Deep-water Processes by G.
Shanmugam
Mechanics of sediment failure and sliding
Sediment failures on continental margins are controlled
by the pull of gravity, the source of the material (bedrock vs. regolith), the
strength of the soil (grain size, mineralogy, compaction, cementation, etc.),
the weight of the material, the slope angle, the pore-water pressure, and the
planes of weaknesses. In order to evaluate sediment failures in general, one
needs to conduct a slope stability analysis for describing the sediment
behavior and sediment strength during loading or deformation.
The role of excess pore-water pressure
Terzaghi (1936) first recognized hat
pore-water pressure controls the frictional resistance of slopes, which has
remained the most important concept in understanding landslide behaviour. A founding principle of slope stability is
that a rise in pore-water pressure reduces the shear strength of the soil
(Skempton, 1960). The shear strength of soil, in particular
clays, is controlled by the frictional resistance and interlocking between
particles (i.e., physical component), and interparticle forces (i.e.,
physicochemical component) (Karcz and Shanmugam, 1974; Parchure, 1980; Hayter et al., 2006). A rise in pore-water
pressure occurs when the saturated soil is stressed, and when the porosity
cannot increase or the pore fluid cannot expand or escape through fractures.
The excess pore-water pressure has been considered a vital factor in explaining
the origin of subaerial mass-transport processes (Johnson, 1984; Anderson and
Sitar, 1995; Iverson, 1997, 2000; Iverson et al., 1997; Jakob and Hungr, 2005).
Soil strength and slope stability
The most fundamental requirement of slope stability is
that the shear strength of the soil must be greater than the shear stress
required for equilibrium (Duncan and Wright, 2005; Shanmugam, 2014). The two
conditions that result in slope instability are (1) a decrease in the shear
strength of the soil and (2) an increase in the shear stress required for
equilibrium. The decrease in the shear strength of the soil is caused by
various in situ processes, such as an increase in pore-water pressure, cracking
of the soil, swelling of clays, leaching of salt, etc. The increase in shear
stress is induced by loads at the top of the slope, an increase in soil weight
due to increased water content, seismic shaking, etc.
A
common method for calculating the slope stability is the ‘Limit equilibrium
analyses’ in soil mechanics. A stable slope can be maintained only when the
factor of safety for slope stability (F)
is larger than or equal to 1 (Duncan and Wright, 2005, their equations 6.1 and
13.2):
A sediment failure is
initiated when the factor of safety for slope stability (F) is less than 1
(Figure 1B). In other words, the sliding motion along the shear surface
commences only when the driving gravitational force exceeds the sum of
resisting frictional and cohesive forces. Initial porosity of the sediment
plays a critical factor in controlling the behavior of the shear surface
(Anderson and Riemer, 1995). Based on an experimental study on landslides
initiated by rising pore-water pressures, Iverson et al. (2000) reported that
even small differences in initial porosity had caused major differences in
mobility. For example, wet sandy soil with 50% porosity contracted during slope
failure, partially liquefied, and accelerated to a speed of over 1 m s-1,
whereas the same soil with 40% porosity dilated during failure, slipped
episodically, and traveled at a slow velocity of 0.2 cm s-1.
Finally, soil strength differs between drained and undrained conditions
(Terzaghi et al., 1996; USACE, 2003; Duncan and Wright, 2005).
Figure 1. A. Plot showing that the shear strength of the soil
(s) is composed of frictional (φ) and cohesive (c) components. B. Conceptual diagram
showing that a stable slope can be maintained only when the factor of safety
for slope stability (F) is larger
than or equal to 1. The sliding motion of failed soil mass commences along the
shear surface when the factor of safety (F) is less than 1. From Shanmugam,
2014).
The landslide problem (Shanmugam, G., 2015)
Abstract The synonymous use of the
general term ‘”landslide”, with a built-in reference to a sliding motion, for
all varieties of mass-transport deposits (MTD), which include slides, slumps, debrites,
topples, creeps, debris avalanches etc. in subaerial, sublacustrine, submarine,
and extraterrestrial environments has created a multitude of conceptual and
nomenclatural problems. In addition, concepts of triggers and long-runout
mechanisms of mass movements are loosely applied without rigor. These problems
have enormous implications for studies in process sedimentology, sequence
stratigraphy, palaeogeography, petroleum geology, and engineering geology.
Therefore, the objective of this critical review is to identify key problems
and to provide conceptual clarity and possible solutions. Specific issues are
the following: 1. According to
"limit equilibrium analyses" in soil mechanics, sediment failure with
a sliding motion is initiated over a shear surface when the factor of safety
for slope stability (F) is less than 1. However, the term landslide is not
meaningful for debris flows with a flowing motion. 2. Sliding motion can be measured in oriented core and outcrop, but
such measurements are not practical on seismic profiles or radar images. 3.
Although 79 MTD types exist in the geological and engineering literature, only
slides, slumps, and debrites are viable depositional facies for interpreting
ancient stratigraphic record. 4. The use of the term landslide for
high-velocity debris avalanches is inappropriate because velocities of
mass-transport processes cannot be determined in the rock record. 5. Of the 21
potential triggering mechanisms of sediment failures, frequent short-term
events that last for only a few minutes to several hours or days (e.g.,
earthquakes, meteorite impacts, tsunamis, tropical cyclones, etc.) are more
relevant in controlling deposition of deep-water sands than sporadic long-term
events that last for thousands to millions of years (e.g., sea-level
lowstands). 6. The comparison of H/L (fall height/runout distance) ratios of
MTD in subaerial environments with H/L ratios of MTD in submarine and
extraterrestrial environments is incongruous because of differences in data
sources (e.g., outcrop vs. seismic or radar images). 7. Slides represent the pre-transport disposition of strata and their
reservoir quality (i.e., porosity and permeability) of the provenance region,
whereas debrites reflect post-transport depositional texture and reservoir
quality. However, both sandy slides and sandy debrites could generate blocky
wireline {gamma-ray) log motifs. Therefore, reservoir characterization of
deep-water strata must be based on direct examination of the rocks and related
process-specific facies interpretations, not on wireline logs or on seismic
profiles and related process-vague facies interpretations. A solution to these
problems is to apply the term “landslide” solely to cases in which a sliding
motion can be empirically determined. Otherwise, a general term MTD is
appropriate. This decree is not just a quibble over semantics; it is a matter
of portraying the physics of mass movements accurately. A precise
interpretation of a depositional facies (e.g., sandy slide vs. sandy debrite) is
vital not only for maintaining conceptual clarity but also for characterizing
petroleum reservoirs.
Key words debris flows, landslides, mass-transport
deposits (MTD), reservoir characterization, slides, slumps, soil strength,
triggering mechanisms
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