interaction between geological and geotechnical investigations of a sandstone residual soil
TRANSCRIPT
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Engineering Geology
Interaction between geological and geotechnical investigations
of a sandstone residual soil
Flavia Burmeister Martinsa, Pedro Miguel Vaz Ferreirab,
Juan Antonio Altamirano Floresc, Luiz Antonio Bressanic,
Adriano Virgılio Damiani Bicac,*
aUniversidade do Vale do Rio dos Sinos, Sao Leopoldo, BrazilbUniversidade Federal do Rio Grande do Sul, Porto Alegre, Brazil
cUniversidade Federal do Rio Grande do Sul, CEP 90210 Porto Alegre, Brazil
Received 23 January 2003; received in revised form 12 October 2004; accepted 22 October 2004
Abstract
This paper discusses results of geological and geotechnical investigations carried out on a residual soil originated from the
weathering of a southern Brazil sandstone, the Botucatu formation. Triaxial compression tests with local strain measurement
were used for the evaluating stiffness and strength parameters of this soil. Its microstructure was investigated using both optical
and scanning electron microscopy. Important differences observed in relation to the expected geomechanical behaviour of
structured soils are explained by considering details of soil fabric shown by these microscopy analyses.
D 2004 Published by Elsevier B.V.
Keywords: Residual soil; Sandstone; Triaxial compression test; Microstructure
1. Introduction
From a geotechnical point of view, the presence of
microstructure in soils has been considered a relevant
research subject since the classical work of Leroueil
and Vaughan (1990). In that work, the authors
highlighted some important characteristics of the
geotechnical behaviour of structured soils and showed
that these characteristics are quite common in many
0013-7952/$ - see front matter D 2004 Published by Elsevier B.V.
doi:10.1016/j.enggeo.2004.10.003
* Corresponding author. Fax: +55 51 3316 3999.
E-mail address: [email protected] (A.V.D. Bica).
natural soils. Based on such concepts, research into
transitional soil/rock materials has intensified in the
last two decades (Cuccovillo and Coop, 1997). The
presence of microstructure has been identified even in
freshly deposited clays (Burland, 1990) and sands
(Schmertmann, 1991).
In this relatively new area of knowledge, both
geotechnical and geological concepts are essential in
order to build a common basis for understanding and
modelling the behaviour of structured soils. Consid-
ering the similar geomechanical behaviour shown by
some types of structured soils—as pointed out by
78 (2005) 1–9
F.B. Martins et al. / Engineering Geology 78 (2005) 1–92
Leroueil and Vaughan (1990)—it is important to
recognise the different behaviours that can result from
differences in soil microstructure. In this paper, both
geological history and microscopy analysis were
taken into account to explain some peculiarities
observed in the geomechanical behaviour of a residual
soil classified as a structured sand. According to
Barton (1993), transitional soil/rock behaviour is
better understood in clayey materials than in sandy
materials due to the relative facility with which
undisturbed samples of clays can be obtained from
boreholes. Despite this difficulty, Barton (1993)
observed that both interlocking and cementing can
bring cohesion to sands in a sand/sandstone transition
during a lithification process. On the other hand,
Dobereiner and de Freitas (1986) showed that a
structured sand can result either from incipient
consolidation of a freshly deposited sand or from a
weathering process acting on a sandstone. The
residual soil described in this paper derives from the
weathering of a southern Brazilian sandstone called
the Botucatu Formation. Although this material has
been classified as a soil, the original layered structure
of the parent rock is still quite visible. Its behaviour
during geomechanical tests, especially its stiffness and
strength characteristics, is better understood by taking
into account some geological factors. Emphasis is
given in this paper on the description of some
petrologic aspects of this residual soil and its relation
to observed stiffness and strength characteristics.
2. The Botucatu sandstone
The Botucatu sandstone is present across a wide
area of South America. This sandstone is of eolian
origin andwas deposited under desertic conditions over
an area of 1.5 million km2. It consists of a superposition
of paleodunes with typical cross-bedding stratification.
These sedimentary rocks are present in most of
Southern Brazil and are covered by thick Cretaceous
basaltic flows of the Parana Basin. In some places, the
Botucatu sandstone is more than 100 m thick (Scherer,
2000). This sandstone is found not only below the
basaltic flows but is also found between flows, being
related to the Lower Cretaceous period. Erosive
processes acting intensely on the border of these flows
exposed the Botucatu sandstone along a comparatively
narrow east–west strip of land located about 30 km
north of Porto Alegre, Brazil. An extensive inves-
tigation of the geology of Botucatu sandstone was
conducted by Scherer (2000). The author identified the
original process of formation of dune bodies by an
association with observed stratigraphic patterns. The
natural stratigraphy of Botucatu sandstone was related
to wind direction, wind intensity, age and the conse-
quent movement of dune bodies. According to Scherer
(2000), the Botucatu sandstone preserves its original
structure intact. It is one of the most representative
formations originated by an eolian sedimentation
deposition process.
A significant difference can be found between
geological and geotechnical terminologies used to
describe the residual soil of Botucatu sandstone (here
denoted as BRS soil). From a geological point of view,
since this material preserves the original fabric through
depths of tens of metres, it is called a rock. On the other
hand, from a geotechnical point of view, this material is
called a soil (C horizon) due to its low strength, as it is
easily excavated by hand. Different from most tropical
residual soils, in which original features of the parent
rock, like fabric and cementation, are preserved only
within a comparatively narrow C horizon that marks
the transition between soil and rock, BRS soils are
remarkable for showing very thick C horizons. The B
horizon is about 3 m thick, and the A horizon only
about 0.5 m thick. This contrasts with local residual
soils originated from the weathering of basalt, which
usually present a very thick B horizon and a compa-
ratively narrow C horizon. It also contrasts with local
residual soils of granite, which are often less than 3 m
thick. The thick C horizon observed in BRS soils is
explained by its porous fabric, which has a medium
hydraulic conductivity (kN10�7m/s). The consequent
good drainage is responsible for making the Botucatu
formation one of the largest aquifers of the world.
However, weathering of this sandstone did not act so
efficiently as to form a thick B horizon (as in residual
soils of basalt), probably due to the quartzitic nature of
its clasts. At the Vila Scharlau site, about 20 km north of
Porto Alegre, the BRS soil (C horizon), with its clearly
visible original fabric, is over 30 m thick. This paper
shows results of triaxial compression tests performed
on samples taken from the C horizon at this site
(Martins, 1994, 2001) and also from another site,
denoted as RS239 and located about 10 km from Vila
Fig. 1. Location of investigated BRS soils: (a) Brazilian sedimentary rocks; (b) Rio Grande do Sul geological units.
F.B. Martins et al. / Engineering Geology 78 (2005) 1–9 3
Scharlau (Ferreira, 1998, 2002). See the location map
on Fig. 1.
3. Aspects of behaviour identified by geotechnical
investigations
The presence of a relict microstructure—together
with its good drainage—gives to the BRS soil some
unusual engineering characteristics. For instance, steep
(N608) cut slopes are known to remain stable over
heights of up to 30 m. This is the case at the Vila
Scharlau site where the depth to the water table exceeds
30 m under normal rainfall conditions. However, at the
nearby site of Estancia Velha, a slope failure occurred
in the same type of BRS soil after a particularly intense
rainfall event. The failure caused the deaths of 10
people and some property damage (Bressani et al.,
1995). A conventional stability back-analysis could not
explain the failure adequately when normal unconfined
seepage was considered for evaluating pore pressures.
Failure conditions were simulated properly only when
higher pore pressure values were considered. During
the preliminary site investigation immediately after the
failure, piping holes were observed in the scarp left by
the failure. Water was observed seeping in large
quantity from the scarp surface, particularly from these
piping holes. The presence of a less permeable
compacted fill covering the BRS soil also supports
the assumption of high pore pressures before failure.
The piping holes were therefore considered an impor-
tant feature of this slope failure as they could be
associated with artesian pore pressures in the slip
surface region (Martins et al., 2001). The particle size
distribution of BRS soils shows a characteristic gap in
the silt fraction, which is consistent with a high
erodibility potential; this condition is favourable to
the development of piping holes. Conventional design
approaches must therefore be applied with caution to
geotechnical problems involving BRS soils.
At both sites, undisturbed block samples of BRS
soils were cut from the sloping sides of borrow pits at
a depth of about 7 m below the original ground
surface. Specimens of this type of soil could be easily
trimmed at the natural water content (wc12–14%),
using a soil lathe and a steel wire. However, when
submerged in water, these specimens disintegrated
completely, showing the weak nature of the cohesive
bonds. The unconfined compression strength of Vila
Scharlau soil at the natural water content was about
F.B. Martins et al. / Engineering Geology 78 (2005) 1–94
200 kPa. According to Dobereiner and de Freitas
(1986), sandstone samples that do not disintegrate
with submergence and full saturation show a saturated
unconfined compression strength of 500 kPa or
greater. These authors suggest that an important
difference between a weak sandstone and a structured
soil is that the latter does not have enough cohesion to
support its own weight near saturation.
Particle size distribution analyses were carried out
using the sieving and hydrometer sedimentation, with
samples previously submitted to mechanical disinte-
gration and pretreatment with sodium hexametaphos-
phate (A.S.T.M., 1998a). Atterberg limits were
obtained according to A.S.T.M. (1998b). As shown in
Fig. 2, samples of BRS soil from both sites have a
similar grading. This soil is a nonplastic granular
material composed of nearly 70% fine to medium sand,
18% silt and 12% clay-sized particles. The predom-
inance of the sand fraction is consistent with its eolian
origin. There is a marked gap in the silt fraction (which
is also observed in other residual soils of sandstone).
The BRS soil can be described as a silty, clayey
medium to fine sand. At the Vila Scharlau site, the field
void ratio was about 0.7. At RS239 site, it was varying
from 0.6 to 0.7. The hydraulic conductivity is of the
order of 10�7 m/s (da Cunha, 1997). Suction values
were measured directly in the field using a miniature
suction transducer (Ridley and Burland, 1995).
Although the suction at the natural water content was
only about 40 kPa, it helps to sustain the soil fabric in
Fig. 2. Particle size distribution of BRS soil
the unconfined state. Table 1 presents some of main
physical characteristics identified for these samples.
An extensive series of triaxial compression tests
with local strain measurement were performed on
undisturbed samples from both sites by Martins
(2001) and Ferreira (2002). These tests revealed the
existence of well-defined yield surfaces (Fig. 3). Such
yield surfaces are similar in shape to those found in
other geomaterials due to overconsolidation and/or the
presence of microstructure. As described by Leroueil
and Vaughan (1990), if a stress path is imposed on a
structured soil so that it reaches the yield surface, a
loss of stiffness is noted as a result of the breakage of
particle bonds. This loss of stiffness can be observed
on the stress–strain curve of each test and is associated
with the yield point. Each yield point then represents
structural damage caused by loading of the soil
specimen. The yield surfaces shown in Fig. 3 were
obtained by the interpolation of yield points deter-
mined from several triaxial tests. The RS239 soil
(with initial void ratios between 0.66 and 0.7) shows a
yield surface considerably larger than the Vila
Scharlau soil (initial void ratio of 0.7). Both yield
surfaces are associated with the BRS soil micro-
structure. A discussion about the origin of these
differences is presented in Section 4.
Fig. 4 shows results of an isotropic compression
test performed with an undisturbed sample of BRS
soil, with the specimen axis perpendicular to the
bedding planes. In this test, both axial and radial local
s from RS239 and Vila Scharlau sites.
Table 1
BRS soil main physical characteristics
Field void ratio (eo) 0.59–0.72
Field water content (wo) 10–14%
Specific gravity (G) 2.65
Plasticity Index nonplastic
Dry unit weight (cd) 15.4–16.8 kN/m3
Hydraulic conductivity coefficient (k) c10�7 m/s
Field suction (S) 35–45 kPa
Fig. 4. Isotropic compression test on BRS soil: (a) void ratio vs
mean effective stress curve; (b) radial strain vs. axial strain curve.
F.B. Martins et al. / Engineering Geology 78 (2005) 1–9 5
strains were measured using Hall effect displacement
transducers (Clayton et al., 1989). On Fig. 4(a), the
void ratio vs. mean effective stress curve shows a
clear yield point around p’=300 kPa; corresponding to
an axial strain of about 0.3%. Note on Fig. 4b the
large difference in magnitude between the measured
radial and axial strains, especially during the first
stage of isotropic loading. During this stage, the BRS
soil is clearly much stiffer in the radial direction than
in the axial direction. In addition, a sharp change of
slope is observed in the radial strain vs. axial strain
curve after ea~0.1% is reached. This change suggests
the occurrence of a collapse in the radial direction.
This pattern of behaviour could not be identified when
the test data were plotted on the usual void ratio vs.
mean effective stress curve. Axial and radial local
strain measurements were therefore essential as they
also revealed the anisotropic behaviour of the BRS
Fig. 3. Yield surfaces of BRS soils from RS239 and Vila Scharlau sites [ pV=(rV1+2rV3)/3; q=(rV1�rV3)].
.
Table 2
Average point counting composition of BRS soil
Mineral Percen
Monocrystalline quartz 29.8
Policrystalline quartz 12.8
Porous secondary clay matrix (pores+clays) 29.5
Mudstone and siltstone fragments 7.3
F.B. Martins et al. / Engineering Geology 78 (2005) 1–96
soil. This anisotropy is clearly related to soil micro-
structure (involving fabric and cementation). Triaxial
tests on samples of BRS soil showed further
anisotropic aspects in the stiffness and strength
behaviour. This is discussed in more detail by Martins
(2001) and Ferreira (2002).
Opaques and turmalines 3.5
Iron oxides 9.9
Metamorphic/micas/schists 7.2
4. Aspects of behaviour identified by geologicalinvestigations
In the BRS soil, it is possible to identify by the
unaided eye an alternate sequence of white and pink
bands approximately 1 mm thick (bedding planes).
Optical microscopy analysis and point counting were
conducted on thin sections of this soil. Air-dried
samples were subjected to impregnation with a blue-
dyed epoxy resin, and thin sections were afterwards
cut perpendicular to the bedding planes. When a thin
section is observed under a cross-polarized light (Fig.
5), it is possible to identify the presence of clay
particles filling the voids between the quartz grains.
These particles are recognised in Fig. 5 as multiple
small refraction plans in the clay matrix. Based on this
analysis, it is possible to state that the BRS soil
consists essentially of detritic quartz grains in a clay
matrix of secondary origin. Point counting average
results are given in Table 2.
Considering the eolian origin of this soil, it is
interesting to ask about (1) the formation of the clay
Fig. 5. Undisturbed BRS soil under cross polarised light (XPL)
showing subrounded monocrystalline quartz grains (Qtz) and a
porous region. In this region, it is possible to observe multiple
refraction plans, identified as kaolinite particles (Kln) in the voids
between quartz grains (Qtz).
Fig. 6. Photomicrograph of undisturbed BRS soil under natural ligh
(PPL): iron oxides contours (IOC) marking the previous existence
of K-feldspar grains.
t
particles, and (2) how these particles contribute as a
cementing agent to the geomechanical behaviour of
BRS soils. Fig. 6 provides the answer to the first
question. It shows another thin section where it is
possible to identify contours of iron oxides marking
the preexistence of feldspar grains. These contours are
called feldspar phantoms. Clay particles in the void
spaces between the quartz grains therefore derive from
the weathering of feldspar grains of detritic origin. For
the Vila Scharlau soil, it was not possible to identify
any intact feldspar grains as the weathering process
had completely transformed the feldspar in clay. This
was not the case for the RS239 soil, where it is not
difficult to find feldspar grains.
X-ray diffraction and Scanning Electron Microscope
(SEM) analyses showed that clay particles in the BRS
soils are mainly of the kaolinite type. Fig. 7 shows an
electron photomicrograph in which it is possible to
observe theweathering process that transforms a feldspar
grain in kaolinite. As a result of this, sand-sized feldspar
t
Fig. 8. (a) Reconstituted BRS soil showing kaolinite particles (Kln
covering a fine quartz (Qtz) grain; (b) undisturbed BRS soi
showing its structured character associated with kaolinite (Kln) and
iron oxide bonds (Fe).
Fig. 7. Undisturbed BRS soil: weathering of K-feldspar grains (Kfs)
originating kaolinite particles (Kln).
F.B. Martins et al. / Engineering Geology 78 (2005) 1–9 7
grains are transformed directly into clay particles without
forming intermediate silt-sized particles. This helps to
explain the gap in the silt fraction shown by the particle
size distribution of these soils (Fig. 2).
With respect to the importance of the clay matrix as a
cementing agent, Fig. 8 compares two samples of BRS
soil. Fig. 8(a) shows the electron photomicrograph of a
sample of BRS soil that was reconstituted at the field
void ratio (eo=0.7). In the reconstituted sample, kaolinite
particles are seen covering the surface of quartz grains.
An entirely different fabric can be identified in the
undisturbed sample (Fig. 8(b)). In this sample, clay
particles are not randomly distributed inside the void
spaces but instead form bridge-like clusters that link
quartz grains. These clusters appear to be reinforced by
an iron oxide coating that gives a uniform aspect to the
secondary clay matrix. Individual void spaces are bigger
in the undisturbed sample than in the reconstituted one
(despite the same void ratio for both samples). The
secondary clay matrix is thus very porous and fills the
void spaces between quartz grains as a cementing agent.
So, clay and iron oxides can be regarded as neo-formed
cements in this residual soil. These cements are super-
imposed on the fabric inherited from the parent rock,
which in turn resulted from deposition.
Another type of cement can be identified by
examining the thin section shown in Fig. 9. It consists
of an autigenic quartz growth. Its presence is less
important for the Vila Scharlau soil. For the RS239 soil,
however, optical microscopy analyses showed a sig-
nificant presence of this cement. This highlights the
main difference between the cement components of
BRS soils. A more siliceous cementation is present in
the RS239 soil, while iron oxides coating a clay matrix
constitute the major cement component of the Vila
Scharlau soil. The cement component of the RS239 soil
can therefore be regarded as stronger in comparison
that of the Vila Scharlau soil. This can explain the larger
yield surface observed for the RS239 soil (Fig. 3).
Anisotropic aspects were identified in stiffness,
shear strength and hydraulic behaviour. Undisturbed
BRS soils are stiffer and stronger when loaded in a
direction parallel to the bedding planes. Some
interesting details of the geomechanical behaviour of
)
l
Fig. 9. Photomicrograph of undisturbed BRS soil under cross-
polarised light (XPL): autigenetic quartz growth (qtz) acting as a
cementing component around the detritic quartz (Qtz). The contact
between the quartz grain and the cement is defined by an iron oxide
(Fe) cover.
F.B. Martins et al. / Engineering Geology 78 (2005) 1–98
this residual soil could only be understood after its
microscopic fabric was investigated. Figs. 5 and 9
show the stratigraphic pattern of the original fabric.
Due to cyclic time variation of the transport agent
competency—the wind—the original eolian sediment
was formed as an alternate sequence of thin fine and
medium sand layers. It is interesting to note that the
fine sand layers appear to show a less porous
arrangement of grains and iron oxides (black coatings
on Figs. 5 and 9). This observation suggests that it
may have a lower hydraulic conductivity. The
observed greater concentration of iron oxides in this
layer could result from water retention by capillary
action, a condition that favours the precipitation of
iron oxides. Therefore, due to better drainage, weath-
ering processes could act more intensely in the
medium sand layers. For these layers, the number of
contacts between grains is clearly much smaller in
comparison with the fine sand layers. Such contacts
are essentially punctual, while tangential contacts are
seen in the fine sand layers.
5. A mechanistic picture based on geological and
geotechnical considerations
Based on the geological investigation of BRS soils,
it is possible to state that their fine sand layers can be
expected to show a stiffer and stronger behaviour in
comparison with their medium sand layers. But how
exactly do these geological concepts explain the
observed geomechanical behaviour?
For the first stage of the isotropic compression
loading shown in Fig. 3, radial strains are much lower
than axial strains. In a direction parallel to the bedding
planes (i.e., the radial direction of this specimen), both
fine and medium sand layers have to deform in order
to allow any change of specimen diameter. But the
larger concentration of iron oxides and/or siliceous
cement components within the red bands of this soil
means that the fine sand layers act as a reinforcement,
helping to prevent the development of significant
strains in the radial direction. Along the vertical axis,
the less stiff medium sand layers can deform
independently of the fine sand layers.
For axial strains higher than 0.1%, a well-defined
yield point can be associated with the increase of
radial strain. It is possible to assume that the yield
point identified on the first stage of the compression
loading is associated with the breakage of cemented
particle bonds. But even after this yield point is
crossed, radial stiffness remains significantly larger
than axial stiffness. It seems that fabric also has an
important influence on soil behaviour in addition to
cementation. The denser packing of the fine sand
layers acts in order to prevent radial strains. The
observed anisotropic behaviour is associated with the
pattern of soil fabric shown by BRS soils (i.e., the
alternation of fine and medium sand layers) and
especially with the significant stiffness differences
between these layers.
The origin of piping holes in BRS soils can also be
better understood based on this microscopy analysis.
The erosive process that takes place in this soil is not
only associated with its sandy character but essentially
with its grading. The gap observed in the silt fraction
results in a granulometric internal instability. This gap
allows—for a sufficiently high hydraulic gradient—
clay particles to move through the sand-sized pores.
6. Conclusions
Optical and electron microscopy investigations that
were carried out on a sandstone residual soil (BRS soil)
helped to explain some important aspects of the
geomechanical behaviour observed with triaxial tests.
F.B. Martins et al. / Engineering Geology 78 (2005) 1–9 9
The BRS soil is a structured sand that intercalates thin
white and pink layers that are different not only in
colour but also in grading, composition and porosity.
The colour of the white layers is given by kaolinite
particles whose origin is associated with the weathering
process acting on detritic feldspar grains. This process
was of major importance in these white layers due to
the larger size of the voids and the consequent higher
hydraulic conductivity in comparison with the pink
layers. Fine sand grains compose these pink layers,
forming a denser packing. As a result of its lower
hydraulic conductivity, the pink layers show higher
iron oxides and/or siliceous cement concentrations.
Due to both the denser packing and the higher iron
oxides and/or siliceous cement concentrations in the
pink layers, the BRS soil is stiffer in the direction of
these layers. The pink layers work as a reinforcement of
the soil and both cementation and fabric have a
significant influence on stiffness and strength.
BRS soils from two sites were studied. These soils
are similar in porosity, mineralogy, grading and fabric.
Despite these similarities, significant differences were
found between the corresponding yield surfaces
(structural damage) identified for these two soils.
These differences could only be explained by micro-
scopy observations and analyses that showed that the
cement components of these soils are essentially
different. A siliceous cement prevails in the RS239
soil, while iron oxides cement is more important in the
Vila Scharlau soil.
Acknowledgements
The work of F.B. Martins has been supported by
CAPES; P.M.Ferreira and L.A.Bressani have been sup-
ported by CNPq, Brazilian research sponsoring agencies.
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