Snow and perennial ice

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FRAGILE ALPINE ENVIRONMENTS

Frost heaves and thaw settlement | Strength and creep

Mechanics

Frost heaves and thaw settlement

As supersaturated permafrost, which is mainly ice lenses (sergrgation ice), originates frost heaves occur and when the ice melts thaw settlement starts.

Two types of frost heave are distinguished:

primary heave		secondary heave
ice lens building at the freezing front		ice enrichment behind the freezing front

The most important factors are:

temperature gradient (speed of freezing)
water availability (ground water level, inpermeable layers)
ground characteristics (permeability, effects of capillarity)
shut-off pressure

1 - Frost blister in the Tien Shan mountains, China (184K)

2 - Frost mound in the Tien Shan mountains, China (124K)

3 - Sorted circles, Spitzbergen, Savalbard (128K)

Strength and creep

Frozen soil is typically stronger then unfrozen soil or ice, but it displays a time-dependent creep behaviour similar to ice. The strength of frozen ground can be considered as consisting of the cohesion of the ice matrix and frictional resistance of the soil particles. The viscoplastic strength and deformation characteristics may be attributed largely to the presence of the ice matrix and cementation bonds. However, the composite behaviour of the frozen soil may not be a simple sum of the structural components, and some complex interaction apparently exists between the ice and the soil components.

The characteristic nature of creep behaviour is most easily appreciated by considering a sample of ice-rich frozen soil to which is applied a constant compressive stress, that due to an applied weight, for example. The general form of the creep curve, plotted according to long-established conventions, is shown in Figure 4. The designation of primary, secondary and tertiary phases of creep is traditionally somewhat arbitrary. The immediate response to the application of the stress is a strain, a shortening of the sample. If the stress is not too great and is quickly removed, the original length may be immediately restored.

Such a deformation is an elastic one. If the stress is large enough and is applied for a considerable time, deformation continues, but at a decreasing rate. Conventionally, all this is called primary creep. After a period of perhaps hours, the deformation may assume (B in Figure 4) a fairly steady rate for a considerable length of time. If conditions (especially temperature) do not change, then after a certain period (which could be day, weeks or much longer, depending on the applied stress) the strain rate will again increase (acceleration, C in Figure 4) leading inevitably to failure. The constant rate of strain is referred to as secondary creep, and the following acceleration stage as tertiary creep.

classical creep cuve

4 - Classical creep curve


A	B	C	D	E
5 - Horizontal downslope borehole deformation in five boreholes in the Eastern Swiss Alps:
A Murtèl-Corvatsch, borehole 2/1987:1987-1995 B and C Pontresina-Schafberg, boreholes 1/1990 and 2/1990: 1991-2000, 1994-1999 D and E Muragl, boreholes 3/1999 and 4/1999: 1999-2000
Source: Arenson et al. (2002)

6 - Creeping permafrost (rock glaciers) in the Pontresina-Schafberg area, Upper Engadin, Switzerland (116K)

7 - Rock glacier flowing from cirque into Kathleen lake (Yukon, Canada) (92K)

8 - Rock glacier close to Dezadeash lake (Yukon, Canada) (80K)

29 August 2011 © ALPECOLe 2002-2007