9. Slow mass movement of soil (Soil Creep)

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Soil Creep
The downslope movement of the soil mantle.
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How to measure soil creep?
Using Rudberg pillars (disadvantage, can only do once).
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Evidence of soil creep (reference)
Boulders may move downslope. Monuments, headstones, posts etc tilt downslope. Broken retaining wall (Stahler and Stahler)
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4 types of soil creep
1. Rhealogical creep (continuous creep) 2. Seasonal creep 3. Random (biogenic) creep 4. Solifluction
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Rheological soil creep?
Clay-rich soils deform under the downslope component of their own weight.
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What are recorded surface rates for rheological soil creep?
10-223 mm/yr. May not seem like much, but if there is a house on top..
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Seasonal creep
Creep by soil wetting and drying
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Seasonal Creep process
Clay minerals in soil absorb water and expand. When it dries out clay minerals in soil contract. Creates net movement down slope as soil expands and contracts.
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What influences soil creep
Gradient, frequency of w/d cycles, depth of w/d, soil composition: clay minerals
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Freezing and thawing (frost creep)
Water undergoes 9% expansion when frozen.
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Potential Frost Creep. pfc = h tan a
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Factors affecting creep rate
1. Gradient 2. Depth and frequency of freeze-thaw (arctic; not important as only one frost cycle) 3. Moisture availability during soil freezing 4. Frost susceptibility of soil (sand doesn't freeze)
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Needle ice creep (requirements and surface movement
1. requires up to -2C and abundant soil moisture 2. Surface movement up to 0.3 m/yr but declines to zero at a few mm depth. Just thin carpet being moved.
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Random (biogenic) creep
Downslope movement of soil particles disturbed by biological activity
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Occurs when:
Cattle trampling wet soil. Downslope movement of soil particles into voids created by worm casts, burrows and rotting roots.
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Occurs only in cold environments.
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2 components:
1. Frost creep 2. Gelifluction (localised flow of frost-susceptible soil during ground thaw)
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Mechanism of gelifluction
Thaw consolidated layer is where ice lenses are thawing out - a lot of water. Where water going to go? Not down - still ice (impermeable). Situation where water pressure will build up because it can't escape down or up.
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SR of soil is
SR = C + ( W cos a - u) tan φ
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Card 2


How to measure soil creep?


Using Rudberg pillars (disadvantage, can only do once).

Card 3


Evidence of soil creep (reference)


Preview of the front of card 3

Card 4


4 types of soil creep


Preview of the front of card 4

Card 5


Rheological soil creep?


Preview of the front of card 5
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