the causes of the high friction angle of dutch organic soils.pdf
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The causes of the high friction angle of Dutch organic soils
X.H. Cheng a ,, D.J.M. Ngan-Tillard a, E.J. Den Haan b
a Department of Geotechnology, TU Delft The Netherlandsb GeoDelft Delft The Netherlands
Received 24 May 2006; received in revised form 28 February 2007; accepted 7 March 2007
Available online 12 April 2007
Abstract
Dutch organic soils have been found in past experiments to possess extremely high effective strength parameters. Since this
finding is not expected and the phenomenon has yet to be explained, the high yield strength value is not used in practice.
Understanding the abnormal properties of Dutch organic soils would thus be beneficial from the practical point of view. A
programme aimed at understanding the unusual properties of Dutch organic soils, non-peat soils in particular, was performed on the
representative organic soils in Dutch nature reserve park, Oostvaardersplasen (OVP) near Almere. Highly variable fabric of these
organic soils was characterized by Computed Tomography X-ray scanner and environmental electronic microscope. Recognized
fabric is in line with the geology of the OVP site. The multi-scale investigation as presented eventually identified the major role
played by subhorizontal laminae and other non-organic microstructural elements (microfossil skeleton) in the high values of
OVP organic soils. Deformation mechanisms of the microstructural elements are proposed and these make the unusual
geotechnical properties explainable. Organics as involved were believed to have a primary contribution in increasing Atterberg
limits and compressibility, and to allow the generation of high pore water pressures and low effective confining pressures during
shearing. It has been also observed that high value is always correlated to the low effective confining pressure.
2007 Elsevier B.V. All rights reserved.
Keywords: Organic soils; Microfabric; Laminated soils; High friction angle
1. Introduction
Past experiments involving a large number ofundrained triaxial tests conducted on Dutch organic
soils have led to the conclusion that the soils possess
extremely high effective strength parameters which
increase while the bulk density of the soil decreases
(Den Haan, 1995). Since this finding is not in line
with expectations and the phenomenon has yet to be
explained, the high yield strength value is not used in
practice.
Organic soils cover a large part of the Netherlands.Infrastructure lines are founded on top of organic
deposits and dikes are made of organic soils. Under-
standing the abnormal properties of Dutch organic soils
would thus be beneficial from the practical point of
view. Depending on the mechanisms responsible for the
high effective strength of organic soils, less conservative
values could be used in the design of embankments, and
more realistic estimations of the strength of dikes could
be made. One might also think of mimicking nature to
strengthen soils.
Engineering Geology 93 (2007) 31 44
www.elsevier.com/locate/enggeo
Corresponding author.
E-mail address:xiaohuicheng@hotmail.com(X.H. Cheng).
0013-7952/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2007.03.009
mailto:xiaohuicheng@hotmail.comhttp://dx.doi.org/10.1016/j.enggeo.2007.03.009http://dx.doi.org/10.1016/j.enggeo.2007.03.009mailto:xiaohuicheng@hotmail.com -
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An international literature review reveals that organic
soils with a high effective friction angle, are not
unique to the Netherlands:
a) An apparent of 34 was considered as particu-
larly high for a slightly organic clay known as theBothkennar clay in Britain (Hight et al., 1992). The
dominant angular silt fraction was thought to be
responsible for the high-, and the 2 to 4% organic
content was found to increase Atterberg limits.
During testing, the effective mean stress ranged
between 25 and 150 kPa.
b) Relatively highbetween 25 and 40 are reported
for the reconstituted Osaka bay clay (Tanaka and
Locat, 1999). A microfossil identified as diatom was
accounted for both the high- and high Atterberg
limits. The role of 2 to 4% organic content was notemphasized. An effective friction angle of 44 was
also found for the diatomaceous fill that has a low dry
density and high moisture (Day, 1995). The reasons
for this high effective friction angle were given by
the interlocking and rough surface features of
diatoms at low stress level. Effective mean stresses
between 100 and 300 kPa were recorded during
testing. This is higher than the stress levels reported
by others for clays with a higher organic content.
c) The 10% to 60% organic content of Juturnaiba
organic clays in Brazil was found to increase the
of natural samples according to the followingempirical relationship: = 23 + 0.5766 OC with
OC, the organic content expressed in percentage
(Coutinho and Lacerda, 1989). Angles were found to
increase up to 57 with effective mean stress
pressures decreasing from 300 to 50 kPa.
d) A very high ranging between 60 and 90 was
found for Swedish clayey gyttja with 10% organiccontent (Larsson, 1990). It was related to the
abundance of micro-fibers discovered by electron
scanning microscopy. However, whether the micro-
fibers are organics or was not established. Effective
mean stresses up to 100 kPa and as low as 15 kPa
were recorded during testing.
e) Organic clay from Cubzac-les-Ponts in France with
organic contents up to 25% and bulk densities in the
range 12 to 16 kN/m3, did not reveal particularly
high angles (Shahanguian, 1981). From the
reported CIU tests on normally consolidated clayunder effective mean stresses between 30 and 70 kPa,
back calculation reveals only 2834 or less.
f) Krieg (2000) studied the geotechnical properties of
various organic clays from Schwerin, Berlin and
Rotterdam, and found values ofranging from 44
to 74. Bulk density varied from 1.2 to 1.5 t/m3 with
organic contents up to 30%. Diatoms and remains of
plant fibers were thought to be the cause of the high
strength values. During testing, recorded minimum
effective mean stresses were above 50 kPa.
The multi-scale fabric of organic soils is highlyvariable and the fundamentals of their behaviour are not
Table 1
Geotechnical classification and index properties of OVP organic soils
Depth
(G.L., m)
Geotechnical classification
and identification.
N(%) sa D50
(m)
Samples for triaxial compression
No. Depth (m) Wn (%) n (g/cm3) a/r
1.162.06 (a) Very organic brown silty clay,
with very closely spaced thin laminae
of fine grained silt (Almere deposits)
1020 2.2552.455 5 CIU10A 1.181.35 144 1.332 1.0
CIU10B 1.351.53 167 1.295 1.5
2.062.78 (b) Very organic clayey dark brown silt,with very closely spaced thin laminae of
medium-grained silt (Almere deposits)
30 2.085 12 CAU11B 2.062.26 203 1.226 CAU11D 2.262.44 212 1.210
CIU11C 2.642.84 257 1.145 1.7
2.562.93 (c) Dark brownblack peat, with
significant amounts of fine wood fibers.
Spongy structure
2.933.40 (d) Very organic light gray clayey silt,
with very closely spaced thin laminae
of mediumgrained silt and with
vertical rootlets
33 2.039 8 CIU12C 3.043.24 303 1.149 2.0
CIU12A 3.243.42 148 1.264 1.0
3.404.58 Mixed layers of very organic soils: (c) peat soil+ 70 1.602 CIU14A 3.413.51 355 1.074 2.0
CIU14B 3.513.61 360 1.074 1.5
(d) Very organic clayey SILT CAU13 4.024.22 323 1.198
a
Calculated by the relation 1/s=N/1.365+(1
N)/2.695. This relation was obtained for Dutch organic soils according to the method used bySkempton and Petley (1970)for organic soils. The temperature used in loss-on-ignition test (N) was 550 C.
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well understood. A programme aimed at understanding
the unusual properties of Dutch organic soils, non-peat
soils in particular, was launched in the 1990's. The
Dutch nature reserve park in Oostvaardersplasen (OVP)
near Almere, the Netherlands, was chosen as sampling
site. The OVP Holocene soil deposits are believed to berepresentative of Dutch organic soils, which consist of
layers of organic clay and silt and peat.
The geology of the OVP site can be schematised
usingVan Loon and Wiggers (1975). After the sea level
rose at the end of the last ice age (Weichselien up to
8000 BC), a peat layer was formed above Pleistocene
sands. Sea transgressions and regressions succeeded
each other. During transgressions large areas of peat
were eroded and clay was formed. During regression,
peat was formed. At the end of the subboreal (3000 to
900 BC) several sea transgressions led to the formation
of the Flevo lake. Peat was eroded by wave action and
the lake expanded. A detritus-gyttja layer started to format about 1250 BC from the eroded peat in the fresh water
environment of the lake. Around 0 BC, the connection
of the Flevo lake to the Wadden Sea to the North became
wider. In the fresh to slightly brackish waters of the
newly formed Zuiderzee lagoon, the silts and clays of
the Almere deposits were formed. They contain an
upwards diminishing amount of organic matter derived
Fig. 1. Stress paths and radial strain changes (a) silts b and d (b) clay a and peat c.
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from peat and gyttja and several laminations of various
thickness and gradation. The laminations are believed to
be the result of wave and storm actions (Van Loon andWiggers, 1976). At about 1600 AD, the connection to
the Wadden Sea rapidly widened even further. The water
became brackish to salty and the marine deposits of the
Zuiderzee were formed. The Zuiderzee was closed by a
dike in 1932. Lake Ijssel was created and then reclaimed
to form the Zuiderzee polder in 1968. Human activities
resulted in the formation of a thin and reworked cover of
fresh water soils: the Ijsselmeer deposits. The OVP are a
nature reserve in this polder where little reworking has
taken place. The upper layer has gone through several
stages of weathering in recent decades.
From the results of a preliminary laboratory testingprogramme which focussed on the soft organic clays of
the Almere deposits, the samples were found to fail in
undrained triaxial compression with values of 50 to
60. The values were calculated assuming zero
cohesion and the mobilized effective angle of internal
friction was at maximum. The high values were
found to remain irrespective of the sampler, sample size
and consolidation history (Den Haan, 2003). The latter
results eventually stimulated the multi-scale investiga-
tion of the OVP organic soils, excluding peat, by using
advanced microscopy technologies. The results arepresented hereafter. In the study, one attempts to relate
the presence of microfossils, observed by means of
electronic microscope, and the presence of silty lamina-
tions, recognized by Computed Tomography X-ray
scanner and electronic microscope to the high resistanceof the OVP soils. The use of these technologies is shown
to be decisive for the comprehension of the behaviour
of heterogeneous soils like OVP organic soils, even if
it is not new. X-ray CT scanner allowed a major
breakthrough in the understanding of shear strain
localisation in sand specimens subjected to triaxial
testing (Desrues et al., 1996). It also appeared suitable
for the study of strain localisation in clayey materials
in few occasions, when compacted shear bands form
under large confining pressures (Tillard, 1992) or
existing fissures affect macro-cracking (Sun et al.,
2004). To discover the real-time fabric change ofclayey soils at different loading states, environmental
scanning microscopy (ESEM) is useful, particularly
when equipped in the ESEM chamber with a micro-
loading module (Cheng et al., 2004 and Cheng,
2004).
2. Triaxial compression of OVP organics soils:
Macro-level observations
2.1. Profile description
Samples down to 5 m below the ground level were
recovered using the Delft Continuous Begemann
Table 2
Triaxial compression properties of OVP organic soils
Soil type Test No. End of consolidation Failure, i.e. maximum shear stress state
a() v () r() 3 (kPa) s (kPa) a() r() t(kPa) s (kPa) 3,f(kPa) () assuming c = 0
Organic silt CIU11C 0.15 0.33 0.09 83 83 0.29 0.02 40 57 17 45
CIU12A 0.05 0.17 0.06 39 39 0.17 0 17 25 8 43CIU12C 0.19 0.38 0.09 121 121 0.30 0.04 49 81 32 38
CAU11B 0.20 0.16 0.02 30 45 0.26 0.05 27 34 7 52
CAU11D 0.16 0.18 0.01 19 32 0.22 0.02 23 28 5 56
CAU13 0.11 0.09 0.01 25 38 0.13 0.02 19 23 4 56
Organic clay CIU10A 0.05 0.15 0.05 41 41 0.13 0.01 22 31 9 46
CIU10B 0.12 0.27 0.08 83 83 0.35 0.03 35 57 22 38
Peat CIU14A 0.08 0.17 0.04 40 40 0.17 0 22 25 3 63
CIU14B 0.15 0.33 0.10 79 79 0.31 0.01 41 49 5 65
Table 3
Variation in and c with organic soil type and effective confining stress
OVP a very organic silty clay 2 tests:
CIU10A and CIU10B
OVP b, d very organic clayey silt OVP c peat 2 tests:
CIU14a and CIU14b3 CIU tests CIU11C,
CIU12C and CIU12A
3 CAU tests CAU11B,
CAU11D and CAU13
() 29.7 35.8 (R2= 0.974) 44.0 (R2= 0.990) 64.3
c(kPa) 7.7 4.5 (R2= 0.974) 4.6 (R
2= 0.990) 0
n
(g/cm3) 1.314 1.186 1.211 1.074
3,f(kPa) 8.6; 22 8.1; 16.7; 31.6 4.0; 4.6; 7.1 3.0; 5.3
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sampler. FromDen Haan (2003), it can be expected that
they are high quality samples which behave as Laval
samples during consolidated triaxial testing. Visual
inspection of the OVP cores and index properties
(bulk density, specific density, natural water content,
loss-on-ignition and particle size distribution) weredetermined. The results from the analysis are summa-
rized inTable 1. Sand layers and the top deposits are not
considered.
The bulk density of OVP soils was found to decrease
and fluctuate with depth. As expected, geotechnical
indices of OVP organic soils were found to be highly
variable at the centimeter scale of resolution of routine
test measurements. For example, the water content of
two neighboring samples CIU12A and CIU12C was
found to deviate by 155%. Using XRD analysis, size
distribution analysis and petrographic analysis on 30 mthick thin sections and natural samples, it was shown
that OVP organic soils are mixtures of clay, silt and fine
sand-size fractions. The clay-size fraction consists of
illite, quartz, calcium carbonate and organics. The
medium silt-size fraction dominates in the mixture and
is made of quartz, calcium carbonate and organics. The
fine sand-size fraction consists of quartz, pyrite, shells
and organics. Organics present in the OVP soils are
derived from eroded peat and gyttja formed in an earlier
period. Wood fragments, stems and rootlets co-exist in
the OVP soils with micro-organisms such as algae and
plankton, amorphous organics and silicate and calcium
carbonate microfossils. Some thin laminations of
medium to coarse silts could be observed with the
naked eye. In the dry state, the OVP soils do not
disintegrate under light to moderate finger pressure due
to the binding effect provided by their amorphous
organic and clay fractions.Denomination of the OVP organic soils has not been
possible due to the diversity of classifications proposed
in the literature to distinguish true peats from organic
soils. Progressive transition from one soil type to
another renders the segmentation of the organic soils
profile even more difficult. However, disregarding the
top and bottom sandy layers, the soil profile was
schematised as shown in Table 1 and four types of
organic soils (a to d) have been identified accordingly.
2.2. Description of testing programme
After classification, 10 samples were selected. Samples
66 mm in diameter and 150 mm in height (whenever
sufficient material was available) were subjected to
undrained triaxial tests after isotropic and anisotropic
consolidation, abbreviated CIU and CAU respectively.
Application of a back pressure of 200 kPa ensured
saturation to a satisfactory level withB-values greater than
0.94. The pore pressure was monitored in the traditional
way at the sample ends and further by using a needle probe
in the middle of sample. The ratio of the axial to the radial
strain was calculated at the end of the isotropic
Fig. 2. CT-numberdepth profile.
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consolidation. The radial strain was derived frommeasurements of axial strain and volumetric strain made
from the start to the end of the isotropic consolidation
phase. Special attention was paid to the effects of shear
rate and remoulding process on shear strength. Two
undrained creep tests on anisotropically consolidated
specimens were carried out under constant deviatoric
stress to examine the possible reduction of strength due to
an extremely low shear rate. One sample was brought to
failure by undrained shear and subsequently subjected to a
few cycles of unloadingreloading. The purpose of the so-
called remoulding process in place was to examine the
degradation of strength due to de-structuring.
Fig.1 illustrates the stress pathsfollowed during testingin the deviatoric stress (t=1/2(13))effective mean
stress (s =1/2(1+3)) plane as well as the change in
radial strain as function of the effective mean stress.
The observed strength results are summarized in
Table 2per soil type and consolidation path. Initial soil
bulk density and effective confining stress at failure
(3,f) are also indicated.
2.3. Results and discussions
As can be seen inTable 1, the very organic clayey
silts (soils b and d) form the important constituent of the
Fig. 3. Laminations in sample 10B (a) CT-number profile (b) sample reconstitution after test (c) nebulous material in bright lamina, 5.0 cm from
bottom. d) shell-like structures in zone in between bright laminae, 7.0 cm from bottom.
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OVP organic soils. Despite some deviation in water
contents, all b and d specimens present more or less the
same loss-on-ignition. For the sake of simplicity, soils b
and d will not be distinguished later on, as far as strength
parameters are concerned. The presence of vertical
rootlets in soil d caused several aborts during the triaxial
tests and explains the divergence observed between the
deformation behaviour of soils b and d.
Values of N or s, Wn, n and a/r are listed in
Table 1. They are the loss-on-ignition, the specific
density, the natural water content, the bulk density and
the ratio of axial strain to radial strain at the end of
isotropic consolidation. They indicate respectively that
all organic OVP soils are very organic, very light and
slightly anisotropic. Despite having such properties, the
position of their failure envelopes in the t-s' plane
Fig. 4. Failure mode of Sample CAU11D: CT images are enhanced pictures with the threshold range of CT-number values in [400,680].
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(Fig. 1) are extremely high and the following observa-
tions can be made on their strength and their
deformation characteristics, i.e., ratio of axial strain toradial strain at the end of isotropic consolidation and
rebound of radial strain at the near failure state.
The amount of binding (cohesion) is scattered and
remains limited to a few kPa.
If cohesion is ignored, values of' increase of 5 to
15 and range between 38 and 56 for non-peaty soils
and 65 for peaty soils (see Tables 2 and 3).
The remarkably high values of were found to
increase in the order of very organic clay, silt and
then peat soils.
The high of non-peat soils was found to beslightly related to the anisotropy of the samples in
terms of the ratio of axial to radial strain measured at
the end of isotropic consolidation (seeTable 1) and
independent of the loading rate and remoulding
process (CIU12A and CAU11A inFig. 1).
As far as the peat soil samples (CIU 14A and 14B)
are concerned, the failurewas reached without any
subsequent decrease in shear stress, which eventually
led to a constant Maximum at the post-failure
state. This post-failurebehaviour can be explained
by considering the sample geomet ry and theboundary conditions at the moment of failure: the
samples became very flat after around 30% vertical
compression, and possible activation of permanent
shear bands was limited by the end restraints on flat
samples. This stress feature is sufficient to distin-
guish peat soil from other organic soils, in addition to
the abundance of plant remains.
All samples were found to fail with high at a
considerably low effective confining stress (b35 kPa).
This observation is in agreement with the trend
observed byKenney et al. (1967), for clayey soils up
to a normal stress of 100 kPa. But, values of the
OVP soils were found to be significantly higher than
those reported by Kenney et al. and close to those
observed by Coutinho and Lacerda (1989) and Larsson
(1990) for organic soils. Microscopic observations
presented in next section will clarify this point.
The influence of consolidation history on strength forvery organic clayey silt is noticeable. The triplet of
CAU tests on OVP soils b and d led to a lower
effective confining stress and a 8 higher than the
triplet of CIU tests as indicated inTable 3.
All samples were strongly compressed during shear,
and failed near the point where the sample diameter
was restored to its initial size by the lateral extension
during shear, i.e. at around r=0% in the graphs of
Fig. 1a and b, seeTable 2. Sample CIU12C (shown in
Fig. 1a) was an extreme exception, probably due to
the inclusion of several vertical rootlets. Thisdeformation feature indicates that all organic soil
samples at their initial state had little possibility of
lateral extension except to collapse. Nevertheless,
both vertical and lateral compressions were allowed
with slight anisotropy as mentioned earlier. Micro-
scopic observations presented in the next section will
elucidate this point.
3. Unusual minifabric of OVP organic soils at
submillimeter scale
The observation of high variability of the OVPindices and the inverse correlation between their high-
values and bulk density have given reason to study the
density distribution at submillimeter scale (0.10.5 mm).
A medical Computed Tomography X-ray scanner
(Cheng and Ngan-Tillard, 2006) was used to inspect
undisturbed sample cores in depth (see Fig. 2) with a
resolution of 0.290.291 mm3. The attenuation of
the X-rays is called the CT-number and is measured in
Hounsfield units (HU) which are defined as HU=1000
(water) /water where and water are the linear
attenuation coefficients of the material and water,respectively. Variations in CT-numbers are known to
correlate to either changes in bulk density or chemistry
or both. Six samples as listed in Table 1 were also
scanned after the triaxial tests in order to visualize the
failure mode.
Scans of 1 mm thick 66 mm diameter soil slices were
made at 1, 4 or 100 mm intervals. In Fig. 2, each solid
point corresponds to the average CT-number of four 1 mm
thick slices and the arrows indicate the positions of two
samples CIU10B and CAU11D subjected to detailed
scanning after shearing. The images of the specimens after
the test are displayed inFigs. 3 and 4respectively. The
Table 4
Correlation of CT-numbers, bulk densities and of all scanned
samples
Scanned
sample
()
n
(g/cm3)
Mean of
CT-number
values PHU
Standard
deviation of
CT-number
values
CAU11A
(undrained creep)
1.307 498 91
CIU10B 29.7 1.405 619 133
CAU13 44 1.22 440 236
CAU11D 44 1.25 432 87
CIU11C 35.8 1.219 370 128
CIU12C 35.8 1.204 208 211
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CT-number profile of sample CIU10B is shown in Fig. 3a.
Changes of CT-number profiles of sample CAU11D
caused by triaxial testing are shown inFig. 5.
From the CT analysis, the following observations can
be made on the material variability:
The CT-numbers of the OVP soils oscillate but
gradually decrease with depth
Several magnitudes and lengths of oscillations are
observed both in overall CT-number profile (Fig. 2)and
at the meso-scale within the samples (Figs. 3 and 5).
Peaks in CT-number profiles correspond to the
presence of light subhorizontal laminae in the CT
images.
A light subhorizontal laminae appears about every
3 cm in sample CIU10B. Its cross-section is
characterized by the presence of a nebula of moreattenuating material (seeFig. 3c). Visual inspection
of sample CIU10B after failure allowed to correlate
the light subhorizontal laminae of the CT images to
23 mm thick silty layers. Other slightly darker
laminae are visible on the CT images of sample
Fig. 5. Statistical analyses of CT-numbers over sample CAU11D.
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CIU10B and CAU11D but cannot be detected with
the naked eye on the failed samples. Lenses of attenuating material can be found in cross-
sections recorded outside the light and slightly darker
laminae. This is the case for the slice recorded at a
distance of 5.0 cm from the bottom of sample
CIU10B and shown in Fig. 3d. Lenses can be
oriented vertically rather than horizontally.
The CT analysis also allowed to visualize the 3D
geometry of the failed samples.
The failure mode of sample CAU11D combined an
inclined shear plane in the upper part with a major
vertical crack at the bottom (Fig. 4). The shear plane
formed across several dense laminae while the
vertical cracking detached the weak part at the
bottom. In the cross-section at 8 cm from the bottom
where the shear band is present (Fig. 4d), lenticular
elements are noticeable, even at a micro-CT levelshown in Fig. 4(e). One cross-section of sample
CAU11D changed due to triaxial shear as visualized
in Fig. 4(b) and (c). It can be seen that around this
section more vertical cracks (the dark zones in the
image) were developing when the sample failed. So,
the splitting failure mechanism cannot be excluded in
the inter-laminate zones especially when the effective
lateral stress is low. The role played by weak zones
present in the sample before testing (Fig. 4b) will
have to be clarified.
Fig. 6. Microstructure of marked denser lamina of sample 10B (after
triaxial test).
Fig. 7. Microstructure of marked denser lamina of sample CAU11D(after triaxial test).
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Sample CIU10 B turned out to be exceptional. A
continuous increase of shear stress during triaxial
testing was observed as shown in Fig. 1 with a
bending point marked with a star in theFig. 1b. Thissample would have reached a much higherthan if
the maximum shear stress had been reached. Its
mobilized is 29.7 at bending point against 60 at
the end of the test. Bright subhorizontal laminae were
found in abundance in the CT images and no
permanent shear band could be identified across the
sample in the CT images. The exceptional stress
behaviour might be correlated to the CT observations.
A statistical analysis of CT-numbers for all the
scanned samples has made possible the correlation of
the mean and deviation values of CT-numbers per
sample together with the bulk densities (after triaxial
compression) and -values measured in the laboratory.
Results are gathered in Table 4. Assuming a homoge-
neous chemistry for all samples, the following relation-
ship has been derived by fitting all the data in the table,
which enabled the transformation of average CT-numbers (PHU) to the bulk density (
n) for any volume
(minimum volume: 0.30.31 mm).
gn 0:0006P
HU1 3:1
The relationship has an R-square of 0.84 and is valid
for OVP soils with a bulk density ranging from 1.05 to
1.35 g/cm3. Similar correlations are available in the
literature (Cortellazzo et al., 1995) but could not be used
for the OVP soils as they were established for materials
of different nature and structure and using different CTparameters.
The six samples scanned after testing were grouped in
pairs according to the soil type and the bulk density level
in Table 4. It may be seen that a clear correlation of
density to does not exist. Neither do more
homogenous samples, i.e. the ones with less deviations
of CT-numbers necessarily correlate with higher
values. But more detailed analysis of CT-numbers over
sample CAU11D enables a better understanding of local
deformation. The increased CT-number values shown in
Fig. 5a after the test are obviously the consequence of the
consolidation that made the sample denser. By analyzingthe length changes of three parts (bottom, middle and
top, each of them 5 cm long) the axial strains at the end of
test are calculated to be approximately 20% for both
bottom and middle part and 40% for the top part
respectively. The mean of these three local deformations,
i.e. 27%, well agrees with the axial strain value measured
by means of LVDT during the test. The CT-number per
voxel over different parts of this sample as shown in
Fig. 5b does not follow a normal distribution since the
histogram envelopes are skewed and present several
peaks especially for the middle part of the samplebecause of the presence of laminae. The volume of three
different parts after test are determined by fitting the
maximum square inside each CT image of the core. They
are 121.5, 132.8 and 112.3 cm3 for the low, the middle
and the top parts respectively. With respect to the initial
volume of 160.8cm3 ( =6.4 cm,H=5 cm) of each part,
the relative volume changes are 24%, 17% and 30% for
the three parts. As the average of these ratios of 24% does
not agree with average volume change of 18.6%
measured at the end of triaxial test, the corrected relative
volume change with respect to the measured average
volume change are used to represent the local volume
Fig. 8. (a) Representative microfabric of OVP non-peat organic soils:
deflocculated matrix of fine flat/porous silts (before consolidation test,
broken diatoms as indicated). (b) Organics: plant remains in the debris
aggregate, possibly wood cellular walls indicated by arrows (after
consolidation test).
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strain of different parts of sample CAU11D, i.e. 18% forthe bottom, and 13% for the middle and 23% for the top.
Local radial strain of the bottom, middle and top parts
then follow equal to 1%, 3.5% and 8.5%, all in extension.
These values of local deformations confirm that denser
laminae as more presented in bottom and middle parts
than in the top part could more restrict axial compressive
and radial extensional deformation.
In conclusion, the unusual minifabric of the OVP non-peat soils has been studied by means of the CT-scanner.
Two types of ministructures are thought to affect the shear
strength and deformation of OVP samples: subhorizontal
light and slightly darker laminae and, outside laminae
light lenses. Zones containing more laminae were found
to deform less both axially and radially. In the weakest
part of the samples, the splitting failure mechanism cannot
Fig. 10. Schematic diagram of micro-deformation mechanisms of observed different layers (inside and outside denser laminae.
Fig. 9. Intact diatom embedded in amorphous organics (Existence of much carbon in EDAX results indicates the presence of organics and calciferous
matter together with Ca as indicated. The sulphate may be also present as a relatively high amount of S indicates).
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be excluded when confining stress is low. The micro-
structure of the laminae and lenses is analyzed in the
following section to better understand their contribution
to the strength of the OVP soils.
4. Microfabrics of OVP soils (up to a few tens ofmicrometers across)
Optical and electron microscopy equipped with EDAX
(energy diffraction analysis of X-rays for chemical
element identification) were used for the close-up
observation of the areas marked in Fig. 3(b) (sample
CIU10B) andFig. 4(a) (sample CAU11D). The denser
lamina of sample 10B was dominated by angular, platy
and elongated siliceous silts, while organic micro-fibers
were also identified with EDAX (Fig. 6). More or less the
same microstructures were found for the bright laminabesides the shear band zone in sample CAU11D, shown in
Fig. 7, with the impression that more elongated and
lenticular siliceous silts (broken diatoms) were present.
Outside the laminae, a looser microfabric was
identified in which much finer silts and clay dominated
the soil matrix with a much higher content of organics.
Fig. 8a illustrates such a microfabric of the organic silts of
soil type d. This chosen microfabric is believed to be
representative. The silts generally included quartz frac-
tions, broken diatoms, shell and faeces fractions while
organics were present in variable size and form (seeFigs.
8b and 9). This kind of deflocculated matrix is capable ofholding much water and contributes greatly to the high
compressibility of the soil. Lenses found on the CT
images outside the laminae can be microfossils (diatom or
shell or unknown microfossils) or other silicates etc.
The close observation of the material inside and
outside the dense laminae allows to postulate the fol-
lowing micro-mechanisms as sketched inFig. 10, which
are considered to be responsible for the high friction angle
of OVP organic soils. The medium-coarse siliceous silts
inside denser laminae and the fine siliceous/carbonate silts
outside interlock during deformation due to their shapes,either angular or elongated or even lenticular. The inter-
locking mechanism prevails inside denser laminae
because of the higher contact opportunity of silts. But it
can also take place in the looser material when samples
have gone through a consolidation process. Main siliceous
silts involved in OVP organic soils were broken diatoms
with lighter specific density (1.92.2 g/cm3) than quartz
(2.65 g/cm3). Broken diatom-related silts have very rough
surfaces because of their nano-pores. Broken diatoms led
to many elongated or lenticular silt-size fractions in OVP
organic soils. Although the micro-mechanical properties
of lenticular elements deserve further study, one can
imagine that they can sustain considerable amount of
tensile stress rather than compressive stress.
5. Concluding discussions
1) From geotechnical classification point of view, naturalOVP organic soils contain three types of soft soils, e.g.
very organic clay, very organic silt and peat soil. Despite
a large spread in index properties, all OVP organic soils
exhibit extremely high values of during consolidated
undrained triaxial compressions. The macro-mechani-
cal properties of OVP organic soils depend on details of
multi-level fabric to varying degrees. The multi-scale
investigation as presented bridged the gap between
laboratory observations and microstructure, which
eventually identified non-organic microstructural ele-
ments serving as internal confinement in OVP organicssoils. Deformation mechanisms of the microstructural
elements are proposed and these make the unusual
geotechnical properties explainable.
2) On one hand, the presence of dense subhorizontal
laminae is revealed. The laminae are shown to be rich
in angular and platy particles of medium silt size which
are believed to interlock during deformation and
contribute to the high strength of the OVP soils. We
have defined a programme focused on the influence of
laminae on the strength and deformation of OVP soils
by testing samples with and without laminae in simple
shear and Ko oedometer. On the other hand, lense-likestructures are observed at several scales outside dense
laminae and are believed to have a role of self-
confinement in much the same way as horizontally
orientated plant fibers in peat. We propose to elucidate
their role in the high strength of the OVP soils in a
Distinct Element Modelling environment.
3) In the OVP tested soils organic materials were
believed to have a primary contribution in increasing
Atterberg limits and compressibility. But their role in
increasing strength was not evident except in peat
soil. Coexistence of organics and microfossils madeidentification of organics difficult. On the other hand,
unlike in fibrous peat the fibrous microstructural
elements were difficult to detect in a back-swap light
microscopy equipped in several geotechnical labo-
ratories, simply because of their non-organic nature.
4) Organics and microfossils are associated to high
water content and allow the generation of high pore
water pressures and low effective confining pressures
during shearing. It has been observed that high
value is always correlated to low effective confining
pressure. Moreover samples show little possibility of
lateral extension other than collapse.
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5) Further investigation of the mechanics of microfab-
rics will enhance the understanding of the fundamen-
tal behaviour of soft organic soils. It may also enable
the creation of innovative ground improvement
techniques based on smart geomaterials.
Acknowledgements
This research was supported by Delft Earth Research
Centre. Part of the experimental work was performed
with the CT scanner purchased by TUDelft within the
framework of the STW project entitled Control of flow
in porous media using gels.
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