Mechanics of Thermal Contraction Cracks and Ice-Wedge Polygons in Permafrost

In August 1978, a large tundra lake was drained to study the aggradation of permafrost into newly exposed lake-bottom sediments. Ice-wedge growth, which started in the first winter following drainage, had ceased in most of the lake bottom within about twelve years. The gradual cessation of thermal contraction cracking can be attributed to rapid vegetation growth, snow entrapment, an increase in winter ground temperatures, and a decrease in the linear coefficient of thermal contraction associated with freezethaw consolidation of the initially saturated lake-bottom sediments. The tilt and separation of markers in the active layer revealed gradual convergence towards the troughs even after ice-wedge growth had ceased. For the first few years the ice-wedge growth rate was up to 3 cm/a as determined by excavation, drilling, separation of the bottoms of benchmarks installed into permafrost, and divergence of free-floating inductance coils placed on the sides of ice wedges well below the bottom of the active layer. The vertical extent of most ice wedges was probably about 2 m, as deduced from the depths of ice-wedge cracks and the geometries of the wedge tops. Many thermal contraction cracks propagated upward to the ground surface from the tops of the ice wedges rather than downward from the ground surface. Small, upward facing, horizontal steps and vertical slickensided surfaces in permafrost on both sides of an excavated ice wedge near its top indicated that the adjacent permafrost had moved upward, relative to the wedge, from thermal expansion during the warming period.

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Late Holocene syngenetic ice-wedge polygons development, Bylot Island, Canadian Arctic Archipelago

Canadian Journal of Earth Sciences ◽

10.1139/e04-031 ◽

2004 ◽

Vol 41 (8) ◽

pp. 997-1012 ◽

Cited By ~ 66

Author(s):

Daniel Fortier ◽

Michel Allard

Keyword(s):

Late Holocene ◽

Canadian Arctic ◽

Canadian Arctic Archipelago ◽

Thermal Contraction ◽

Current Configuration ◽

Outwash Plain ◽

Bylot Island ◽

Ice Wedge ◽

The Mean ◽

Organic Sediments

The initial configuration of the syngenetic ice-wedge polygons that developed in the outwash plain of glacier C-79 after 6000 BP was modified by the accumulation of wind-blown and organic sediments that began after 3670 ± 110 BP. The late Holocene sedimentation led to an increase in the thermal contraction coefficient of the soil and the formation of third- and fourth-order contraction cracks, partially explaining the current configuration of the polygonal network. The upturning of the sedimentary strata bordering the ice wedges was associated with the summer thermal expansion and resulting internal creep of the soil. The mean annual soil displacement coefficient was in the order of 2.52.7 × 105 /°C at the thousand-year scale. The late Holocene sedimentary strata under the centre of the polygons were undisturbed, which will make it possible to use this sedimentary record in further studies to attempt paleoenvironmental reconstructions from cores.

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The Closing of Ice-wedge Cracks in Permafrost, Garry Island, Northwest Territories

Canadian Journal of Earth Sciences ◽

10.1139/e75-146 ◽

1975 ◽

Vol 12 (9) ◽

pp. 1668-1674 ◽

Cited By ~ 32

Author(s):

J. Ross Mackay

Keyword(s):

Northwest Territories ◽

Crack Width ◽

Crack Spacing ◽

Elastic Model ◽

Thermal Contraction ◽

Underground Cables ◽

Ice Wedge ◽

The Mean

The closing of thermal contraction (ice-wedge) cracks at Garry Island, N.W.T., 150 km northwest of Inuvik, N.W.T., has been measured by means of gauge probes inserted into the cracks and by precise taping between bench marks across ice-wedge troughs. The results show that a simple elastic model fails to explain the time of cracking, the depth of cracking, the crack spacing, and the time of closing. The mean annual ice vein increment, at Garry Island, is probably less than 20% of the mean winter crack width. Thermal contraction cracks are of potential engineering interest, because they may affect underground cables, reservoirs, and other man-made structures.

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