelling stage of test OED 7 the response of the
undisturbed clay to unloading is quite stiff, much stiffer than after
compression to gross yield.
In Figure 6 the whole
test OED 7 is shown, together with another similar test, OED 5, in which the
sample was loaded from the undisturbed state. It can be seen that sample OED 7
gross yields at a lower stress (1800 kPa) than the that exhibited by the clay
in test OED5, as a consequence of the unloading before reloading. Not only does
the swelling process reduce the gross yield stress (hence reducing the YSR),
but it also results in the post-yield compression line differing from that of
the undisturbed clay. This illustrates the level of weakening induced on the
structure of Pappadai clay by a large unloading path and subsequent reloading.
This weakening, although limited, appears to be due to the slow cyclic (rather
than monotonic) unloading-reloading path, which is causing the degradation of
the amorphous calcite film binding the particles.
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