Wave-Induced Liquefaction. General Lecture B. Mutlu Sumer

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Wave-Induced Liquefaction. General Lecture

• B. Mutlu Sumer
• Technical University of Denmark, MEK, Coastal and
  River Engineering (formerly ISVA)
• 2800-Lyngby, Denmark

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Definition

• Soft marine soils under high waves may undergo a
  process
• in which the soil grains become completely free,
  and
• the water-sediment mixture, as a whole, acts like
  a fluid!
• This process is called liquefaction.
• Under the liquefaction condition, obviously the
  soil fails!

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Consequences. With the soil liquefied,

• Buried pipelines may float to the surface of the
  seabed
• Pipelines laid on the seabed may sink in the
  soil
• Large individual blocks (like those used for
  scour protection) may penetrate into the seabed
• Sea mines may enter into the seabed and
  eventually disappear
• Or, an indirect effect As a result of the wave
  motion, structures may execute cyclic motions,
  resulting in local liquefaction around them,
  which may enhance scour, thus leading to the
  instability of the structures
• Sometimes, we use wave-induced liquefaction to
  our end, to compact sand (as was done by
  LICengineering (Denmark), a member of LIMAS, in
  combination with soil replacement in an
  engineering exercise!)

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Waves (for those who are not terribly familiar
with waves)?

• In coastal areas, Wave height O(1-2 m)
• In offshore areas, with water depth of 60-70 m,
  for example, wave height for 50-100 years return
  period O(10-20 m) are not unusual!
• Wave period O(5-15 s)!

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Two kinds of wave liquefaction

• Liquefaction induced by the buildup of pore
  pressure, called the Residual Liquefaction
• Liquefaction induced by the upward-directed
  pressure gradient, called the Momentary
  Liquefaction

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Residual Liquefaction
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Residual Liquefaction

• The result of a lab experiment with a silt
  bottom Two time series
• (1) Surface elevation and (2) Pressure time
  series
• Water depth 42 cm
• Wave height 10 cm
• Wave period 1.6 s
• Pressure measured at depth 16.5 cm in the soil

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Residual Liquefaction

• In this progressive buildup of the pore pressure,
  if the waves are high, the pressure may reach
  such levels that it will exceed the submerged
  weight of the soil above!
• In this case, the soil grains will become unbound
  and completely free, and the soil will begin to
  act like a liquid!
• This process is called the residual liquefaction!

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Watch out The conditions for Residual
Liquefaction

• Soil must be soft, like backfill in a trench
  hole, so that there is room for the grains to
  rearrange
• (A soil with a long history of wave loading is
  unlikely to liquefy because there is not much
  room for the grains to rearrange this is due to
  compaction!)
• Soil must be fine (silt, fine sand) so that all
  pore pressures accumulated during the wave cycle
  would not dissipate as rapidly as they develop
• Waves must be sufficiently high

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Residual Liquefaction. A video film

• The video camera views the soil through the
  glass-side wall of the wave flume.
• On the screen, in the upper left-hand corner, two
  signals in a window One is the surface
  elevation, and the other the pore pressure
  recorded at the depth 12 cm.
• The signals recorded simultaneously with the
  videotaping.
• Soil, Silt d50 0.045 mm Water depth 40 cm
  Wave height 17 cm, Wave period 1.6 s
• Will see a horizontal band in the middle of the
  screen. Do not take any account of this! It is
  silicon used to fill the gap between the side
  wall of the flume and the side wall of the silt
  box.
• Dr. Figen Hatipoglu (She is a Post-Doct at ISVA,
  Tech. Univ. Denmark) made the film.

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Momentary Liquefaction
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Momentary Liquefaction

• Pressure distributions in the soil across the
  depth under the trough!
• For two situations
• (a) The case of a saturated soil (there is no
  gas/air in the soil)!
• (b) The case of an unsaturated soil (there is
  gas/air in the soil)!

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Momentary Liquefaction

• This upward-directed pressure gradient induces a
  lift force on the soil under the wave trough
• If the lift force exceeds the submerged weight,
  the soil will be liquefied!
• This process is called the momentary
  liquefaction!
• (Although there is also an upward-directed
  pressure-gradient force in the saturated case,
  this is apparently too small to cause
  liquefaction even under the highest waves!!)

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Watch out The conditions for Momentary
Liquefaction

• The soil must be an unsaturated soil (the soil
  may be liquefied even with Sr only slightly
  different from 1!)
• Only a shallow, top layer of the soil is
  liquefied because of the large pressure gradient
  experienced (however, under extreme conditions,
  the liquefaction can penetrate to depths as far
  as O(0.5?Wave height))
• Liquefaction occurs during the passage of the
  wave trough
• Waves must be sufficiently high

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Engineering practice

• Be it the residual liquefaction or the momentary
  liquefaction, the question in engineering
  practice boils down to the following
• Given the soil
• Given the waves (50 year, 100 year,..)
• Will there be any liquefaction risk for the soil
  supporting any structure (a pipeline, a gravity
  structure, a breakwater, a pier, a pile, a scour
  protection structure, etc.)?

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To assess liquefaction

• This has stimulated research on the topic in the
  area of coastal engineering over the past 20
  years
• Three approaches have been adopted
• Physical modelling (The ordinary physical
  modelling, and most recently the physical
  modelling involving centrifuge facilities)
• Mathematical modelling
• Deductions from field measurements

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Physical modelling

• The main objective of the physical modelling is
  to get a good understanding of the processes,
  simulated in the lab, under controlled conditions
• It also enables systematic parametric studies
• Furthermore, it provides data for the validation
  of mathematical models, a valuable by-product!
• The down side, however, is that the soil response
  may not be properly extrapolated to the field
  conditions such a lab model may be treated as an
  individual prototype itself!
• To get around the problem (of extrapolating the
  results to the prototype), centrifuge wave
  testing on a soil bed has been tried recently
  (1999, 2001)!

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Mathematical modelling

• The soil is assumed to be a poro-elastic medium
• The model, which governs (1) the soil
  deformation, and (2) the movement of pore water
  (including the pore pressure), is basically the
  Biot equations
• The latter are solved under the boundary
  condition at the seabed

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Mathematical modelling

• The Biot equations good enough to study the
  momentary liquefaction!
• For the buildup of pore pressure and eventual
  residual liquefaction, however, we need some
  additional information
• One such piece of information may be an empirical
  expression for the pressure generated by the
  cyclic shear (the source term!) such as

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Mathematical modelling

• Although this approach does a good job in
  engineering applications, it does not accommodate
  the continuous change of the soil properties, and
  particularly
• it breaks down near liquefaction conditions!
• Recently, sophisticated sand models have been
  developed
• One such model which accounts for the
  contractive/dilative behavior of sand and can
  handle the long-term pore pressure buildup, has
  been adopted by HR!
• HR will present early results of this approach,
  as applied to the wave-induced liquefaction!

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Commercial!

• Great many works have been devoted to the
  physical and mathematical modelling of
  wave-induced liquefaction.
• A detailed account of these works (and its impact
  on scour-related problems) (with over 80
  references) is given in Chapter 10 in the book by
  
• Sumer, B.M. and Fredsøe, J. (2002). The Mechanics
  of Scour in the Marine Environment. World
  Scientific, xiv536 p.

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Deductions from field measurements

• Field measurements not terribly easy to
  interpret. This is largely because we have no
  control over the test conditions!
• Yet, we can make useful deductions from field
  data! (We have one wave-induced-liquefaction
  field study in LIMAS, Université de Pau and had
  one in SCARCOST, the predecessor of LIMAS,
  under EU MAST-III programme)
• In this conjunction, another sensible approach
  would be to simulate the field conditions in the
  lab in a large-scale wave facility (We have one
  such study in LIMAS, Technische Universitat
  Braunschweig)

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http//www.ce.washington.edu/liquefaction/html/wh
at/what1.html
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Centrifuge facilities

• Two types
• (1) Beam type (Sassa Sekiguchi, 1999) and
• (2) Drum type (pretty much the same as a washing
  machine!) (Mark Randolph Liang Cheng, Univ. of
  Western Australia)

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Issues in engineering practice

• Marine pipelines may be buried against heavy
  traffic, or fishing gear. (Dredge a trench Place
  the pipe in it Backfill the trench with the
  excavated material)).
• Question Is the backfill material liquefaction
  resistant? (Otherwise the pipeline will float to
  the surface!) Or should it be replaced with a
  coarser material?

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Issues in engineering practice

• Marine pipelines may also be laid on the seabed.
• Question Will the seabed be liquefied under
  extreme storm events (100 year storm) if it is a
  soft soil?
• (Although there will be very little room for the
  rearrangement of grains, and therefore for the
  residual liquefaction, due to a long exposure of
  waves. However, remember the momentary
  liquefaction!)
• If the bed is liquefied, that will jeopardize the
  pipelines stability!

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Issues in engineering practice

• Marine structures are protected against scour by
  scour protection (rock, armour blocks,..).
• Question Can the supporting soil be liquefied
  under extreme storm events? If yes, to what
  depths do the elements of scour protection sink
  in the liquefied soil?

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Issues in engineering practice

• Marine structures (such as caisson breakwaters,
  gravity structures, piles) execute rocking motion
  under waves.
• This may cause liquefaction around the structure,
  inducing enhancement in scour, and therefore
  endangering the stability of the structure
• Question What is the extent of liquefaction
  around the structure?

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Residual Liquefaction
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Large-time behavior