Permafrost

Permafrost is any earth material that has remained below 32 degrees F (0 degrees C) from one winter through the next. However, much permafrost has been in existence for tens of thousands of years.

Permanently frozen ground prevails throughout most of Alaska, ranging in depth from less than a foot at its southern margin to 2,000 feet at Prudhoe Bay. Local variations in thickness, areal extent, and permafrost temperature depend on differing thermal properties of earth materials and on local differences in climate, topography, vegetation, geology, hydrology, and rate of heat flow within the earth. In many places these local variations mask the regional southward decrease in areal extent and thickness and the southward increase in permafrost temperatures. Areas around large water bodies and thermal springs are generally free of permafrost because of increased heat flow in the ground (Figure 80).

The permafrost table is the upper boundary of permanently frozen ground below the earth's surface. The area between it and the surface is called the suprapermafrost layer. That part of the suprapermafrost zone that freezes in winter and thaws in summer is considered the active layer. When winter freezing does not extend down to the permafrost table, an unfrozen layer remains between the permafrost and the frozen active layer known as talik (Figure 81). Groundwater trapped in taliks may be stored under great hydrostatic pressure. If disturbed, springs may burst to the ground surface and freeze, causing icing and producing a thick and often widespread ice sheet or mound called aufeis. Since water deposits tend to reduce temperature fluctuations from season to season, thawing usually reaches deeper in drier materials. Permafrost thickness is aggrading as it thickens and degrading as it thins.

The extent and thickness of permanently frozen ground in the Northwest Region varies from thick, continuous permafrost in the regions north of Kotzebue Sound, to moderately thick, discontinuous permafrost in the southern part of the region (Figure 83). Climate is a major factor in the formation of permafrost, which generally exists in areas where the mean annual air temperature is 32 degrees F (0 degrees C) or below. However, in Alaska this is complicated by the complex relief characteristics. Seasonal temperature variations are reflected to a depth which depends on a combination of climatic and terrain characteristics. Below that depth permafrost is at its coldest, warming gradually thereafter with depth until it passes the freezing point, indicating the subpermafrost boundary (Figure 82). In the Northwest Region, the boundary between the continuous and discontinuous permafrost zones lies somewhat south of the 17.6 degree F (-8 degrees C) isotherm.

The age of permafrost varies, but near Nome ice wedges probably formed 8,000 to 9,000 years ago (Hopkins, MacNeil, and Leopold 1960). Areas that were ice-covered during the Pleistocene were possibly free of permafrost until the glaciers melted (Hopkins et al. 1955). The ice may have insulated the ground from low air temperatures at the same time that permafrost was aggrading in adjoining ice-free areas. Permafrost in areas that were never ice-covered during the Pleistocene may be much older (Figure 76). Some ice wedges in northern and central Alaska have been dated to 32,000 years ago. After the ice disappeared, permafrost in areas of post-glacial flooding, such as proglacial lakes and marine transgressions, probably dissipated and was not able to regenerate until thousands of years later.

Various terrain conditions have important effects on permafrost growth and thickness (Brown and Pewe 1973). Surface relief directly influences permafrost formation since the amount of solar radiation received depends on the degree and direction of slope. Relief also affects snowfall accumulation and vegetative cover, which in turn affect permafrost thickness.

The type of ground surface is an important determinant of permafrost conditions. The thermal conductivity of silt is about one-half that of coarse-grained sediments and several times less than that of bare rock, which has a high reflectivity value. The active layer is thinnest and the permafrost layer generally thickest in fine-grained soils such as silt and clay.

Vegetation primarily affects permafrost by shading and insulating the ground from solar radiation. Significant removal or disturbance of vegetation lowers the permafrost table.

Moss and peat are specially important insulators. Severe damage occurs when moss or peat cover is disturbed, most commonly from vehicular traffic across the tundra. Disturbance of moss and peat may cause the permafrost to thaw, possibly resulting in local subsidence, flooding, drainage diversion, and erosion.

Permafrost inhibits soil warming during summer, keeping the root zone temperature well below the optimum for growth. This reduces water absorption by plant roots and leads to plant dryness. Root development is substantially retarded, and roots are prevented from growing downward by the impenetrable permafrost layer. They are forced to grow laterally causing many large trees to lean at an angle because of lack of root support. Solifluction, the downward flow of wet soils, disturbs vegetation stability by uprooting or shifting the root zone.

Snowfall conditions and the length of time snow is on the ground influence permafrost distribution and thickness by controlling heat transfer to the ground from the atmosphere. Early, heavy snowfall insulates the ground from severe winter cold, preventing permafrost aggradation. Thick snow cover that lasts late into spring delays spring thaw and can promote aggradation. The thickest and most extensive permafrost and the thinnest active layer exist in areas with little or no snow cover. Variations in snow cover may have more effect on permafrost than variations in vegetative cover.

The influence of fire on permafrost depends on type of vegetation, moisture, and the rate of burning. If a fire moves rapidly through an area, or the surface peat and moss layer is moist, the permafrost may not be significantly disturbed. If the surface vegetation is very dry, it may be almost totally consumed by the fire, robbing the permafrost of its insulative cover. In the discontinuous zone, masses of thin permafrost below a large fire of this type may disappear.

Thermokarst Topography

Thermokarst topography—mounds, sink holes, tunnels, caverns, short ravines, lake basins, and circular lowlands—is caused by local melting of ground ice and the subsequent settling of the ground. It is most common where such massive ice formations as ice wedges and thick segregated ice exist. Melting can result from the disturbance or removal of vegetation or by a warming trend in climate. Even small disturbances, such as a vehicle driven across the tundra, can create thermokarst features.

Thaw Lakes

Thaw lakes, the most common thermokarst feature, often form in tundra regions underlain by continuous permafrost. These lakes range in size from a few yards to several miles in length but are seldom deep.

Slight ground surface depressions, such as low-centered polygons, cause pooling of standing water which begins to thaw the permafrost immediately beneath. Thaw continues along lake margins and the basin extends, often merging with other adjacent lakes and creating a large body of water. Some thaw lakes are connected by stream channels, while others have neither an active inlet nor outlet stream. Since underlying permafrost prevents percolation into underground aquifers, water is trapped until the lake drains. Thaw extension continues until intervening higher ground is breached, creating an outlet channel. The lake drains, leaving behind a low, marshy basin surrounded by the higher topography of the original tundra surface. Repeated formation and drainage of thaw lakes often creates an overlapping series of drained basins with only isolated ridges or platforms of the original surface remaining.

Ice Wedge Polygons

Polygonal or patterned ground, characteristic of permafrost regions, results when ground contracts from the cold. Water and snow collect in the contraction cracks, eventually turning to ice and producing ice wedges that surround each polygon. The addition of ice to the wedges is accompanied by upthrusting of material adjacent to them, both by the expansion of ice during freezeup and of the ground during summer. This tends to form ridges of material on each side of the ice wedge.

With sufficient drainage, ice wedges thaw and form troughs that the drainage tends to perpetuate. Ridge-forming material may slump back into troughs and develop high-centered polygons. In areas of poor drainage, such as marshy, drained lake basins, ice wedges are not eroded, and ridges continue to develop. They stand in significant relief above polygon centers, producing low-centered polygons. Normally, polygon crack systems form at random in areas of uniform temperature stress. However, on receding shorelines, in drained lakes, and in abandoned stream channels, crack systems often form both parallel and perpendicular to the receding shorelines because of horizontal stresses caused by temperature gradients.

Low-centered polygons are uncommon in the inactive wedge region due to lack of ice growth (Figure 84). Inactive wedges do not have a seam or crack extending to the surface in spring and usually exist only in silt, having thawed and disappeared in sand and gravel areas. Most regions of inactive, discontinuous permafrost no longer exhibit patterned ground because they have become covered by recent sediments.

Beaded Streams

A beaded stream consists of a series of elliptical pools, approximately three to 10 feet deep and a few yards in length and width, connected by short, usually straight water courses that generally follow ice wedges. Pools usually form at the intersections of ice wedges.

Pingos

Pingos—small, conical, ice-cored hills—form in two ways. Closed-system pingos develop when tundra thaw lakes drain and permafrost encroaches from the sides. As sediments near the center slowly freeze, massive segregation of ice develops. Volume increases as freezing occurs and pushes the tundra and ice upward, forming a large, ice-cored mound or pingo. As the pingo expands upward a summit crack or fissure often opens, exposing the ice core and allowing part of it to melt and a small lake to form in the crater. Closed-system pingos are characteristic of the continuous permafrost zone (Figure 84).

Open-system pingos form when subpermafrost or intrapermafrost water penetrates the permafrost layer under hydrostatic pressure. A large water-ice lens forms beneath the tundra that heaves up into a mound from a combination of freezing pressure and hydrostatic pressure. Open-system pingos are essentially restricted to the discontinuous permafrost zone (Figure 84). They generally lie near the base of slopes, where water is apparently able to enter the subsurface system from the nonpermanently frozen areas upslope. Almost all open-system pingos exist on south- or southeast-facing slopes of alluvium-filled valleys, and they nearly all lie in areas that have been unglaciated for at least 25,000 years.

Frost Mounds

The frost or ice-cored mound is smaller but related in origin to the pingo. With a maximum relief of about four feet and diameters ranging from 10 to 15 feet, they may exist anywhere on the tundra but are ordinarily in marshy, drained lake basins. Frost mounds consist of a core, composed predominantly of ice, covered with soil or peat. Although some mounds, especially the largest, have cores rooted in permafrost, most probably are restricted to the active layer.

Solifluction Sheets and Lobes

Since permafrost prevents absorption of surface water into the subsurface, the topsoil often remains saturated, causing a downslope movement of water-saturated sediments called solifluction. This can occur on slopes as gentle as three degrees and often creates such forms as lobes, sheets, and terrace-like features.

Stone Nets, Garlands, and Stripes

Stone nets are soil structures with centers of clay, silt, and gravel and roughly circular or polygonal borders of coarse stones. When isolated they are known as stone rings. Their netlike arrangement may extend downward as much as two feet, and they have diameters from a few inches to more than 30 feet. Stone nets are found on flat or nearly flat ground. On slopes of moderate inclination about five to 15 degrees they are drawn out downslope by solifluction into tongue-like or elliptical shapes known as stone garlands. On steeper slopes of from five to 30 degrees, garlands give way to stone stripes—parallel stony and earthy bands.

Engineering Considerations

The widespread occurrence of permafrost requires special engineering considerations for the design, construction, and maintenance of structures and facilities. Permafrost degradation is primarily related to the insulation qualities of the surface layers and the ice content of the frozen ground beneath. Sensitivity is great in the north, where the surface organic layer is thinnest and soil ice content is highest.

Engineering limitations associated with permafrost are not the same for every rock type or sediment. Frozen bedrock with ice in its crevices will present few, if any, construction or maintenance problems. Well-drained, coarse sediments such as gravel may contain little or no ice, even though they are frozen, so again, few problems occur. Major engineering problems arise when permafrost occurs in poorly drained, fine-grained sediments that contain large amounts of ice. Disruption of these deposits results in a change in the thermal balance which causes the ice to melt. Thawing produces excessive wetting and plasticity and makes the sediments unstable (Figure 85). This results in icings, frost heaves, slumping, and subsidence of the ground surface, and in many cases, the sediments flow laterally or downslope.

During winter the active layer freezes from the ground surface down to the permafrost table. In fine-grained materials especially, the formation and resulting expansion of ice causes frost heaving. Thawing of permafrost and cycles of freezing and thawing in the active layer cause extensive damage to highways, railroads, airstrips, and other facilities. Computer models are now being used to predict the interaction of permafrost and manmade structures.

The first step in permafrost engineering is adequate investigation of the site for construction. Initial reconnaissance must be followed by detailed site and materials availability studies that should include examination and interpretation of aerial photography and drilling and sampling of the terrain to define the type of permafrost terrain and the degree of potential instability that must be considered in structural design.

A proper foundation is the most crucial element in permafrost engineering. The difficulty and expense of successful foundation engineering directly relates to substrate composition and the distribution and amount of interstitial ice. Foundation design that preserves the underlying permafrost is by far the most common for permanent construction. Standard methods have been described by Linell and Johnson (1973).

Spread footings, continuous footings, raft or mat foundations, and post and pad construction have been successfully used in both thaw-stable and thaw-unstable permafrost. Most of these utilize sand and gravel as fill materials, although wood or artificial materials are occasionally used in areas with no coarse sediments.

Pile foundations that incorporate an air space between the supported structure and the ground surface are the most common design for thaw-unstable permafrost. Creosoted wood and steel pipe or H-section steel piles are usually used, though precast or cast-in-place concrete piles are occasionally found. Refrigerated piles are used in marginal permafrost to assure the permanently frozen state of the sediments around the piles (Long 1973). Anchors for footings, guyed towers, or pipelines present special difficulties because they are subject to uplift, thrust, and creep.

Roads and airfields must be built on insulated basements thick enough to protect underlying permafrost (Figure 86). Sand and gravel fill is the most commonly used construction material, but scarcity of supply in many areas has necessitated experimentation with artificial insulating materials such as styrofoam, sulfur foams, and the new rigid and flexible polyurethane foams in conjunction with natural fill materials. The runway at Ralph Wien Memorial Airport at Kotzebue was constructed using four inches of Dow Styrofoam Hi. The foam was countersunk below the tundra surface to provide bearing strength, then covered with gravel base course and tarmac.

Gas and oil pipelines and power utilities are generally either raised on elevated utilidors, laid on gravel foundations or pilings, or buried and sufficiently insulated to prevent permafrost degradation. Raised utilidors and pipelines present road crossing problems, physical barriers to certain animal movements and migrations, and are often both functionally and aesthetically undesirable. Underground methods, however, require difficult excavation and favorable subsurface conditions and must be designed to be safe against damage from thermal cracking, permafrost degradation, and flooding.

Erosion

Wind and water erosion generally takes place very slowly on arctic land because streams and ground surface are frozen for up to nine months a year. Even during summer, erosion takes place very gradually. The frozen ground thaws slowly and to shallow depths. Only during spring breakup with extensive flooding and high flow velocity do great quantities of sediments move downstream. Aided by the scouring and flotation action of large slabs of ice, even the coarsest deposits can be carried off. As streams thaw, their exposed bars, beaches, and banks become sources of sand and silt for wind action. Wind deflation blowouts are common along stream and lake banks.

Thermal Erosion

Normal water erosion can only reach as deep as the thawed layer, but thermal erosion adds to the amount of thawed material. In permafrost areas where rivers or seas lap against banks or bluffs containing ground ice, undercutting of the banks commonly takes place. As water comes in contact with the ice-cemented sediments, heat transfer causes the ice to melt. The deposits are released and carried away by the water, creating a cavity or thermo-erosional niche below an overhanging tundra mat. Eventually, undercutting proceeds to the point that the overhanging portion collapses, breaks up in the water, and is carried away. This exposes a fresh ice surface, and the process begins again.

Coastal Erosion

Normal shoreline erosion and transport cease during the nine months each year when seas bordering the Northwest Region are frozen. Seasonally frozen ground and permafrost cement particles together to a rocklike consistency, providing further erosion resistance along the beach. During the short summer the ice melts, breaks up, and drifts out to sea. The beaches thaw to a depth of from five to 10 feet in the summer months, which allows normal erosion.

During summer, mechanical erosion of thawed beaches and thermal erosion of coastal banks and bluffs proceed at such a rate that coastal retreat is a continuing problem throughout the region. Recession rates are greatest in banks and bluffs containing fine-grained sediments and least in better-drained banks of coarse sand and gravel.

At Point Hope the large cuspate spit on which the community is built has been receding in recent years. Measurements have shown that erosion is occurring on the north side of the spit with accretion building out the south side. Overall southward migration of the spit appears to be about 10 feet a year, and the community is threatened by the retreat of the shoreline. The retreat seems to be due principally to natural causes.

Erosion and sedimentation on the beaches are slow and steady, but sudden large movements of material can take place during major storms. During summer and fall the coast is open to wave, and sometimes, ice attack during storms. Strong onshore winds that blow across a long fetch of open water in the Bering or Chukchi Seas can cause disastrous storm surge tides along the coastal areas especially in Norton Sound, which is ice-free for a much longer period than further north. Large wind-generated waves are usually superimposed on storm surges, multiplying the erosive and destructive power of the high water. Floodwaters often reach well inland from the shore, damaging property and eroding the landscape.

Storms of this type have been recorded periodically in the region. In October 1946 a coastal storm caused wind-generated waves—surge—estimated at more than nine feet above normal. Many of the streets of Nome were inundated, flooding buildings and property. Coastal erosion was so severe that several nearshore buildings were undermined and collapsed. As a result of these problems, a rock-mound seawall was constructed by the U.S. Army Corps of Engineers in 1951 to protect the shore, replacing a smaller-scale shore protection attempt utilizing 55-gallon oil drums.

Three storms approached the Northwest Region simultaneously in November 1974 and combined to create extreme southerly onshore winds, estimated as high as 70 mph locally, which threatened all south- and southwest-facing coasts from Point Hope to Norton Bay. Nome experienced floodwaters three to five feet high on Front Street, and flooding occurred inland as far as the airport. Ice blocks were found stranded on the runways during the latter part of the storm. Blocks of ice up to 19 inches thick were responsible for the most severe damage in Unalakleet, which was flooded at least 100 yards from shore, inundating buildings and property. Shishmaref, Deering, Kotzebue, and St. Michael also experienced some flooding and erosion, while Wales and Teller had high water but no flooding.

The problem in Shishmaref has become so great that residents have considered relocating the entire community. An engineering study conducted by Dickinson-Oswald-Walch-Lee (April 1975) for the State Division of Community Planning stated:

[Alaska Regional Profiles, Northwest Region, pp. 74-83]

[Alaskool Home]