TOWARDS AN APPROACH FOR THE ASSESSMENT OF RISK ASSOCIATED
WITH SUBMARINE MASS MOVEMENTS
S. LEROUEIL
Dept. of Civil Engineering, Université Laval, Québec, Canada, G1K 7P4
J. LOCAT, C. LEVESQUE
Dept. of Geology and Geological Engineering, Université Laval, Québec,
Canada, G1K 7P4
H.J. LEE
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 USA
Abstract
With the growing development of offshore natural resources, use of sea-floor transport
and communication routes, considerations for the environment and the effects of global
climate changes, and will for protecting populations and their infrastructures, the need
for assessing risk associated with submarine mass movements is increasing. The present
paper proposes an approach for the assessment of risk associated with submarine mass
movements based on geotechnical characterisation.
Keywords: Mass movements; submarine environment; geotechnical characterisation;
hazard; risk assessment.
1. Introduction
Submarine mass movements may have important consequences on the submarine
environment itself, offshore structures such as oil and gas platforms, and coastal
infrastructures such as harbours and cities. With the growing development of offshore
natural resources, use of sea-floor transport and communication routes, considerations
for the environment and the effects of global climate changes, and will for protecting
populations and their infrastructures, the need for assessing risk associated with
submarine mass movements is increasing.
An interesting approach for examining inland mass movements was proposed by
Leroueil et al. (1996). This approach, named “Geotechnical characterisation of mass
movements”, considers mass movements defined in geomorphological classifications,
10 types of geomaterials defined on the basis of their mechanical behaviour and 4 stages
of movements, namely the pre-failure stage, the failure stage, the post-failure stage and
the reactivation stage. The information concerning the slope movement is divided in
three classes (Champetiers de Ribes, 1987): predisposition factors, triggering or
aggravating factors, and revealing factors. Because it splits the complex problem of
mass movements into more focussed aspects, the Geotechnical characterisation of mass
movements appears to be a powerful tool for assessing risk associated with mass
movements (Leroueil & Locat, 1998).
59
Leroueil et al.
60
Submarine mass movements may differ from sub-aerial slope movements: they can
originate from nearly flat surfaces, with slope sometimes lower than 1°; the causal
factors may be different; the volumes of geomaterials involved may be of several
thousands of km3; and travel distances may reach tens of kilometres (Hampton et al.,
1996; Locat & Lee, 2002). Consequently, the Geotechnical characterisation initially
developed for sub-aerial slopes has to be adapted to the submarine environment.
However, it is thought that the general framework and its application to risk assessment
are still valid.
In the present paper, a general approach for the assessment of risk associated with
submarine mass movements is proposed. In the first part, a geotechnical characterisation
adapted to submarine mass movements is presented. In a second part, considerations on
risk assessment are made.
2. Geotechnical characterisation of submarine mass movements
The geotechnical characterisation of slope movements may be schematised by the 3-D
matrix shown in Fig. 1. The axes of the matrix represent the type of material, the type of
movement and the stage of movement. Not all the elements of the matrix are
representative of real situations. However, for each relevant element, a characterisation
sheet containing the following elements is proposed:
controlling laws and associated parameters;
predisposition factors;
triggering or aggravating factors;
revealing factors; and
consequences of the movement.
T
TS
N
ME
MATERIALS
SLOPE MOVEMENTS
VE
MO
ES
AG
• Controlling laws and
parameters
• Predisposition factors
• Triggering or aggravating
factors
• Revealing factors
• Consequences
Figure 1. Schematic mass movement characterisation (from Leroueil et al., 1996).
Towards an Approach for Risk Assessment
61
The types of material that can be encountered in submarine environments are essentially
the same as those found inland, with the exception of residual soils and unsaturated soils
with continuous gas phase. On the other hand, gassy soils with gas originating from the
decay of organic matter (primarily methane) or from the dissociation of gas hydrates
may be found (see Fig. 2).
Figure 2. Material types considered in the characterisation.
The stages of movement in submarine conditions are the same as those existing in
subaerial conditions: pre-failure, failure, post-failure and reactivation. However, the
reactivation stage associated with movements developing on pre-existing failure
surfaces does not seem to be documented. Pre-failure movements include all the
movements that occur before a first-time failure. They can result from a combination of
phenomena: elasto-plastic deformations associated with changes in effective stresses;
viscous deformations (creep); and strains and displacements associated with progressive
failure (Leroueil, 2001). Such movements, already difficult to observe in onshore
conditions, are poorly documented in submarine conditions. However, Syvitski et al.
(1987) reported several evidences of creep observed offshore Canada, and Locat et al.
(2001) observed fissures at the site of Palos Verdes rock avalanche.
Failure is characterised by the formation of a continuous shear surface through the entire
soil or rock mass. As indicated on Fig. 3, the main types of movement, that link geology
and landslide activity, are similar and present the same characteristics as in onshore
conditions. As discussed later on, some causal factors may, however, be different. Locat
& Lee (2002) give examples of submarine failures triggered by a variety of
mechanisms.
It is at the post-failure stage that movements can differ the most from onshore
conditions. The moving body may remain largely undisturbed. That is generally the case
for rock masses that sometimes may glide in water over long distances (Moore et al.,
1989). In the case of spontaneous liquefaction, the moving mass will continue as a flow.
In the other cases, the behaviour of the moving mass depends on the physical and
mechanical characteristics of the soil and on the potential energy available. If this latter
is large enough to completely remould or destructure the soil, the slide may become a
Leroueil et al.
62
flow. The minimum strength of the material is then defined as the critical state strength
for cohesionless soils; in cohesive soils, it is related to the liquidity index of the clay
(Cur (kPa) = (IL – 0.21)-2; Leroueil et al., 1983). In flows, it is often convenient to consi-
Figure 3. Movement types at the failure and post-failure stages.
der the moving mass as a viscous liquid. Locat (1997) proposed the following
rheological model:
 c 

τ = τ c + ηγ + 
γ +γo 
(1)
in which τ is the resistance to flow, τc is the yield strength, η is the dynamic viscosity
(in mPa·s), γ is the shear rate, γo is the shear rate corresponding to the yield strength of
the bi-linear fluid, and c is a constant with units in kPa·s-1.
Also, Locat (1997) showed that the parameters η and τc from Eq. 1 can be related to the
liquidity index of the soil. For pore-water salinity of about 30 g/l and liquidity index
larger than 1.0, Locat (1997) proposed the following equations:
 9.27 

η = 
 IL 
3. 3
(2)
and
 12.05 

τ c = 
 IL 
3.13
(3)
Above, it is considered that the soil remains at its natural water content. However,
during mass movement in a marine environment, water can be incorporated into the
Towards an Approach for Risk Assessment
63
moving mass and the strength and viscous characteristics of the material vary in
agreement with Eqs. 1 to 3.
If the volume concentration of solid becomes lower than 9% and the movement is
turbulent, the flow becomes turbidity current (Bagnold, 1962; Mulder & Cochonat,
1996; see Fig. 3).
The division of movements into different stages reflects the dynamic aspect of mass
movements. It also evidences the fact that the laws and parameters controlling each
stage are different from one stage to the other.
The main predisposition and triggering or aggravating factors encountered in submarine
environment are listed in Table 1. It is important to separate both predisposition and
triggering factors. For example, presence of gas hydrates constitutes a predisposition
factor; it is their dissociation due to a decrease in pressure or an increase in temperature
that may be a triggering factor. In addition to gas hydrates, one can find storm waves,
underconsolidation, diapir formation and changes in tide level that are specific to the
submarine environment.
The revealing factors provide evidence of mass movements but generally do not
participate to the process. Some are also indicated in Table 1.
Table 1. Main factors influencing or revealing submarine slope movements.
Predisposition factors
•
Zone of seismic activity
•
Weak or weakenable layers
•
Gas hydrates
•
Presence of active volcanos nearby
•
Organic materials in decomposition
•
Artesian pressures
•
Erosion
•
High sedimentation rates
•
Unfavourable layering (dense material onto less dense material; loose sand under low
permeability layer; etc.)
Triggering or aggravating factors
•
Earthquakes
•
Volcanic activity
•
Gas hydrate dissociation : f (pressure, temperature)
•
Gassy soils: f (permeability, rate of organic decomposition)
•
Storm waves
•
Oversteepening: f (erosion, sedimentation)
•
Underconsolidation: pore pressure = f (permeability, sedimentation rate)
•
Diapir formation
•
Changes in tide level or in artesian pressures
Revealing factors
•
Evidence of previous instabilities
•
Evidence of liquefaction due to seismic activity
•
Evidence of creep or presence of cracks
• Pockmarks: springs, gassy soils
• Mud lumps or islands: diapirism
Leroueil et al.
64
3. Risk associated with submarine mass movements
Varnes et al. (1984) defined the total risk RT as the set of damages resulting from the
occurrence of a phenomenon. It can be described by the following equation:
RT = Σ H Ri Vi
(4)
in which H is the hazard or the phenomenon occurrence probability within a given area
and a given time period; Ri (for i = 1 to n) are the element at risk, potentially damaged
by the phenomenon; Vi is the vulnerability of each element represented by a damage
degree comprised between 0 (no loss) and 1 (total loss).
The elements at risk can suffer directly from the phenomenon or from induced
phenomena. The destruction of an offshore oil platform founded on an area affected by
instabilities could be a direct consequence of a landslide whereas destruction of coastal
structures by an induced tsunami would be an indirect consequence. All these elements
must be considered in a risk analysis. In the context of the geotechnical characterisation,
the elements at risk and their vulnerability should be directly or indirectly found in
“Movement consequences”, in Fig. 4.
Type of
movement
Stage of
movement
Material
Controlling laws
and parameters
Predisposition factors
Triggering or
aggravating factors
Revealing factors
Movement consequences
Their probability of
occurrence at a given
level provides the
HAZARD(prefailure,
failure and reactivation
stages)
Help defining the
ELEMENTS AT RISK
and their
VULNERABILITY
Figure 4. Geotechnical characterisation and risk assessment.
When the failure stage is considered, the associated hazard, Hf, is directly the
probability of the triggering factor to reach a critical value leading to failure (Fig. 4).
For the post-failure stage, the hazard Hpf associated with a movement with given
characteristics is more difficult to define as it involves the mechanical and physical
properties of the soil (predisposition factors) as well as the geometrical characteristics of
the moving mass. Vaunat & Leroueil (2002) describe the mobility index, which defines
the travel distance of the unstable geomaterials, as the product of sub-indices associated
with failure, brittleness of the material, ability of the soil to develop pore pressures,
Towards an Approach for Risk Assessment
65
geometry of the moving soil mass and characteristics of the terrain. This division of
post-failure movement into more focussed aspects should help in the determination of
Hpf.
When considering a landslide, two stages are involved in the geotechnical
characterisation: the failure stage followed by the post-failure stage. As a consequence,
the hazard associated with a landslide of given characteristics is the hazard associated
with the possibility of having a failure (Hf) multiplied by the hazard associated with the
possibility that the post-failure stage has specific characteristics (Hpf).
H = Hf · Hpf
(5)
When the possibility of tsunamis has to be considered, the probability that a given coast
be struck by a tsunami with a given amplitude is:
H = Hf · Hpf · Hts
(6)
in which Hf and Hpf have the same meaning as in Eq. 5 and Hts is the probability that a
tsunami of such an amplitude be generated. In general terms, Hts is a function of the
volume, velocity and deformability of the moving mass (characteristics of the
movement at the post-failure stage), of the depth of the landslide and of the distance to
the coast (Lee et al., this volume; Jiang and Leblond, 1992).
The main difficulty in risk assessment comes from the evaluation of hazard. The
problem is often complex and there are uncertainties. Morgenstern (1995) associated
uncertainties to three main sources: parameter uncertainty, model uncertainty and
human uncertainty. The uncertainty on parameters can be subdivided in two parts, well
identified in the geotechnical characterisation: the uncertainty that depends on the
spatial variation of the parameters characterising the material and the predisposition
factors (Fig. 4); the uncertainty due to temporal variation of the triggering or
aggravating factors. It is mostly because of these variations that there can be failure.
Spatial variability is a problem as geotechnical investigations are generally limited and
not of excellent quality due to the difficulty to get high quality samples. It is however
thought that it is possible to combine local investigations and seismic databases to
specify spatial distribution of geotechnical characteristics.
Hazard is extremely difficult to establish with an absolute value. It is thus essential to
use as much as possible the available information: historic information; geological
information; geomorphological and geophysical information; and geotechnical
information. If most of this information is more difficult to obtain in submarine
conditions, it must be recognised that geophysical methods provide, in submarine
conditions, information on soil stratigraphy and structures, as well as on the presence of
gas that is often difficult to obtain onshore. Historic and geomorphological information
must be used with caution as conditions, which were prevailing at the time of previous
movements, may not exist anymore. On the other hand, unfavourable conditions may
have recently developed.
The difficulty to estimate quantitatively the hazard may be overcome by a qualitative or
semi-quantitative evaluation as suggested by Hungr (1997). Also, for a number of prob-
66
Leroueil et al.
Figure 5. Risk management procedure.
Towards an Approach for Risk Assessment
67
lems, the relative evaluation of risk may be sufficient for the selection of location for the
installation of structures or infrastructures offshore or along the coast.
Figure 5 shows the procedure derived from the proposed approach for evaluating the
risk associated with submarine mass movements.
4. Conclusion
In the present paper, a general approach based on the geotechnical characterisation is
proposed for the assessment of risk associated with submarine mass movements. This
constitutes a first step in a relatively new field. However, it seems to be a rational and
powerful approach that we plan to apply in the context of the COSTA-Canada project to
several submarine areas in a near future.
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