LIQUEFACTION POTENTIAL OF GASSY MARINE SANDS
J.L.H. GROZIC
University of Calgary, Dept. of Civil Engineering, Alberta, Canada, T2N 1N4
Abstract
Loose submarine sand deposits, such as those found in the Fraser River Delta,
Canada, are often susceptible to flow or cyclic liquefaction. These deltaic soils
often contain both free and dissolved gas, most commonly methane, which will
alter the soil strength and hence its potential for liquefaction. This paper evaluates
the liquefaction resistance of loose gassy sands using laboratory and constitutive
modeling results and presents a preliminary method for evaluating the liquefaction
potential of gassy sands.
Keywords: Gassy, liquefaction, loose sand, submarine slide, triaxial
1. Introduction
Gas charged sediments are known to be widely distributed throughout the world’s
oceans, and occurrences have been reported in coastal and estuarine regions, across
the continental shelves, and within deep ocean basins.
A gassy soil contains a large amount of gas dissolved in its pore fluid, which will
affects its response to loading and unloading (Sobkowicz and Morgenstern 1984).
The bubbles develop when the volume of gas exceeds the maximum volume that
can be dissolved in the pore liquid at a particular pressure and temperature. This
paper discusses only soils where the water phase is continuous and the gas phase is
discontinuous; in other words, where the gas phase forms discrete bubbles. In
coarse-grained soils, the gas bubbles are relatively small compared with the particle
size and form within the void spaces.
The presence of gas in soils will affect behavior of the soil by altering its
engineering properties such as its shear strength, settlement characteristics, and
potential for flow or cyclic liquefaction. Gas in seabed soils will affect marine
geophysical surveys, foundation design, drilling procedures, slope stability, and
may even have environmental effects. This paper focuses on the effect of gas on the
resistance of loose soils to liquefaction.
2. Liquefaction flow slides
Soil liquefaction is a phenomenon that gives rise to a loss of shearing resistance, or
to the development of excess strains as a result of a transient or repeated
disturbance. Often instabilities in cohesionless sediments are a result of liquefaction
flow slides. Many liquefaction events, both flow and cyclic, have occurred in
37
38
Grozic
submarine slopes. Two submarine liquefaction slides of particular interest are
discussed.
2.1 THE FRASER RIVER DELTA
The Fraser River is the largest river in British Columbia and it forms an actively
prograding delta into relatively open water. The Fraser River delta has been
subjected to reoccurring flow liquefaction slides (McKenna et al. 1992) placing
dykes, jetties, submarine power cables, a lighthouse, and even the Tswwassen ferry
terminal and the Robert Banks coal port at risk of damage due to potential
submarine instabilities. Between 1970 and 1985 five known liquefaction slides
occurred in the silty sand deltaic deposits. An investigation into the 1985
liquefaction slide evaluated the contribution of possible triggering mechanisms such
as sedimentation, surface waves, and tidal drawdown on saturated sediments, and
concluded that neither alone nor combined could any of these mechanisms account
for the deep-seated flow failures that were observed (Chillarige et al. 1997). The
observation of high concentrations of methane gas in the pore fluid of these deposits
(Christian et al. 1997) prompted an examination into the effect of gas. Chillarige et
al. (1997) found that the presence of gas in the sediments could generate residual
pore pressures, which during low tides could trigger submarine liquefaction
failures.
2.2 THE KLAMATH RIVER DELTA
In 1980, a major earthquake (magnitude 6.5 to 7.2) occurred 60 km off the coast of
northern California (Field et al. 1982). Onshore the effects were relatively minor
but offshore commercial fisherman reported the presence of ridges and scarps on
the usually flat seabed off the Klamath River. Within a month of the earthquake, a
marine survey was conducted and when compared to pre-earthquake surveys the
data demonstrated that a large area of the Klamath delta (at least 20 km2) had failed
as a direct result of the earthquake (Field et al. 1982). Features such as a terrace, toe
scarp, compressional ridges, sand boils, and collapse craters indicated that the fine
to medium sand sediments of the delta had experienced earthquake-induced cyclic
liquefaction and subsequent flow failure. Marine geophysical surveys indicated
evidence of gas, both before and after the earthquake (Field and Jennings 1987).
The geophysical data showed that the gas was present in significant quantities
immediately after the earthquake. Five years after the earthquake gas seeping had
returned to pre-earthquake levels. Field and Jennings (1987) postulated that the
earthquake triggered the release of large quantities of gas, which contributed to the
loss of strength and failure of the deltaic deposits.
2.3 OBSERVATIONS
A few observations can be made from the above case histories, as well as from
other submarine liquefaction slides. First, flow liquefaction failures, whether
statically induced or earthquake induced, are a reasonably common phenomenon in
submarine soils, yet only when they damage or adversely affect man-made
structures or activities are we aware of their impact. Second, the presence of gas in
submarine sediments does affect the soil’s engineering properties and behavior,
Liquefaction potential of gassy marine sands
39
specifically its resistance to liquefaction. Third, the presence of gas may contribute
to, or even trigger, flow liquefaction failure.
3. Static Liquefaction of Gassy Sand
3.1 MONOTONIC TRIAXIAL TEST RESULTS
Monotonic triaxial compression tests were performed on saturated and gassy sand
specimens to evaluate the effect of gas on the static liquefaction of loose sand.
Over 20 reconstituted specimens of Ottawa sand were tested in drained and
undrained loading conditions (Grozic et al. 1999). Although all the gassy specimens
were sheared in undrained conditions, the void ratio and degree of saturation
changed during loading due to compression and solution of the gas.
The tests performed on gassy specimens revealed that a strain softening response
occurred if the initial degree of saturation was greater than about 88%. A strain
hardening response was observed when the initial degree of saturation was between
about 88% and 80%, except when the initial void ratio was exceptionally high.
When the initial degree of saturation was less than about 80%, the specimens
responded in a strain hardening manner regardless of the initial void ratio. Figure 1
illustrates the various specimen responses and shows that the range of response of
the gassy specimens is bounded by the saturated tests. The results indicate that the
higher the initial degree of saturation the closer the response to a saturated
undrained test.
1000
Deviator Stress, q (kPa)
A
800
B
Legend
600
C
400
D
200
F
einitial
Sr initial
A
0.83
100% (CD)
B
0.90
80%
C
0.87
86%
D
0.92
80%
E
0.85
91%
F
0.91
100% (CU)
E
0
0
200
400
600
Effective Mean Norm al Stress, p’ (kPa)
800
Figure 1. Monotonic laboratory test results; Effective stress paths.
3.2 CONSTITUTIVE MODELING OF GASSY SAND
The theoretical behavior of loose gassy sand was also investigated using a
constitutive model (Grozic et al. 2002). The model is able to assess the liquefaction
40
Grozic
of loose gassy sands over a wide range of states and loading conditions by taking
into account the compressibility and solubility of the pore gas and liquids. The
initial degree of saturation is needed for model prediction and the coefficient of
volumetric solubility (Henry’s constant) is introduced as a new model parameter.
For simplicity, the model does not take into account the time dependent behaviour
of gas compression and solution. The model was verified using the results of the
monotonic triaxial tests on saturated and gassy sands.
The model was then used to predict the effect of degree of saturation on the
undrained behavior of loose sand. The results as presented in Figure 2, show that as
the degree of saturation is decreased the response becomes more strain hardening.
The stress strain curves and the stress paths also show that strain softening and flow
liquefaction may occur in soils with degrees of saturation greater than
approximately 90%. For soils with degrees of saturation less than 85%, no potential
for flow liquefaction was observed. The model confirmed that gas has the effect of
decreasing, but not eliminating, the susceptibility to flow liquefaction of loose sand.
700
Deviator Stress, q (kPa)
600
80%
500
400
85%
300
90%
200
95%
100
Saturated
0
0
100
200
300
400
Effective Mean Norm al Stress, p’ (kPa)
500
Figure 2. Constitutive modeling results; Effective stress paths. Initial degree of saturation is
indicated adjacent to each test.
For gassy specimens, the liquefaction potential will depend on the degree of
saturation and the gas and pore fluid characteristics, in addition to the soil state,
grain characteristics, and mode and rate of shearing. The laboratory results showed
that the presence of gas shifts the state boundary surface up in a e – logp’ projection.
A higher state boundary surface means that either a higher void ratio or a higher
effective mean normal stress is necessary to trigger collapse. The laboratory and
modeling test results also showed that for compressive loading, at a given initial
density, the potential for flow liquefaction of gassy sand is lower than that for
saturated sand. However, liquefaction can occur in gassy soils provided they are
loose enough and the initial degree of saturation is high enough.
Liquefaction potential of gassy marine sands
41
4. Cyclic Liquefaction of Gassy Sand
4.1 CYCLIC TRIAXIAL TEST RESULTS
Cyclic triaxial tests were performed on saturated and gassy specimens to determine
the effects of gas on the propensity for loose sand to cyclicly soften and experience
cyclic liquefaction. Over 15 reconstituted specimens of Ottawa sand were tested in
undrained loading (Grozic et al. 2000). Similar to the monotonic tests, the gassy
cyclic tests changed in degree of saturation and density during loading due to
compression and solution of the gas.
A significant difference in response between a gassy and a saturated specimen was
observed in the test results. The differing responses showed that a gassy specimen
must reach the failure envelope as a result of an increase in applied load, whereas a
typical saturated specimen reaches the failure envelope as a result of a decrease in
effective mean normal stress caused by an increase in pore pressure. Details of this
phenomenon are given in Grozic et al. (2000).
The laboratory test results revealed that the presence of gas increased the resistance
to cyclic loading, by 200% to 300%. Figure 3 presents a plot of the cyclic resistance
ratio (defined as the cyclic stress ratio to cause liquefaction or a specified amount of
strain) in triaxial conditions versus the number of cycles to failure. The figure
shows that the lower the initial degree of saturation, the higher the cyclic resistance.
During loading, all the gassy specimens increased in density due to gas compression
and solution. As the initial degree of saturation decreases, there is a greater change
in relative density during loading. The change in density can be approximated from
(where Sr is in percent):
∆Dr = -0.98Sr + 0.98
(1)
This approximation is based on the laboratory data and is for loose specimens only;
for dense specimens, little change in density would be expected regardless of the
initial degree of saturation.
For gassy soils, the resistance to cyclic liquefaction will be affected by the initial
degree of saturation and the pore gas and liquid properties, in addition to the applied
shear stress, soil state, and loading characteristics. The cyclic test results show that
the presence of gas increases the resistance to cyclic liquefaction.
42
Grozic
0.35
74%
0.30
76%
79% 83%
87%
84%
(CRR)tx
0.25
78%
0.20
76%
S aturated or S r =100%
0.15
100% 99%
0.10
0.05
Value adjacent to each data point
indicates the initial degree of s aturation.
0.00
0.1
S aturated S amples
Gas s y S amples
1
10
Num ber of Cycles, N
100
Figure 3. Triaxial cyclic resistance ratio versus number of cycles to failure.
6.
Evaluating the liquefaction potential of loose gassy sand
6.1 POTENTIAL FOR FLOW LIQUEFACTION
The concept of flow potential is a useful framework in which to evaluate the
potential for flow liquefaction (Yoshimine and Ishihara 1998). Flow potential can
be represented by the maximum excess pore pressure ratio achieved during
undrained monotonic loading. Flow potential, uf, is defined as:

p′ 
u f = 1 − PT  × 100(%)
p c′ 

(2)
where uf is the flow potential, p’c is the effective mean isotropic confining stress,
and p’PT is the effective mean normal stress at phase transformation including steady
state. A flow potential value of 100% would indicate the specimen was completely
strain softening and a value of 0% would indicate the specimen was completely
strain hardening and therefore not susceptible to liquefaction.
The results of the monotonic laboratory program were analyzed using the concept
of flow potential. Figure 4 shows a plot of initial void ratio, with the corresponding
initial relative density, versus initial degree of saturation. The flow potential value
is indicated beside each data point. A zone of potential liquefaction is shown. The
zone is delineated by a lower solid line that separates the test results, a dashed line
that is an extension of the solid line, and an upper dash-dot line that indicates the
maximum possible void ratio. The upper dash-dot line rises slightly as the degree of
saturation decreases because at low degrees of saturation sample bulking can occur
due to gas exsolution resulting in very loose specimens.
Liquefaction potential of gassy marine sands
43
1.10
-68%
1.00
86
Initial Void Ratio, e
0.95
0.90
0
-48%
POTENTIAL LIQUEFACTION
95
96
0
0
0
62
0
0
-28%
0
0
97
0.85
0
-8%
100
0.80
0
0.75
38
NO LIQUEFACTION
12%
0
96
Initial Relative Density, Dr
1.05
-88%
Value adjacent to each data point indicates
the flow potential in percent.
32%
0.70
52%
0.65
0.60
105%
100%
95%
90%
85%
80%
Initial Degree of Saturation, Sr
75%
70%
Figure 4. Initial void ratio versus initial degree of saturation with flow potential. Liquefaction zones
based on Ottawa sand, triaxial compression, and engineering judgement.
The liquefaction zones presented in Figure 4 show that if the initial degree of
saturation is 100%, then flow liquefaction can occur if the specimen is loose
enough. As the initial degree of saturation decreases, flow liquefaction is only
possible if the specimens are very loose, that is they plot within the potential
liquefaction zone. When the initial degree of saturation drops below about 75%,
liquefaction is not possible regardless of the initial density.
The zone of potential liquefaction shown in Figure 4 is based on triaxial
compression tests. A similar zone of potential liquefaction could be identified for
specimens tested in direct simple shear; the simple shear liquefiable zone would
plot lower than the triaxial compression zone shown in Figure 4. The lines sketched
on Figure 4 are for specific laboratory conditions and are drawn based on
engineering judgment. This figure should be used only to perform a preliminary
assessment of the flow liquefaction potential of gassy sand.
6.2 POTENTIAL FOR CYCLIC LIQUEFACTION
A method to predict the cyclic resistance of loose gassy sand is proposed based on
the results of the cyclic triaxial tests. The method involves two steps; the first step is
to determine the cyclic resistance ratio assuming the soil is saturated and the second
step is to adjust the cyclic resistance ratio to account for the presence of gas.
To determine the cyclic resistance ratio (CRR) assuming the soil is saturated, a
correlation based on initial relative density such as that proposed by Seed et al.
(1985) is used. Seed et al. (1985) developed the limiting line based on field
observations and although it is not directly linked to laboratory data, the saturated
44
Grozic
cyclic tests performed as a part of this research plotted very close to Seed’s limiting
line. If the range of initial densities from very loose to medium is considered, then
the limiting line can be approximated with a linear function. Using the limiting line
and the initial density of the gassy soil, the CRR can be determined for an
equivalent saturated soil from the equation (where Dr is in percent):
(CRR7.5)SS = 0.2515Dr – 0.002
(3)
To account for the effects of gas, the CRR must be adjusted. This is carried out
using the assumption that when the specimen changed density during cyclic
loading, the density and CRR at failure would plot on Seed's limiting line, a
hypothesis confirmed by the cyclic laboratory results. By combining Seed's limiting
line with Equation (1), the increased cyclic resistance due to the presence of gas can
be determined based on the initial degree of saturation from (where Sr is in percent):
∆(CRR7.5)SS = -0.247Sr + 0.247
(4)
The final step in the proposed method is to simply add the increased CRR (4),
which accounts for the gas to the CRR found assuming the soil was saturated (1).
For any initial degree of saturation and density, the resistance to cyclic loading can
be estimated using this proposed method. This procedure is based on specific
laboratory conditions and as such is suggested as a guideline only.
7. Summary and conclusions
Many liquefaction events have occurred in submarine slopes known to contain gas.
This paper presents the summary of a research program aimed at determining how
the presence of gas affects the potential for flow and/or cyclic liquefaction. Results
from monotonic laboratory testing and constitutive modeling indicate that the
presence of gas reduces the propensity for loose sand to strain soften and experience
static liquefaction; provided that the initial degree of saturation is high enough and
sand is loose enough, static liquefaction can still occur. Results from cyclic
laboratory testing indicate that the presence of gas increases the resistance of loose
sand to cyclic liquefaction. For cyclic liquefaction of gassy sand to occur the
applied cyclic load must be almost as great as the load required for monotonic
extensive failure. Methods to evaluate the potential for flow and cyclic liquefaction,
based on the initial density and degree of saturation, are proposed.
8. References
Chillarige, A.V., Robertson, P.K., Morgenstern, N.R., and Christian, H.A. 1997. Seabed instability due
to flow liquefaction in the Fraser River delta. Canadian Geotechnical Journal, 34(4): 520-533.
Christian, H.A., Woeller, D.J., Robertson, P.K., Courtney, R.C. 1997. Site investigations to evaluate
flow liquefaction slides at sand heads, Fraser River delta. Canadian Geotechnical Journal,
34(3): 384-397.
Field, M.E. and Jennings, A.E. 1987. Seafloor gas seeps triggered by a northern California earthquake.
Marine Geology, 77: 39-51.
Liquefaction potential of gassy marine sands
45
Field, M.E., Gardner, J.V., Jennings, A.E., and Edwards, B.D. 1982. Earthquake-induced sediment
failures on a 0.25o slope, Klamath River delta, California. Geology, 10: 542-546.
Grozic, J.L., Robertson, P.K., and Morgenstern, N.R. 1999. The behavior of loose gassy sand. Canadian
Geotechnical Journal, 36(3): 482-492.
Grozic, J.L.H., Robertson, P.K., and Morgenstern, N.R. 2000. Cyclic liquefaction of loose gassy sand.
Canadian Geotechnical Journal, 37: 843-856.
Grozic, J.L.H., Imam, S.M.R., Robertson, P.K., and Morgenstern, N.R. 2002. Constitutive modelling of
gassy soil behaviour. Accepted for publication in the Canadian Geotechnical Journal.
McKenna, G.T., Luternauer, J.L., and Kostaschuk, R.A. 1992. Large-scale mass-wasting events of the
Fraser River delta front near Sand Heads, British Columbia. Canadian Geotechnical Journal,
29: 151-156.
Seed, H.B., Tokimatsu, K., Harder, L.F., and Chung, R. 1985. Influence of SPT procedures in soil
liquefaction resistance evaluations. Journal of Geotechnical Engineering, ASCE, 111(12):
1425-1445.
Sobkowicz, J.C. and Morgenstern, N.R. 1984. The undrained equilibrium behavior of gassy sediments.
Canadian Geotechnical Journal, 21: 439-448.
Yoshimine, M. and Ishihara, K. 1998. Flow potential of sand during liquefaction. Soils and Foundations,
38(3): 189-198.