L5: Frost Heave Flashcards
When water changes to ice its expands and freezes…but by how much?
Phase change is up to a 9% volume change.
What increases the expansion that occurs when water changes to ice?
Segregation (water migration to that ice)
What does expansion release?
Pressure
This occurs in two directions…
- Vertically, as frost heave
- Laterally, as frost thrust
What does general heave involve?
The whole ground
Primary heave from phase change
Secondary heave from water migration
What noticiable qualities does stone heave or “upfreezing of clasts have?
It acts differentially on stones –larger particles are heaved most.
Freezing thawing freezing thawing = patterns occuring
Elongated stones are tilted into the vertical (at an angle)
How does frost push of pebbles occur?
- Heat flux greatest through pebble – cools quickly
- Film of ice forms around cold pebbles, particularly at it’s base
- Ice pushes stone up by ice at base, leaving cavity full of ice crystals which can get bigger and bigger
- With summer thaw, the cavity partially fills with sediment and the stone cannot settle back
- Ice crystals are certainly found in cavity beneath stone, but were they responsible for pushing the stone? Tiny ice crystals are supposed to push the pebble up through an overlying zone that has already frozen!
How does frost pull occur?
- Clast
- Freezing occurring stone starts to cool quicker as the frost front is penetrated- ground surface is rising.
- Clast gets pulled up with the heaving ground (start freezing quicker than the matrix around me)
- Leaving a cavity beneath, ice crystals grow beneath
- Forefront penetrating stones warm up quicker
- Ice melt material falls into cavity
- Stone doesn’t fall back down- mound on top
Cryostatic pressure
- Frost table falls through the ground in Autumn
- Not a perfect horizontal freezing front (some relief) and so as it goes down through frozen material it’s exerting cryostatic pressure ahead of the frost table
- Where the frost table bends, convergent pressure may lift pebble (would have to have warm bit above clast)
What is differential frost heave?
Non-uniform movement of material in some areas than other.
Can initiate an undulating surface that then causes positive feedback processes and ever more differentiation with each freeze thaw cycle.
Ultimately leading to patterning.
What is soil circulation and where does it happen?
Buoyancy driven circulation of the soil (moisture and frozen state type densities, not temperature).
Happens in the thaw (critical period for movement of material).
What is needed for soil circulation?
Will only happen if we have already had segregation ice
What happens during soil circulation?
As ice rich soil melts it consolidates (less dense) and the bulk density decreases with depth causing upwards movement of low density saturated soils in centre and downward migration at edge.
Define cryoturbation
(Singular) A collective term to describe all soil movements due to frost action (repeated freezing and thawing).
(Plural) Cryoturbations refer to the irregular structures formed in soils by deep frost penetration and frost action processes.
What are the cryoturbation processes?
Differential heave- freezing and the formation of segregation ice which causes cryostatic pressure, pushing frozen material into unfrozen material.
Loading- layers of different densities (high will drop down to low density, low density will be forced up).
Cryohydrostatic pressure-
Relict cryoturbation structures
The fine material is active
Lobes of fines injected into surrounding materials suggest high pressures
Sometimes (but not always) the feeder for these lobes is visible
Sometimes diapirs are forced down from above
