Characterization of Micaceous Sand
for Investigation of a Subsea Mass Movement
T. Langford and S. Perkins
Abstract A recent project involved a shallow slope failure of a deposit of loose
sand and a site investigation and laboratory test program was initiated to investigate the cause of the slide and evaluate the likelihood of any further movement.
The sand was found to have an elevated mica content, which affected the density,
compressibility and shearing behavior. Existing correlations between relative density and cone resistance were evaluated to better understand the in-situ density of
the soil. Undrained triaxial tests were used to investigate the static strength and
material anisotropy, while static and cyclic direct simple shear tests helped study
behavior during undrained cyclic loading. The test results are summarised and key
conclusions are presented which are of relevance for sites worldwide where micaceous sands are prevalent.
Keywords Submarine landslide • geotechnical site investigation • laboratory
testing
1
Offshore Investigation
A recent project offshore Norway involved a shallow slope failure of a deposit of
loose sand. An initial geophysical survey of the slope revealed a classic ‘fan’ shape
failure. A detailed site investigation and laboratory test program was initiated to
investigate the cause of the slide and evaluate the likelihood of further movement.
The investigation included geophysical techniques to investigate the bathymetry
and shallow stratigraphy, eight geotechnical vibrocore (VC) samples to obtain
T. Langford ()
NGI, P.O. Box 3930 Ullevål Stadion, NO-0806 Oslo, Norway
e-mail: thomas.langford@ngi.no
S. Perkins ()
Montana State University, 205 Cobleigh Hall, Bozeman MT 59717, USA
e-mail: stevep@ce.montana.edu
D.C. Mosher et al. (eds.), Submarine Mass Movements and Their Consequences,
Advances in Natural and Technological Hazards Research, Vol 28,
© Springer Science + Business Media B.V. 2010
79
80
T. Langford and S. Perkins
Depth below MSL, m
0
100
Distance along slope, m
100 200 300 400 500 600 700 800
slope = 3 to 9°
120
dire
ctio
140
160
180
200
blue line
shows seabed
no
f sl
ide
dashed red line shows
top of underlying till
CPTU/VC locations
Fig. 1 Seabed bathymetry and cross section through slope showing approximate thickness of
overlying sand
material for laboratory testing and 28 Cone Penetration Tests with measurement of
pore pressure (CPTU) to investigate the in-situ properties of the soils. The geophysical investigation clearly revealed a debris fan, as shown in Fig. 1.
A corresponding cross-section after the slide shows how the sand has accumulated at the lower end of the slope. The following describes the general stratigraphy
(descriptions of sand density are qualitative):
Unit Ia very loose to medium dense sand, micaceous, shells and shell fragments
Unit Ib loose to medium dense sand inter-layered with soft to firm clay
Unit II till, consisting of firm to stiff sandy clay with inclusions of sand and
gravel
The geophysical and geotechnical data confirmed the slide had occurred within the
upper loose sand deposit on a shallow gradient of around 5° to 10°. The project’s
main focus was therefore to characterise this loose sand deposit, and investigate its
cyclic behavior for potential seismic loading.
2
General Characteristics of Upper Sand
The fact that the slide occurred in micaceous sand was reminiscent of the welldocumented ‘Jamuna Bridge’ project in Bangladesh (Hight et al. 1998), where a
series of failures occurred due to dredging of shallow slopes formed of loose sand.
Subsequent testing of material from the bridge site revealed how the sand’s behavior was affected by the mica content. Mica particles are flat and platy, resulting in
loose packing and a significant increase in voids ratio when compared with other
‘additives’, as illustrated in Fig. 2.
The upper sand was found to be well graded, with few fines (see Fig. 3).
Examination of the sand from the current project in the onshore laboratory revealed
that it was indeed micaceous. The minimum and maximum void ratios were also
Characterization of Micaceous Sand for Investigation of a Subsea Mass Movement
81
2.5
sand-sized mica
angular silt
kaolin
voids ratio
2.0
1.5
mica inhibits packing
1.0
0.5
0.0
0
5
10
15
20
% mica, silt or kaolin by weight
Fig. 2 Effects of mica content on voids ratio (adapted from Hight et al. 1998)
1.0
Hokksund
Ticino
Ticino
Dogs Bay
80
Percentage passing
L. Buzzard
Quiou
0.6
Current site
0.4
0.2
0
0.5
1.0
1.5
2.0
Voids ratio, e
2.5
3.0
3
20
40
60
L. Buzzard
1
Toyoura
60
Quiou
Current site
40
20
0.0
2
Dogs Bay
0
0.01
2
3
4
Highcompressibility
for current site
0.0
1
0
Penetration, m
0.8
Compression index, Cc
Cone resistance, qc, MPa
100
Hokksund
0.1
1
Grain size, mm
10
5
0
Pore pressure, u, kPa
Fig. 3 Grain size distribution and compression index for different sands; typical CPTU data for
the site
investigated and these are presented in Table 1 together with data for other ‘welltested’ sands, based on information from Lunne et al. (1997).
The voids ratios measured for the current site are high compared with other
sands in the literature, and much higher than those for Hokksund and Ticino
sands which were used to develop the widely-used correlations between CPT
cone resistance and relative density based on calibration chamber testing. Note
that measurement of minimum and maximum voids ratio involves a degree of
uncertainty, depending on the method used. The values in Table 1 are mainly
based on the ASTM approach.
Calibration chamber testing has been performed by several institutions,
including ENEL CRIS (Bellotti et al. 1988), ISMES (Baldi et al. 1986), NGI
(Parkin and Lunne 1982), and Oxford University (Houlsby and Hitchman 1988)
82
T. Langford and S. Perkins
Table 1 Properties of sand for current sites and sands from calibration chamber tests
emin, emax
Mineralogy (for guidance)
Compressibility
Sand name
Gs
2.70
2.70
2.67
2.64
2.65
2.75
2.71
2.70
3.02
2.65
0.9–2.1
0.5–0.9
0.6–0.9
0.6–1.0
0.5–0.7
1.1–1.8
0.5–1.3
0.8–1.6
0.6–1.1
0.6–0.8
Cone resistance, qc, MPa
Cone resistance, qc, MPa
2
4
6
8
0
10 12
1
2
3
4
5
2
4
6
8
Before/outside slide
2
3
4
5
20
40
60
80
Relative density, Dr, %
0
100
0
1
After/within slide
High
Medium
Medium
Low
Low
High
High
High
High
Low
Relative density, Dr, %
0
10 12
0
Depth below seabed, m
Depth below seabed, m
0
Depth below seabed, m
0
10% mica, 40% quartz, 50% shells
35% quartz, 10% mica
30% quartz, 5% mica
90% quartz
Quartz
90% carbonate
74% shell fragments, 12% quartz
Carbonate
Feldspar, quartz, mica
Quartz with feldspar
1
2
3
4
5
20
40
60
80
100
0
Depth below seabed, m
Current site
Hokksund
Ticino
Toyoura
L. Buzzard
Dogs Bay
Quiou
Sand A
H. Mines
Monterey
Before/outside slide
1
2
3
4
5
After/within slide
Fig. 4 Profiles of CPTU cone resistance and relative density before (or outside) the slide and after
(within) the slide
and Southampton University (Last et al. 1987). Although many of the tests were
performed on low to medium compressibility sands, some highly compressible
sands were also tested, such as Dogs Bay and Quiou Sand. Research into highly
compressible sands has generally focused on materials with a large proportion
of ‘carbonate’ or ‘calcareous’ particles, whereas the engineering properties of
the current sand appear to be governed by both the mica and carbonate
particles.
Many calibration chamber test results are presented in Lunne et al. (1997)
together with descriptions of the equipment. Grain size curves for some of the
sands are shown together with the compressibility measured in oedometer tests on
Fig. 3. A typical profile of cone resistance and pore pressure from the slide material
is also shown.
Figure 4 shows the range in CPTU cone resistance measured at locations before
(or outside) the slide and after (within) the slide. Very low values were measured
within the slide and the average profile is much lower than that measured before
the slide.
Characterization of Micaceous Sand for Investigation of a Subsea Mass Movement
3
83
Interpretation of Relative Density
The cone penetration test is an excellent tool for evaluating the variability in soil
strength or density across a site. For sands, relative density is typically used as a
reference parameter, where this is the relationship between the current in-situ
voids ratio and the maximum and minimum voids ratios, emax and emin, referring to
the loosest and densest possible states. The use of relative density as a design
parameter itself is questionable. However, the parameter is used in the geotechnical laboratory during preparation of reconstituted sand specimens to the in-situ
density since it is extremely challenging to obtain an ‘undisturbed’ sand sample
during a site investigation.
Cyclic testing was planned for the current site to investigate liquefaction
susceptibility of the sand and a reasonable estimate of the relative density was
required to reconstitute the specimens, since the vibrocore method caused significant
densification of the original material. The low cone resistance measurements were
initially correlated with the relative density using the general correlation presented
by Baldi et al. (1986) for normally consolidated deposits of clean sand:
Dr =
⎡
⎤
qc
1
. ln ⎢
' 0.55 ⎥
2.41 ⎣157 ⋅ s v 0 ⎦
(1)
Where Dr is relative density, qc is cone resistance and s¢v0 is the in-situ vertical
effective stress. The corresponding profiles of relative density as calculated above
are shown in Fig. 4. The Baldi correlation is probably the most widely used within
current engineering practice and therefore serves as a reference for other correlations
between cone resistance and relative density.
The relative density profiles inside the slide are greatly reduced, as are the cone
resistance profiles shown in Fig. 4. The lowest interpreted relative densities for
some locations are below 10% for some locations between 1 and 4 m penetration.
These values are extremely low when compared with previous experience in both
hydraulic fills and natural soils. Several studies suggest a minimum relative density
around 25% in sand deposits (e.g. Lee et al. 1999). The same general figure applies
in the laboratory where it is difficult to prepare a stable specimen in a looser state.
These low calculated densities were therefore treated with caution.
The Baldi correlation is accepted to give reasonable results for deep deposits of
uniform soils that match well with ‘standardised’ and less compressible materials
in the literature (i.e. Ticino and Hokksund sands). Unfortunately, the correlation is
less reliable for other types of soil. Differences in grain size distribution, angularity
and especially mineralogy will affect the measured cone resistance for a given relative density. Furthermore, the existing correlations generally provide a less reliable
indication of actual soil density for shallow soils (the upper 5 m) since the lowest
confining stress used in the calibration chamber tests was 50 kPa.
Compressibility has previously been used as a qualitative factor to explain some
of the scatter in the data from calibration chamber testing. Jamiolkowski et al. (1985)
84
T. Langford and S. Perkins
show how correlations between relative density and cone resistance are seen to
change with varying compressibility. The results confirm the intuitive trend that
more compressible sand will give a lower cone resistance for a given relative density when compared with less compressible sand. Kulhawy and Mayne (1990)
suggest an empirical correlation accounting for compressibility:
Dr2 =
qc1
305 ⋅ QC ⋅ QOCR ⋅ QA
(2)
Where qc1 is the normalised cone resistance = (qc/pa)/(s¢v/pa)0.5, QC is a compressibility factor, QOCR is an overconsolidation factor = OCR0.18 and QA is an ageing
factor. The compressibility factor is based on qualitative evaluation as opposed to
any specific measurement with suggested values of 0.91 for high compressibility,
1.0 for medium compressibility and 1.09 for low compressibility, corresponding to
the trend mentioned above.
Wehr (2005) presents another correlation based on a series of recent calibration
chamber tests from Karlsruhe University (unpublished), including compressible
calcareous sands. The correlation modifies the cone resistance using a ‘shell correction factor’, fshell. The updated cone resistance is then given by qc,shell = qc . fshell
and the relative density may be recalculated.
fshell = 0.0046 ⋅ Dr + 1.3629
(3)
These relationships are compared on Fig. 5 with data for different sands from calibration chamber testing. The Baldi correlation tends to the lower bound of relative
density for low values of normalised cone resistance (qc1 < 50).
110
90
Relative Density, Dr
Range from
Kulhawy and Mayne
Correlations for
more compressible
(calcareous) soils
100
80
70
60
Range from
Jamiolkowski et al
50
CC data
40
Baldi
K&M
30
Wehr
20
Jamiolkowski
10
0
50
100
150
200
250
300
Normalised cone resistance, qc1
Fig. 5 Relative density versus normalised cone resistance, comparing empirical relationships
with calibration chamber test data
Characterization of Micaceous Sand for Investigation of a Subsea Mass Movement
85
Figure 5 shows clear scatter in test data which may be predominantly attributed
to sand type. The high compressibility of the sand for the current site is demonstrated in Fig. 3. Uncertainty on how to reliably interpret the relative density for
laboratory testing meant that the project chose to re-evaluate calibration chamber
test data, including more recent experience with calcareous sands (see Table 1). The
data points from Figure 5 are divided into sand type on Fig. 6 using a logarithmic
plot to provide more detail at low values of normalised cone resistance.
Figure 6 suggests that the data for more compressible sands (Dogs Bay, Quiou
and Sand A) indeed plot ‘above’ the Baldi line, agreeing with the trend indicated
by the various published correlations. Furthermore, a fitted line through all data for
low, medium and high compressibility sands also plots above the Baldi correlation,
which is based predominantly on Ticino and Hokksund sands. Since no calibration
chamber testing was performed for the current site, this ‘best-fit’ correlation is
compared with the Baldi correlation in Fig. 7 for the range of CPTU data shown
previously in Fig. 4.
Figure 7 shows the mean for all locations together with the mean ±1 standard
deviation for the Baldi and ‘best-fit’ correlations. There is relatively little difference in the relative density for CPTUs performed before or outside the slide zone
where the average is around 70% (medium dense). However, there is a marked
increase for the CPTUs performed within the slide zone where the relative density
is significantly lower. This fits with the trend shown on Fig. 6, where the intersection between the Baldi correlation and the logarithmic fit to calibration chamber
test data is for a relative density of around 70% (or a normalised cone resistance
just over 100).
Based upon the profiles shown in Fig. 7, the project agreed that the likely in-situ
relative density of the sand was higher than predicted by the Baldi correlation.
110
Toyoura
L Buzzard
Hokksund
Correlations for
Ticino
more compressible
Dogs Bay
(calcareous) soils
Quiou
Sand A
Log fit (all data)
K&M
Wehr
Baldi
Jamiolkowski
100
Relative Density, Dr
90
80
70
60
50
40
30
20
10
0
5
50
Normalised cone resistance, qc1
500
Fig. 6 Relative density versus normalised cone resistance, showing data for different sand types
86
T. Langford and S. Perkins
Relative Density, Dr %
0
20
40
60
80
Relative Density, Dr %
0
100
0
20
40
60
80
100
0
Best fit, av
Baldi, av
1
1
Best fit, +/– stdev
Depth, z, m
Depth, z, m
Baldi, +/– stdev
2
3
4
2
3
4
After/inside
Before/outside
5
5
Fig. 7 Profiles of relative density before (or outside) the slide and after (within) the slide, based
on Baldi and ‘best fit’ correlations for all calibration chamber test data
Nonetheless, a range of 20% to 25% was conservatively applied for much of the
advanced laboratory testing due to remaining uncertainties.
4
Laboratory Testing of Sand
An extensive laboratory test program at NGI and Montana State University was
undertaken to investigate the behavior of the sand. A series of Anisotropically
Consolidated Undrained triaxial (CAU) tests sheared in compression and extension
were used to evaluate anisotropy. Cyclic constant volume Direct Simple Shear
(DSS) tests were used to investigate the liquefaction potential.
The CAU triaxial tests were performed at the lowest achievable relative density,
ranging between 25% and 45%, with a consolidation stress coefficient K0 around
0.5. Figure 8 shows normalised stress paths for a total of ten tests, with six sheared
in compression and four in extension. The mean and shear stresses have been
normalised by the mean consolidation stress, p¢0.
The results are compared with the range given by Hight et al. (1998) for Jamuna
Bridge sand with a mica content of 0% to 40%. The results agree well with these
previous findings, showing clear anisotropy with a much weaker response during
initial shearing in extension than compression. According to the Jamuna Bridge
results, the addition of a relative small proportion of mica (less than 2.5% by
weight) resulted in a more unstable response, whereas larger proportions actually
increased the stability of the sand. No detailed quantitative evaluation of the mica
content was performed for the current site, but an estimate was made of 10% content by volume which is equivalent to a much lower proportion of the weight. Hight
et al. (1998) discuss the importance of sample preparation and grain orientation on
results. Moist tamping was used exclusively for preparing samples for the current
site, although care was taken to keep the relative density as uniform as possible,
Characterization of Micaceous Sand for Investigation of a Subsea Mass Movement
87
1.0
Normalised shear stress, τ/p'0
0.8
Range in behaviour
from Hight et al. (1998)
for sands with different
mica content (0 to 40%)
0.6
0.4
CAUC-1
0.2
CAUC-2
CAUC-3
0.0
CAUC-4
CAUC-5
CAUC-6
-0.2
CAUE-1
CAUE-2
-0.4
CAUE-3
Results show significant anisotropy, much weaker
response in extension than compression
-0.6
0.0
0.2
0.4
0.6
0.8
1.0
CAUE-4
1.2
1.4
1.6
Normalised mean stress, p'/p'0
Fig. 8 Normalised stress paths from anisotropically consolidated undrained triaxial tests
with the lowest practical achievable relative density around 25% for the triaxial
specimens and 20% for the DSS tests.
The results for the current site suggest static stability would be low for a trigger
mechanism involving unloading at the base of the slope, similar to the Jamuna
Bridge case. However, in the absence of the trigger mechanism, the slope stability
was found to be adequate.
Cyclic DSS tests were performed for two levels of relative density, namely
around 20% and 50%. The samples were sheared with different levels of normalised cyclic shear stress, tcy /s¢vc, and an average shear stress tcy /s¢vc = 0. Results are
summarised on Fig. 9 in terms of normalised cyclic shear stress versus number of
cycles to failure. The scatter in data around tcy /s¢vc = 0.25 is thought to be due to
natural variability in the material.
The resulting ‘strength’ of the material with the relative density of 50% is much
greater than that with the lower relative density. The DSS testing was used to evaluate
the liquefaction potential of the soils during seismic loading, where this can be
evaluated by comparing the Cyclic Stress Ratio (CSR) and Cyclic Resistance Ratio
(CRR) so that the resulting factor of safety is given by CRR/CSR.
The CSR represents the relative shear stress caused by the seismic event, whereas
the CRR represents the relative shear resistance of the soil to a given event.
Seed and Idriss (1971) suggest a procedure for evaluating the CSR (here excluding
the depth reduction coefficient):
T. Langford and S. Perkins
Normalised cyclic shear stress, τcy/σ'vc
88
1.4
Dr around 50%
Dr around 20%
1.2
1.0
0.8
N for gcy = 10%, then
static shear, estimated
N = 15 for gcy = 15%
0.6
0.4
0.2
0.0
1
10
100
1000
10000
Number of cycles to failure, N
Fig. 9 Summary of results from cyclic DSS tests at two levels of relative density
CSR =
th
a
s
= 0.65 ⋅ max ⋅ v'
g sv
s v'
(4)
Where th is the horizontal shear stress caused by the seismic event, s¢v is the vertical
effective stress, sv is the vertical total stress and amax is the seabed acceleration.
Two seismic events were evaluated for the site: a 100 year event and more
extreme 10,000 year event. This allowed the CRR to be evaluated from the cyclic
DSS tests. The 100 year event assumed an equivalent number of cycles (Nequiv) of 3,
giving the CRR as 0.4. The Peak Ground Acceleration (PGA) at bedrock was given
as 0.02 g. The 10,000 years assumed Nequiv as 5, giving the CRR as 0.2. The PGA in
this case was given as 0.1 g. A site response analysis suggested the accelerations in
the sand layer could be amplified by a factor of around 1.0 to 1.5. The total unit
weight of the soil is taken around 16 kN/m3, which is equivalent to a relative density
around 20%.
The CSR for the 100 year event is found to be between 0.04 and 0.06 for the
100 year event, which is far lower than the CRR of 0.4. The corresponding CSR
for the 10,000 year event is found to be between 0.18 and 0.28 compared with the
CRR of 0.2. The resulting safety factor suggests a possibility of failure for the
extreme 10,000 year seismic event. However, additional analyses showed the predicted maximum displacements resulting from the extreme seismic event to be
minimal, which suggest that a catastrophic failure would be unlikely. Furthermore,
the DSS results presented here are based upon a conservatively low relative density.
Characterization of Micaceous Sand for Investigation of a Subsea Mass Movement
89
The prior evaluation of calibration chamber CPTU data suggests that these low
values are likely to underestimate the in-situ field density.
The very shallow (<5°) slope at the base of the slide area where the loose sand
is currently deposited gives an additional normalised average shear stress tav/ s¢v =
sin 5° = 0.07. This shear stress acts permanently on the slope and can therefore be
taken to act drained. Although no specific tests were performed in this case, experience with other sands indicates that a small degree of drained average shear stress
does not reduce the resistance to undrained cyclic stress.
Additional tests were performed to investigate the post-cyclic static strength of
the sand, in order to evaluate the sand’s behavior after the unlikely event of seismic
failure. Figure 10 shows the normalised cyclic shear stress versus shear strain and
normalized effective vertical stress. The tests were run as before up to a limit of
10% cyclic shear strain at which point the cyclic stress was set to the zero point.
Without allowing any intermediate drainage, the specimens were then sheared statically to failure in either the ‘forwards’ or ‘reverse’ directions.
The cyclic tests show the typical trend of increasing shear strains with number
of cycles, which would usually lead to failure for a cyclic test. The post-cyclic static
test appears to follow a similar path as the final cycle of the cyclic tests. The average effective vertical stress for the cyclic tests gradually reduces towards zero,
although the response of the static test for larger strains forms an envelope due to
dilation of the sand. This suggests that the sand would develop additional capacity
at larger strains. This means that even if a slide were to occur due to a seismic trigger, the slope would likely stabilize after limited displacements.
The results from the in-situ and laboratory testing were then implemented in analyses
to evaluate the static and cyclic stability of the slope as part of an ongoing investigation.
Initial results suggest that the current stability is within acceptable levels.
5
Conclusions
A detailed site investigation and laboratory testing program was used to study the
properties of a loose micaceous sand involved with a shallow slope failure offshore
Norway. The soil exhibited elevated voids ratios and higher compressibility than
many other documented sands. A subsequent re-evaluation of existing correlations
between cone resistance and relative density suggested that the common Baldi correlation would not be valid, and a higher range of relative densities was used for
reconstitution of samples in the laboratory.
Anisotropically consolidated undrained triaxial test results showed that the sand
responded in a similar fashion to the well-documented Jamuna Bridge sand. The
elevated voids ratio and platy mica particles promote a significantly anisotropic
response during static undrained shearing. Direct simple shear testing revealed that
the current sand would develop additional capacity at larger strains during postcyclic static shear.
90
T. Langford and S. Perkins
0.6
Normalised shear stress, t/s'vc
0.5
0.4
0.3
0.2
0.1
0.0
–0.1
–0.2
–0.3
DSS-12-cy
–0.4
DSS-13-cy
DSS-12-st
–0.5
DSS-13-st
–0.6
–14 –12 –10 –8 –6 –4 –2 0
2
4
6
8 10 12 14
Shear strain, g, %
0.6
Normalised shear stress, t/s'vc
0.5
0.4
0.3
0.2
0.1
0.0
–0.1
–0.2
–0.3
–0.4
–0.5
–0.6
–0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Normalised effective vertical stress, s'v/s'vc
Fig. 10 Results from cyclic and post-cyclic static DSS tests in forward and reverse directions
Characterization of Micaceous Sand for Investigation of a Subsea Mass Movement
91
For the Jamuna Bridge project, the failure was found to be initiated by a trigger
mechanism with excavation at the base of the slope. A similar mechanism is anticipated to be responsible for the failure at the current site; an investigation is ongoing
and the laboratory testing results have been implemented in analyses to evaluate the
static and cyclic stability of the slope. The results for this material may also be
relevant for other sites worldwide where micaceous sands are encountered.
Acknowledgments The authors would like to acknowledge the input of David Hight and Steve
Kay for advice and initial discussions during the planning and testing stage, based on experience
with the Jamuna Bridge project. Colleagues at NGI and Montana State University are also thanked
for assistance with laboratory testing and interpretation. The reviewers are acknowledged for
timely and useful comments to the draft manuscript.
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