deformation they can flow like liquids, with a very low
internal friction. In Scandinavia, Holocene marine
clays which have been uplifted by glacio-isostatic
rebound have been slowly weathered by percolation
of rainwater so that sodium has been leached out,
making them more prone to landslides and to form
mud flows.
Systematic surveys using modern coring and
remote sensing techniques, such as underwater
cameras, side scan sonar and 3D seismic time slices,
have shown that large-scale debris flows are rather
common on continental slopes.
On the eastern slope of the Norwegian Sea a huge
slide (the Storegga slide) occurred about 8,000 years
ago, involving about 3,500 km
3 of sediment. The slope
scar stretches for nearly 300 km and parts of the flow
extended up to 800 km across the deep ocean floor.
The transport mechanism was chiefly debris flow,
where the sediments were riding as a plug on a
wedge of water that reduced the friction against the
bottom.
2.13 Grain Flow
Grain flow is flow of relatively well-sorted sediment
grains which remain in a sort of suspension above the
substratum due to collisions between the grains. We
see this if we make a little landslide in a dry sandpit or
pour sugar out of a bag. Grain flow can develop only
when the initial flow is near the angle of repose (about
34
). Bagnold (1956) described how collisions
between sediment grains led to a dispersive stress.
However, this stress is only significant near the base
of a flow, where we have rapid variation in flow
velocity as a function of height above the base
(dv/dh). Here grains with very different velocities
will strike one another, and transfer velocity
components to one another. Higher up in the flow the
dispersive stress due to collisions between grains will
be considerably less as the grains have far more similar velocities, despite turbulence. The dispersive pressure developed near the base cannot support a thick
layer of overlying sediment and therefore grain flows
have an upper thickness limit of about 5 cm.
Sand grains which avalanche down the lee side of
sand dunes form small grain flows and are probably
one of the few significant examples of natural pure
grain flow. Grain flow may also occur on beaches and
in shallow marine environments.
2.14 Liquefied Flow
Liquefaction is the name given to a process whereby
sediments lose most of their internal friction, and
consequently act almost like fluids. This is the case
when the pore pressure is equal to the weight of the
overburden. When sediments are deposited, they have
a high water content and the sediment grains are
packed in an unstable manner. As the overburden
increases, the stress on the grain contacts increases,
and the framework of the sediment grains may collapse suddenly. Earthquakes produce tremors which
may cause this structure to collapse, but it can also
take place purely as a result of stress (loading). When
the packed framework of grains which was formed
during deposition is destroyed, the grains can pack
more closely together. For this to be able to happen,
however, water must flow out of the bed as the porosity decreases. This leads to an upward flow of
porewater and fine sediment particles, which may be
as great as or greater than the settling velocity of the
grains. This process is called liquefaction. The force of
gravity, acting on the contact between the grains, is
therefore neutralised, and friction between the grains
tends towards zero, resulting in liquefaction. If we
measure the pressure in the porewater, we find that it
increases during settlement (compaction) when the
unstable grain framework is destroyed. At one stage
the pore pressure will be approximately as great as the
weight of the overlying sediments. We can then use
Coulomb’s Law:
τ ¼ C þ σ v À P
ð
Þtan φ;
where τ ¼ shear strength, C ¼ cohesion, σ v is the
weight of the overlying sediment, P ¼ pore pressure
and φ is the angle of friction (about 34
). (σ v À P) is
the effective stress. When the pore pressure, P,
approaches the weight of the overlying sediments
(σ v ), the friction component (σ v À P) tan φ,
approaches zero. Fine-grained sediments like clay
have considerable cohesion (C), and this will often
prevent clay sediments from sliding even if there is
little friction. However, once a deformation plane
forms, there will often be movement mainly along it
48
K. Bjørlykke
internal friction. In Scandinavia, Holocene marine
clays which have been uplifted by glacio-isostatic
rebound have been slowly weathered by percolation
of rainwater so that sodium has been leached out,
making them more prone to landslides and to form
mud flows.
Systematic surveys using modern coring and
remote sensing techniques, such as underwater
cameras, side scan sonar and 3D seismic time slices,
have shown that large-scale debris flows are rather
common on continental slopes.
On the eastern slope of the Norwegian Sea a huge
slide (the Storegga slide) occurred about 8,000 years
ago, involving about 3,500 km
3 of sediment. The slope
scar stretches for nearly 300 km and parts of the flow
extended up to 800 km across the deep ocean floor.
The transport mechanism was chiefly debris flow,
where the sediments were riding as a plug on a
wedge of water that reduced the friction against the
bottom.
2.13 Grain Flow
Grain flow is flow of relatively well-sorted sediment
grains which remain in a sort of suspension above the
substratum due to collisions between the grains. We
see this if we make a little landslide in a dry sandpit or
pour sugar out of a bag. Grain flow can develop only
when the initial flow is near the angle of repose (about
34
). Bagnold (1956) described how collisions
between sediment grains led to a dispersive stress.
However, this stress is only significant near the base
of a flow, where we have rapid variation in flow
velocity as a function of height above the base
(dv/dh). Here grains with very different velocities
will strike one another, and transfer velocity
components to one another. Higher up in the flow the
dispersive stress due to collisions between grains will
be considerably less as the grains have far more similar velocities, despite turbulence. The dispersive pressure developed near the base cannot support a thick
layer of overlying sediment and therefore grain flows
have an upper thickness limit of about 5 cm.
Sand grains which avalanche down the lee side of
sand dunes form small grain flows and are probably
one of the few significant examples of natural pure
grain flow. Grain flow may also occur on beaches and
in shallow marine environments.
2.14 Liquefied Flow
Liquefaction is the name given to a process whereby
sediments lose most of their internal friction, and
consequently act almost like fluids. This is the case
when the pore pressure is equal to the weight of the
overburden. When sediments are deposited, they have
a high water content and the sediment grains are
packed in an unstable manner. As the overburden
increases, the stress on the grain contacts increases,
and the framework of the sediment grains may collapse suddenly. Earthquakes produce tremors which
may cause this structure to collapse, but it can also
take place purely as a result of stress (loading). When
the packed framework of grains which was formed
during deposition is destroyed, the grains can pack
more closely together. For this to be able to happen,
however, water must flow out of the bed as the porosity decreases. This leads to an upward flow of
porewater and fine sediment particles, which may be
as great as or greater than the settling velocity of the
grains. This process is called liquefaction. The force of
gravity, acting on the contact between the grains, is
therefore neutralised, and friction between the grains
tends towards zero, resulting in liquefaction. If we
measure the pressure in the porewater, we find that it
increases during settlement (compaction) when the
unstable grain framework is destroyed. At one stage
the pore pressure will be approximately as great as the
weight of the overlying sediments. We can then use
Coulomb’s Law:
τ ¼ C þ σ v À P
ð
Þtan φ;
where τ ¼ shear strength, C ¼ cohesion, σ v is the
weight of the overlying sediment, P ¼ pore pressure
and φ is the angle of friction (about 34
). (σ v À P) is
the effective stress. When the pore pressure, P,
approaches the weight of the overlying sediments
(σ v ), the friction component (σ v À P) tan φ,
approaches zero. Fine-grained sediments like clay
have considerable cohesion (C), and this will often
prevent clay sediments from sliding even if there is
little friction. However, once a deformation plane
forms, there will often be movement mainly along it
48
K. Bjørlykke