Environmental Engineering Reference
In-Depth Information
and well delineated. Boulders and gravel are randomly distributed in a finer-grained
cohesive matrix. Muddy debris flows are very frequent in the Alps.
- Granular debris flow . Although the size distribution is wide, the material is
poor in fine (clay-like) particles. Bulk behavior is expected to be frictional-collisional
[ANC 97, CHE 87, JEN 94, TAK 91]: it is mainly governed by collisions and friction
between coarse particles. Energy dissipation is usually much larger for granular debris
flows than for muddy debris flows and thus, granular debris flows require steep
slopes ( > 15%) to flow. Presumably, as for very large rockfalls, a granular debris flow
involving a very large amount of materials may travel large distances over more gentle
slopes. In the field, deposits are easily recognized by the irregular chaotic surface.
Deposits are generally graded, with coarser debris forming mass deposits and finer
debris transported downstream (due to drainage).
- Lahar-like debris flow . The particle-size distribution is narrow and the material
contains only a small proportion of clay-like materials. This type of debris flow is
typical of volcanic soil areas (soils made up of fine ash), but it can be observed
on other terrains (e.g. gypsum and loess) [WAN 94]. Bulk behavior is expected
to be frictional/viscous: at low shear velocities, particles are in sustained frictional
contact and bulk behavior may be described using a Coulomb frictional equation.
At high shear velocity, due to dilatancy and increased fluid inertia, contacts between
coarse grains are lubricated by the interstitial fluid [ANC 99]. In the laboratory,
such materials exhibit very surprising properties: at rest, they look like fine soil
(silts) but once they have been stirred up, they liquefy suddenly and can flow nearly
as Newtonian fluids. Contrary to muddy debris flows, the yield stress is low, and
therefore, lahars can move over gentle slopes of < 1%. Deposits are very thin and
flat and look like alluvial deposits.
1.4.2. Rheometry
Natural suspensions are made up of a great diversity of grains and fluids. This
observation motivates fundamental questions: how to distinguish between the solid
and fluid phases? What is the effect of colloidal particles in a suspension composed
of coarse and fine particles? We shall see that when the particle size distribution
is bimodal (i.e. we can distinguish between fine and coarse particles), the fine
fraction and the interstitial fluid form a viscoplastic fluid embedding the coarse
particles, as suggested by Sengun and Probstein [SEN 89a]; this leads to a wide
range of viscoplastic constitutive equations, the most common being the Herschel-
Bulkley model. The bimodal-suspension approximation usually breaks for poorly
sorted slurries. In that case, following Iverson and his colleagues [IVE 97, IVE 05],
we will see that Coulomb plasticity can help understand the complex, time-dependent
rheological behavior of slurries.
Over the past 20 years, a large number of experiments have been carried out to
test the rheological properties of natural materials. The crux of the difficulty lies
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