The causes of the high friction angle of Dutch organic soils.pdf

8/13/2019 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:[email protected](X.H. Cheng).

0013-7952/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.enggeo.2007.03.009
mailto:[email protected]://dx.doi.org/10.1016/j.enggeo.2007.03.009http://dx.doi.org/10.1016/j.enggeo.2007.03.009mailto:[email protected]


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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.

32 X.H. Cheng et al. / Engineering Geology 93 (2007) 3144


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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.

33X.H. Cheng et al. / Engineering Geology 93 (2007) 3144


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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.

References

Cheng, X.H., Janssen, H., Barends, F.B.J., Den Haan, E.J., 2004. Acombination of ESEM, EDX and XRD studies on the fabric of

Dutch organic clay from Oostvaardersplassen (Netherlands) and its

geotechnical implications. Applied Clay Science 25, 179185.

Cheng, X.H., 2004. Localization in Dutch Dune Sand and Organic

Clay. Delft university press, The Netherlands.

Cheng, X.H., Ngan-Tillard, D.J.M., 2006. X-ray CT Study of

Mini-fabrics of Organic Soils (Oostvaardersplassen). Advances in

X-ray Tomography for Geomaterials. ISTE Ltd, Grenoble, France,

pp. 399406.

Cortellazzo, G., Simonini, P., Bellis, G.B., Della Vedona, B., Ramigni,

R., 1995. Soil properties by computed tomography and needle probe

method. Proc. 11th ECSMFE, Copenhagen. Danish Geotechnical

Society, Copenhagen, pp. 331336.

Coutinho, R.Q., Lacerda, W.A., 1989. Strength characteristics of

Juturnaiba organic clays. Proc. 12th ICSMFE. Balkema, Rotterdam,

pp. 17311734.

Day, R.W., 1995. Engineering properties of diatomaceous fill. Journal

of Geotechnical Engineering 908910 (December).

Den Haan, E.J., 1995. Theme report on special problem soils/soft

rocks. Part I: Peats and Organic Soils. Proc. 11th ECSMFE,

Copenhagen, vol. 9, pp. 139156.

Den Haan, E.J., 2003. Sample disturbance of soft organic Oostvaarders-

plassen clay. Proceedings 3rd International Symposium on Defor-

mation Characteristics of Geomaterials, vol. 1. Lyon, pp. 4955.

September 2224.

Desrues, J., Chambon, R., Mokni, M., Mazerolle, F., 1996. Void ratio

evolution inside shear bands in triaxial sand specimens studied bycomputed tomography. Gotechnique.

Hight, D.W., Bond, A.J., Legge, J.D., 1992. Characterization of the

Bothkennar clay: an overview. Gotechnique 42 (2), 303347.

Kenney, T.C., Moum, J., Berre, T., 1967. An experimental study of

bonds in natural clay. Proceedings of the Oslo Geotechnical

Conference on Shear Strength of Natural Soils and Rocks, vol. 1,

pp. 6569.

Krieg, S. (2000). Viskoses Bodenverhalten von Mudden, Seeton undKlei. Ph.D. thesis. Verffentlichtungen des Institutes fr Boden-

machanik und Felsmechanik der Universitt Fridericiana, Heft

150, Karlsruhe.

Larsson, R., 1990. Behaviour of Organic Clay and Gyttja. Report,

vol. 38. Swedish Geotechnical Institute, Linkping.

Shahanguian, S., 1981. Dtermination exprimentale des coubes d'tat

limite de l'argile organique de Cubzac-les-Ponts. Rapport de

recherche LCPC, vol. 106.

Skempton, A.W., Petley, D.J., 1970. Ignition loss and other properties

of peats and clays from Avonmouth, King's Lynn and Cranberry

Moss. Gotechnique 20 (4), 343356.

Sun, H., Chen, J., Ge, F., 2004. Deformation characteristics of silty

clay subjected to triaxial loading, by computerised tomography.

Gotechnique 54 (5), 307314.Tanaka, H., Locat, J., 1999. A microstructural investigation of Osaka

Bay clay: the impact of microfossils on its mechanical behaviour.

Canadian Geotechnical Journal 36, 493508.

Tillard, D., 1992. Etude de la rupture dans les gomatriaux cohsifs.

Application la marne de Beaucaire. University Grenoble 1,

Grenoble France, p. 297.

Van Loon, A.J., Wiggers, A.J., 1975. Holocene lagoonal silts (formerly

called sloef) from the Zuiderzee. Sedimentary Geology 13,

4755.

Van Loon, A.J., Wiggers, A.J., 1976. Primary and secondary

synsedimentary structures in the lagoonal almere member

(Groningen formation, Holocene, the Netherlands). Sedimentary

Geology 16, 8997.

Further reading

Den Haan, E.J., El Amir, L.S.F., 1994. A simple formula for final

settlements of surface loads on peat. Advances in Understanding and

Modelling the Mechanical Behaviour of Peat. Balkema, pp. 3548.

Mitchell, J.K., 1992. Fundamentals of Soil Behavior. Wiley & Sons

Inc, New York.

Paul, M.A., Barras, B.F., 1999. Role of organic material in the

plasticity of Bothkennar Clay. Gotechnique 49 (4), 529535.

Tigchelaar, J., De Feijter, J.W., Den Haan, E.J., 2000. Shear tests on

reconstituted Oostvaardersplassen clay. Proc. Conf. Soft Ground

Technology. Noordwijkerhout, The Netherlands, pp. 6781.


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