Permafrost

Definition

Any rock or soil material that has remained below 32° F continuously for two or more years. The two-year minimum stipulation is meant to exclude from the definition the overlying ground surface layer which freezes every winter and thaws every summer (called the "active layer" or "seasonal frost").

By type

Cold permafrost — Remains below 30° F, and which may be as low as 10° F as on the North Slope; tolerates introduction of considerable heat without thawing.

Ice-rich — 20% to 50% visible ice.

Thaw-stable — Permafrost in bedrock, in well drained, coarse-grained sediments such as glacial outwash gravel, and in many sand and gravel mixtures. Subsidence or settlement when thawed is minor, foundation remains essentially sound.

Thaw-unstable — Poorly drained, fine grained soils, especially silts and clays. Such soils generally contain large amounts of ice. The result of thawing can be loss of strength, excessive settlement and soil containing so much moisture that it flows.

Warm permafrost — Remains just below 32° F. The addition of very little additional heat may induce thawing.

Depth along pipeline route

From a few inches to 2,230 ft. approx.

Location, definitions

Continuous Zone — Permafrost is found almost everywhere in the zone, as the name implies. Includes all of the North Slope and most of Western Alaska.

Discontinuous Zone — Permafrost is found intermittently. Includes much of the interior of the state.

Sporatic Zone — Permafrost is found in isolated, small masses of permanently frozen ground.

Pipeline, affected areas

Approximately 75% of the line passes through permafrost terrain. The line traverses the continuous zone on the North Slope and through the Brooks Range; it then encounters the discontinuous and sporatic zones and passes through areas of no permafrost in the immediate vicinity of Valdez.

 

 

Problems

Frost-heaving — When the active layer freezes, ice forms, pushing the ground surface upward.

Frost-jacking — When heaving occurs as described above, if a structure imbedded in the ground is not properly anchored to resist such movement, the structure will be forced upward along with the ground surface. In most cases, the structure does not return to its original position when the active layer thaws during the following summer. The net upward movement is called "jacking." This phenomenon can occur whenever there is seasonal freezing and thawing of the active layer, and is not limited to permafrost areas.

Thaw settlement — Structures founded on "thaw-unstable" permafrost may settle if the large amounts of ice in the thaw-unstable permafrost are melted. Melting is typically caused by heat from the structure or changes to the natural thermal conditions.

Design solutions

 

The pipeline design is based primarily on the soil conditions encountered along the right-of-way. There are three principal design modes.

Above-ground pipeline — Where thaw-unstable permafrost was encountered, problems associated with melting permafrost were avoided by placing the pipeline above ground on an elevated support system. VSMs (pilings) used to support the line were designed to resist frost-jacking forces. To allow animals to cross, twenty-three sections were conventionally buried line-wide, each about 200 feet long.

Below-ground pipeline, conventional burial — Where either unfrozen or thaw-stable permafrost were encountered the pipeline was buried in the conventional manner with no special provisions for permafrost.

Below-ground pipeline, special burial — Where thaw-unstable permafrost was encountered, but where the pipeline had to be buried for highway, animal crossings, or avoidance of rockslides and avalanches, protection of the permafrost from heat of the pipeline was provided by insulation around the pipeline. Some special burials include ground refrigeration systems along with pipe insulations.

Special burial locations — (about four miles)

• MP 645-649 approx. — caribou crossing
• MP 681 approx. — highway crossing (Glenn Highway)
• Two sections (about one mile) in Atigun Pass were buried in insulated boxes to provide protection from rock slides and avalanches.

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