STABILITY ANALYSIS OF THE HUDSON APRON SLOPE, OFF NEW JERSEY,
U.S.A.
J. LOCAT, P. DESGAGNÉS,
Dept. of Geology and Geological Engineering, Université Laval, Québec, Canada
S. LEROUEIL
Dept. of Civil Engineering, Université Laval, Québec, Canada
H. J. LEE
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 USA.
Abstract
As part of the STRATAFROM project, the Hudson Apron area was selected for a detailed
slope stability analysis. Results indicate that high pore pressure is necessary to trigger a
failure. Under normal conditions, an excess pore pressure of more than 90% would be
required for failure. On the other end, the actual strength profile would indicate a remaining
marginal stability. Aggravating factors were the high sedimentation rate cyclicity and
resulting layering inducing high excess pore pressures, and potentially gas pressures and
earthquakes.
Keywords: New Jersey Margin, mass movements, sedimentation rates, stability, strength
1. Introduction
The Hudson Apron area is located on the East Coast of the United States on the continental
shelf boarding the Hudson Canyon at a distance of about 200 km offshore of New Jersey
(Figure 1). This area has extensively been investigated by the USGS as part of the EEZ
surveys of the early 1980’s. Evidences of slope failures were observed in the area by Booth
et al. (1993) and McHugh et al. (2002). They identified many facies related to many types
of mass movements transport and deposition mechanisms.
Figure 1. Location map of the Hudson Apron slide region in (a), slide area in (b) extent and frequency of mass
movements taken from Booth et al. (1993).
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Failure mechanisms which have been invoked in this area are: presence of sea-floor gas
hydrates (Kayen and Lee 1993), high sedimentation rate (Coleman et al. 1993), excess pore
pressures caused by groundwater sapping (Robb 1984), and large submarine canyons
(Carlson et al., 1993). Dugan and Fleming (2000 and 2002) focused their studies on the
potential role of the sedimentation rates as a mechanism for triggering of sediment failures
in the area during early Pleistocene time. Earthquakes and cyclic stresses could also
increase pore pressure (Locat and Lee 2002).
For this, the Hudson Apron area offers a unique opportunity, as part of the STRATAFORM
project (Nittrouer 1999, Locat and Lee 2002), to study the geotechnical aspect of mass
movements because of the already existing large amount of stratigraphic data and physical
properties available from the Ocean Drilling Program (ODP) leg 174A (Austin et al. 1998),
preliminary seismic investigations and interpretation (Pratson et al. 1995, Austin et al.
1998), the sidescan sonar surveys of the United States Exclusive Economic Zone (Lee et al.
1993) and the geotechnical investigation of the ODP 1073 core (Dugan et al. 2002). As a
result of these previous studies, this site was selected for coring by the French research
vessel Marion Dufresne II which enabled the STRATAFORM researchers to recover cores
from the Hudson Apron area (Figure 1). General details about our work on the Hudson
Apron can also be found in Desgagnés et al. (2000 and 2001) and in Desgagnés (2003).
This paper presents the results of our back analysis of the Hudson Apron site. We conclude
that large variations in the sedimentation rate, in response to glacial periods during the
Pleistocene, created layering conditions such that the consolidation could not fully take
place resulting in remaining high excess pore pressure in the underlying older strata and that
these high pore pressures most likely still remain now.
2. The Hudson Apron site
As indicated by Booth et al. (1993) and by McHugh et al. (2002) there are many slides and
flows along the New Jersey margin. One of them had been isolated for a more detailed study
(Pratson et al. 1995) and is referred to as the Hudson Apron slide. This mass movement is
located on the slope about 200 km offshore the coast of New Jersey, near the Hudson
Canyon (Figure 1). The actual slide morphology is best observed from the seismic section
because the postglacial sedimentation along the Hudson Apron has smoothed out the surface
morphology (Dugan and Fleming 2002).
Another explanation is provided by Driscoll (1997) who relates this to the effect of canyons
development on both sides of the Hudson Apron which would locally limit the formation of
gullies by intercepting the groundwater flow. This part of the U.S. Atlantic margin was not
covered by glaciers during Wisconsinian time but was rather an area of high sedimentation
rates during that period (Austin et al. 1998). This high sedimentation rate is evidenced by
sediment seaward pro-gradation as seen in Figure 2b.
The ODP 1073 core, shown on Figure 2b, provides both geotechnical and stratigraphic
information along a 664m deep section. The Marion Dufresnes core went to a depth of 38m
and represents about 50 000 years of sedimentation. Therefore, most of the sediment
Stability analysis of the Hudson Apron Slope, off New Jersey, USA
269
collected in the first 38 m were deposited during Wisconsinian time (late Pleistocene) and
they are mostly detritic, i.e. resulting from glacial and river erosion on the mainland. Still,
large river discharge inputs occurred during much of the Pleistocene, particularly during
the melting phases of the various ice sheets during which time the sedimentation rates were
much higher than during the Pliocene. Wei (2001) investigated the microfossils in core
1073 and found that the depth of 120m corresponded to an age of 260 000 yBP, and the
depth of 70m to an age of 85 000 yBP, representing an average sedimentation rate of 85
cm/ky, which is considered very high for such an environment.
Figure 2. Seismic profile and location of ODP core 1073 near the head of the Hudson Apron slide area. (modified
after Austin et al. 1998).
The Hudson Apron area which will be used for the slope stability analysis is taken from a
description provided by Partson et al. (1995). It ranges from a water depth of about 600
mbsl to at least 1500 mbsl (Figures 1 and 2). From the escarpment, it extends over more
than 10 km on a slope at about 4o, and is about 5 km wide. The height (Figure 2b) of the
escarpment is about 100-150 m and the visible length of the failure surface is more than 10
km (Figure 2; Pratson et al. 1995). The potential failure plane, shown in Figure 2a, is
considered to mostly follow a sandy horizon located at a depth of about 200m to 250m.
From our analysis of the geophysical line, it appears that we could define some failure
surfaces that could involved the upper strata (Figure 2a) and this would mean that the
deformation of the slope took place in recent times even tough Dugan and Fleming (2002)
consider that the overall stability would have increase since about 300 000 years.
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3. Methodology
The samples tested as part of this study were obtained with the Marion Dufresne II which is
equipped with the long Calypso piston corer (up to 60 m). Three cores were taken on the
Hudson Apron and along the same isobath, i.e. 632 to 639 mbsl (MD992211: 39o 13.92N
72o 15.58W; MD992212: 39o 13.98N 72o 15.53W; MD992213: 39o 13.39N 72o
17.04W). Cores MD992211 and MD992212 were aimed to be outside the slide area while
core MD992213 was targeted to be within the mass movement area. Core MD992212
(37.12 meters long) was brought to Laval University (Québec) for geotechnical testing and
analyses while cores MD992211 (30.05 m) and MD992213 (29.74 m) were taken to the
Lamont-Doherty Earth Observatory (NY) for stratigraphic analyses.
Laboratory work included CATSCAN profiling in order to help plan the laboratory testing
program. The program included index properties and physico-chemical properties
determination, scanning electron analysis, oedometer testing, SEDCON (Locat 1982)
testing and triaxial testing. All tests were carried out according to ASTM or Bureau de
Normalisation du Québec (BNQ) standards.
Figure 3. Result of physico-chemical and microstructural analyzes on core MD992212.
4. Hudson Apron Sediments Properties
At the location of borehole 1073 (Figure 1b), a 664m long section has been investigated in
detail and the reader is referred to the work of Dugan et al. (2002) who looked at various
Stability analysis of the Hudson Apron Slope, off New Jersey, USA
271
physical properties. Hereafter, we will focus on the Pleistocene section which is the one
mostly involved in the instability analysis and we consider that the results obtained on core
MD992212 (Figures 3 and 4) are also representative of this unit.
4.1 NATURE AND MICROSTRUCTURE
The nature of the near surface Hudson apron sediments is illustrated in Figure 3. The
plasticity index varies from 25 to 40%, the specific surface area form 50 m2/g to 90 m2/g
and the cation exchange capacity from 6 to 13 meq/100g. The activity is considered low to
medium with a value between 0.5 and 1 (Figure 3a). The main source of the sediments is
from the Appalachian mountains. The mineralogy is typical of sediments derived from
Appalachian rocks and similar to Québec postglacial clays also derived from the
Appalachian rocks (Locat 1995). The clay fraction consists mostly of illite, chlorite and
kaolinite with traces of smectite (or swelling clays). The chlorite content appears more
important than the kaolinite content as shown by a greater peak for chlorite at about 25°
(Figure 3b). For the sections studied within a depth of 38 meters, the sediment
microstructure appears well flocculated with only traces of microfossils (coccolith in Figure
3d).
4.2 STRENGTH CHARACTERISTICS
The evaluation of the stability of the Hudson Apron slope requires the knowledge of the
strength parameters. Strength and other physical properties obtained for core MD992212
and OPD data base have been assembled in the geotechnical profiles provided in Figures 4
and 5. The section between 9.5 and 12.0 m, in Figure 4, has not been investigated because
of the high sample disturbance.
For the near surface horizon (0-38m) investigated with the Marion Dufresne core, the
Cu/σ’vo ratio decreases from about 0.5 near the surface to 0.15 at the base. A few oedometer
tests were carried out on intact specimens and the Cu/σ’p ratio is also within the same range
(see Figure 4). The water content down this profile, which is also well reflected by the
density profile obtained from the MST, shows an overall decrease with depth until about
31m followed by a significant increase which may explain the lower strength ratio near the
base. The liquidity index follows a similar trend decreasing with depth from about 1.5 near
the surface to 0.5 at a depth of 38m. Here the liquidity does not change as much near the
base probably indicating that the increase in the water content may reflect the presence of
finer sediments at the base (31-38m in Figure 4).
Water content and strength data from the ODP 1073 core are quite unique. Very shortly
down in the strength profile (~50m), the undrained shear strength drops below values for
normally consolidated sediments. This is illustrated by the dashed line given for a Cu/σ’vo of
0.05 (Figure 5b). This is a very low strength ratio which is also reflected by changes in
water content which even increase within the depth interval between 120 and 250 mbsf, to
potential depth of to the failure plane. Even below this depth, the Cu/σ’vo can be as low as
0.02. One would be inclined to believe that this could be due to sample disturbance. This
may be possible for the undrained strength, but not very likely for the water content. As it
will be discussed later, this profile is believed to represent a situation where a high water
content may result from high sedimentation rate and groundwater flow preventing
consolidation (Dugan and Fleming 2002).
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Locat et al.
Figure 4. Composite geotechnical profile of core MD992212 including MST and CATSCAN profiles.
Stability analysis of the Hudson Apron Slope, off New Jersey, USA
273
Figure 5. (a): Water content and mean grain size (MGS), (b): undrained strength data from ODP 1073. and (c):
triaxial tests results on cores from site MD992212 (BP: base of Pleistocene; BoFP: base of failure plane).
The mean grain size of the sediments at site 1073, taken from Hoyanagi and Omura (2001),
are plotted in Figure 5a. The variation is also reflected by the water content which shows
more or less an inverse relationship. The water content variability has been divided into
zones from the lower portion of the Pleistocene section to the top, numbered from 1 to 5
and representing the repetition of similar patterns. We could speculate here that both the
water content and the mean grain size variation represent the various cycles of melting of
the continental ice sheets, during Pleistocene time, which have induced the observed
layering.
Sedimentation Consolidation (SEDCON) tests were carried on two reconstituted sediments
of the Hudson Apron site. These tests provide the relationship between the water content
and effective stress (i.e. assuming hydrostatic pore pressure conditions). Results are also
plotted on Figure 5a and were extrapolated down to a depth of about 200m. The SEDCON
test predicts fairly well the water content down to a depth of about 100 mbsf after which
there is an increasing difference between the predicted and observed water content values.
This aspect will be discussed later but we can already consider that the in situ conditions at
the site of borehole ODP 1073 are such that the strength of the material is very low.
Some samples were taken for more advance testing into a triaxial cell to measure the
strength parameters. The four tests required to determine the failure envelope is provided in
Figure 5c where the friction angle is found to be 31° with little or no cohesion.
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Locat et al.
5. Back analysis of the Hudson Apron failure
As reported in Figure 1c and 1d, the Atlantic Margin of the United States has experienced
many types of mass movements during the Pleistocene. More than 50% of these mass
movements took place on slopes less or equal to about 4°, with a majority of them covering
areas from 5 to 50 km2 (Booth et al. 1993) and the Hudson Apron is part of that group. The
back analysis will be made using limit equilibrium methods assuming a factor of safety (F)
of 1.0 for failure to take place.
The following analysis is made by considering that the slide took place near the interface of
the sandy layer identified in ODP borehole 1073 which also represents a clear reflector on
the seismic profile (see Figure 2b). The back analysis was made by considering 1D and 2D
conditions, the 2D analysis being carried out using SlopeW software. For the 1D analysis,
the failure plane is considered to be at a depth of 200m. For the 2D analysis the cross
section shown in Figure 6 will be used. It represents a simplified 2D profile of the Hudson
Apron slope and stratigraphy where the main reflectors, shown in Figure 2, are taken to
define the schematic shape of the failed mass.
Figure 6. Schematic topographic and stratigraphic cross section of the Hudson Apron showing the geometry of the
failed mass use in the back analysis of the failure (BoP: base of Pleistocene).
5.1 ANALYTICAL APPROACH
The infinite slope (or1D) analysis has been done by considering the following elements.
First, the slide is taking place under water so that only buoyant specific weight (γ’) is taken.
The limit equilibrium equation for such an infinite slope, using effective stress parameters
(called drained case hereafter), is:
F=
(
)
FR
c’+ γ ’z cos 2 β − ∆u tan ϕ ’
=
FM
γ ’z sin β cos β
[1]
where FR and FM are the resisting and destabilizing forces respectively, z, the depth to the
failure plane (taken here as 200m) and β the slope angle. The friction angle (ϕ’) is taken at
Stability analysis of the Hudson Apron Slope, off New Jersey, USA
275
31° and the cohesion (c’) to 0 kPa, and the buoyant specific weight for the silt and sand
layers were respectively at 8.4 kN/m3 and 10.0 kN/m3. The existence of in situ high pore
pressure will be introduced as a stress reduction factor equivalent to the pore pressure ratio
ru* such as:
ru* =
∆u
γ ’z cos 2 β
[2]
where ∆u is the pore pressure in excess of hydrostatic.
Figure 7. Parametric analysis linking the slope angle and the pore pressure ratio required for a factor of safety (F) of
unity along the Hudson Apron slope using both infinite (1D) and 2D limit equilibrium methods.
For the case using the undrained shear strength (called undrained case hereafter), we still
apply the ru* factor to the reduction of the stress while using the undrained shear strength
for the resisting forces. The undrained shear strength is obtained by considering a range of
Cu/σ’vo. The limit equilibrium for undrained condition (Fu) is approximated by:
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Locat et al.
Fu = k (cos 2 β − ru *) /(sin β cos β )


[3]
where k = Cu/σ’vo and would typically vary between 0.2 and 0.3 for a normally
consolidated sediment.
The 2D stability analysis has been carried by incorporating the cross section shown in
Figure 6 into SlopeW and by considering also the excess pore pressure using ru* , the other
parameters being the same as for the 1D analysis (for more details, see Desgagnés 2003).
5.2 RESULTS OF THE STABILITY ANALYSES
From a limit equilibrium approach it is clear that for a failure to take place on such a small
slope, the shearing resistance must be very low. Using Eqs.[1] to [3], and considering
various slope angles and a range of k values, the required excess pore pressure for a factor
of safety of one can be computed. The computation results are reported in Figure 7 far all
cases considered. As expected for such a small slope angle (4°), the required excess pore
pressure to bring the factor of safety to unity is quite high, from 0.65 to 0.9, if the Cu/σ’vo
ratio is within the range of the normally consolidated sediments, i.e. between 0.2 and 0.3.
In all conditions, the drained case (i.e. using effective stress parameters) requires a higher
excess pore pressure, at the same slope angle, to approach failure conditions (F = 1).
On the other end, if the Cu/σ’vo ratio is low, it reveals the presence of high excess pore
pressures. For example, if the strength values measured in the horizon just above the
proposed failure plane are correct, i.e. at about 100 kPa, and if the soils had initially been
normally consolidated (i.e. 0.2<k<0.3), it would indicate that the in situ effective stress (σ’vo
= Cu/0.25) should be about 400 kPa. The actual buoyant stress (i.e. excluding hydrostatic
condition pressures) at a depth of 200m being around 1600 kPa (200m x (18-10) kN/m3), it
would indicate an excess pore pressure of 1200 kPa, i.e. a ru* of 0.75. This analysis
indicates that, based on the in situ undrained shear strength data, the actual slope is in a near
equilibrium conditions with a marginal factor of safety.
Investigating the strength profiles in Figures 4 and 5 also reveals that the Cu/σ’vo , within
the upper 30m, remains at a high value, i.e. greater than 0.2, which would also suggest that,
in this area, shallow failures are less likely to occur (see also Booth et al. 1984). This may
also explain also the low intensity of gulleying in that area compared to the region just to
the south. It also suggests that with burial, the Cu/σ’vo decreases as drainage boundaries
and stress conditions changes.
6. Discussion
The above analysis has indicated that the actual Hudson Apron slope is in a marginal state
of stability if the measured strength and water content values measured in the deeper
sections in ODP core 1073 are correct. We can consider the factors that could lead to such
an equivalent high pore pressure in the sediments.
Stability analysis of the Hudson Apron Slope, off New Jersey, USA
277
6.1 PREDISPOSITION AND REVEALING FACTORS
An analysis of sedimentation rates has revealed that this portion of the continental margin,
because of its position along the pathways of melt-waters from continental glaciers, has
experienced very high sedimentation rates of the order of 80 cm /ky during the Pleistocene
(Wei 2002) resulting in strong layering contrast between periods of high and low
sedimentation rates. As indicated by Dugan and Fleming (2000 and 2002) this
sedimentation has influence the drainage boundary conditions giving rise to excess pore
pressure where the rates were the highest but with flow path towards the lower part of the
slope. Our analysis also indicates that it resulted in under-consolidation of the sediments in
that portion of the slope.
The revealing factors of signs of instability were obtained as a result of the mapping of the
various mass movements (Booth et al. 1993, McHugh et al. 2002). In addition, evidence of
a deep groundwater aquifer, observed by Robb (1984), on the sea floor at the toe of the
slope, also reveals potentially high pore pressures possibly at the contact between the basal
Miocene sediments and the overlying sediments. Other evidence of instability has been
observed to the south by Driscoll et al. (2000) who described some fissures near the crest of
the slope.
Of interest is also a comparison with the nearby Hudson Canyon. It is deep and has slopes
of about 10° to 15°, quite a contrast with the Hudson Apron slope. Reasons for this
difference are not clear but could be due to different drainage pattern, different lithologies
and rate of sedimentation. This is also true for the gullied slope to the south that is now well
drained and most likely more stable.
6.2 STRENGTH AND WATER CONTENT: ANY HAZARD ISSUES?
As indicated above, the ODP 1073 profile is quite peculiar with high values of water
content and low values of undrained shear strength. The Cu/σ’vo, assuming hydrostatic
conditions, is decreasing rapidly below a depth of 30 m to values below 0.1 to remain more
or less around a value of 0.05 (down to about 250 mbsf) which would indicate an
overestimation of σ’vo and pore pressures much higher than hydrostatic. One way we can
think of maintaining such a high water content is because of the reduction in the drainage
efficiency during consolidation and this impact has been well described by Dugan and
Fleming (2000 and 2002). If the expected high in situ excess pore pressure were only the
result of groundwater flow from the underlying aquifer such a pressure would have had a
tendency to generate shallow failures in the sediments being deposited just over the
discharge area (e.g. sapping, Robb 1984). So, in that sense, we would favor the role of high
sedimentation rates in the context of a stratified system which would maintain high pore
pressure near some horizons with, most likely, some influence of the deep groundwater flow
system in the Miocene coarser sediments.
With such a low strength, is yielding or creep possible? We can approach this by
considering the slope sediments like a Bingham material and taking the measured intact
strength as the yield strength (τc), and using the following equation from Hampton (1972):
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Locat et al.
 τ
H c =  , c
 γ sin β




[4]
where Hc is the critical height above which flow takes place. Using Eq.[4] one finds that for
a slope of 4°, and yield strengths of 75 and 125 kPa, the respective values of Hc are 134m
and 224m respectively, which corresponds to the depths at which such strength values were
measured. Although the direct link to a Bingham-like material may not be so simple, e.g.
effect of sampling disturbance, it does indicate that if the actual strength values are correct,
it could lead to slope deformation in a way similar to creep features observed by Driscoll et
al. (2000) further to the south of the New Jersey Margin.
This study also indicates that if a stability analysis was only done with samples taken with
typical 10m piston samplers, we would have concluded, like Booth et al. (1984), that the
factor of safety of the slope, against failure, is well above unity. A longer geotechnical
profile (Figure 4) appears to bridge with the deeper one (Figure 5) by indicating, like also
pointed by Dugan and Fleming (2002), that the factor of safety is decreasing with depth, at
least within the Pleistocene section of the slope.
Clearly, if the measured strength and water content values are valid, and so far we are
incline to believe that they are, then we must consider that the actual Hudson Apron slope is
in a very marginal state of equilibrium. Under such conditions, little changes in the
environment, or continuing sedimentation at a relatively high rate, could cause other
significant mass movements or slope deformation. Therefore, any major seabed activities
involving in situ development (e.g. oil and gas exploitation) would require in situ
measurements of both the strength and the pore pressures.
7. Conclusions
From our back analysis of the Hudson Apron slope, we can propose the following
conclusions;
• High pore pressures are required to generate instability along the Hudson Apron
slope.
• The potential main contributing factor is considered to be the high sedimentation
rate in the context of a layered system and groundwater seepage.
• From the strength data, there are remaining high excess pore pressures in the slope
so that any future development in this area should call for in situ measurement of
both the strength and pore pressures.
• The low strength values may reveal some potential for creeping or yielding of the
slope.
Stability analysis of the Hudson Apron Slope, off New Jersey, USA
279
8. Acknowledgements
The authors would like thank the U.S. Office of Naval Research for their continued support
via the STRATAFORM and EuroSTRATAFORM projects. We would also like to thank the
Natural Sciences and Engineering Research Council of Canada for their support into
COSTA-Canada. We appreciate to cooperation of G. Mountain of Lamont-Doherty Earth
Observatory. We also thank Progrm Image for their support on the Marion Dufresne II and
T. Robert for his work on the geotechnical analyses of the MD992212 core.
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