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Thermal effects and properties of selected materials





Road, Airfield, and Island Embankments.

Permafrost

Permafrost exists in a thermal equilibrium between the environmental conditions of the region and the insulating effects of the active layer. Whenever the active layer is altered by a construction project its insulating properties are changed. Usually this results in a loss of protection and increased heat flow into the permafrost. It results in an inevitable melting of the frozen soil and subsidence of the surface if frozen moisture contents for the soil involved are greater than the soil can retain after thawing. If the project is a road or airfield the integrity of the project is jeopardized. When the road or airfield embankment must be built on permafrost, precautions should be taken to avoid degradation of the permafrost, which can cause serious problems to the structure. It should be noted that some permafrost degradation always takes place, and when a structure or facility is constructed at a permafrost site, some degradation is tolerated and even expected. The structure is periodically brought back to its original grade as part of routine maintenance. The amount of tolerance is subject to the structure and its use. Airports tolerate less than roads which tolerate less than dikes etc.

But permafrost is not the only problem facing road and airfield construction in the north. Many sites lack suitable materials for embankment construction and suffer from seasonal freezing and thawing effects. The temperatures of the region have an adverse effect on many commonly used materials such as asphalt. Thaw weakening of embankments inhibits the load carrying capability of the embankment for several weeks of the year during spring breakup. So it will necessary to discuss these problems and some of the current solutions that have been used to address them. Some of the solutions such as insulation are still largely experimental, but are gaining acceptance and becoming more commonly used as more information and experience is gained. Words and expressions:




Permafrost Вечная мерзлота

Thermal equilibrium Термальное равновесие

Heat Тепло

Melting Таяние

Soil Почва

Road (airfield) embankment Дорожная (аэродромная) насыпь,

дамба.

Degradation Размытие

То tolerate Допускать, дозволять.

То freeze Замерзать, покрываться льдом.

То thaw Таять

Insulated roads

The thermal resistance of the original materials in. the active layer at a permafrost location and the evaporative cooling effects of wet soil surfaces reduce the amount of heat that flows into the permafrost during the summer and out of it during the winter. In addition there is the insulating effect of the snow cover in the winter. The depth of the top of the permafrost (the permafrost table) is determined by the balance of winter cooling and summer heating. Whenever the thermal resistance or moisture transport effects of this layer are modified (for example, during a forest fire or clearing for construction), the depth of the permanently frozen layer changes until the balance between winter cooling and summer heating is restored. Likewise during a year with a warmer than average summer or colder than average winter, the permafrost table responds accordingly, moving up or down until the balance is once again restored. Climate changes are usually very gradual and the resulting changes are very small. Larger changes result when the thermal resistance is altered dramatically, by the forest fire mentioned above or by construction activity.


It is difficult to change the climatic influence on an embankment, but the overall thermal resistance of the active layer is within our control. The construction of an embankment over ice-rich permafrost causes several changes to take place.



Often the final result of all of these changes is an increase of thermal resistance above the permafrost table. This causes the level of the frozen layer to rise above its previous depth until once again summer heating and winter cooling approach a balance.

Other factors also enter into play. The reflectivity of the surface, for example, plays an important part in how much energy is absorbed from the sun, and thus what the surface temperature will be. The area available for heat to move in and out of the surface will be different and, like the cooling fins on an air cooled engine, will increase heat flow both in and out. But regardless of the changes, a temporary thermal balance will eventually be reached, and the depth of the permafrost layer will be altered either up or down.

In the far north, the thickness of the gravel embankment often provides enough additional insulation to cause the permafrost to rise into the gravel embankment except at the edges. If the embankment materials are non-frost -susceptible, small changes in the new permafrost table do not involve melting of massive ice forms and the associated detrimental effects.

In general, as the locality of the site moves south, summer heating increases and winter cooling decreases. Therefore, the thickness of the embankment that is needed to establish a thermal resistance that will protect the permafrost becomes larger and larger until it is no longer economically feasible to use gravel alone to protect the underlying permafrost. At this point, thermal insulating materials that have a lower thermal conductivity than gravel must be incorporated into the embankment to achieve the overall thermal resistance needed. It should be recognized that there is no location at which gravel alone is no longer adequate to protect the permafrost. Many secondary factors enter into


consideration such as evaporation, direction of exposure, and other climatic conditions. These factors combine to create a region in which insulation must be added at some sites but not others.

Words and expressions:

Layer Слой

Evaporative Испаряющий, парообразующий

To reduce Сокращать

Insulating Изоляционный

Resistance Сопротивление

To reflect Отражать

Thickness Толщина, слой

Detrimental Приносящий убыток, вредный.

Exposure Местоположение

Climatic conditions Климатические условия

Insulation materials

Several materials have been used in experimental road embankments. The most successful road insulation material has been foamed plastic board. Both polyurethane foam and polystyrene foam have been tried experimentally. The experiment showed urethane foam to be undesirable due to its high moisture absorption (as much as 72% by volume) and compression under soil pressures.



In a long term experiment (a roadway over permafrost) located in south central Alaska near Chitna the results have shown that the degradation of the permafrost can be slowed significantly by using foam insulation in the embankment. Thermally, the best location for insulation is at or near the top of the embankment. Insulation results in lower maintenance and longer life for the road embankment that must be constructed on permafrost.

Early experiments in road insulation used foamed sulfur, but researchers


found it absorbed too much water. McHattie, Esch and Reckard experimented with the insulating effect of artificially placing a peat layer beneath a section of road 50 miles southeast of Fairbanks, Alaska. They reported decreased thaw depths in the sections where the peat materials had been placed as compared to the control sections where the embankment had been placed directly on the mineral soil. However, in their final report they concluded that the advantage of peat as an insulation was not sufficient to warrant its general use as a standard design feature. The use of peat should be considered only in areas in which differential heave or settlement is expected to be severe. When peat is used, it should be placed well in advance of paving so that the peat layer can consolidate before paving. This may require paving to be delayed for a construction season.

The most durable foam insulation material at this time for direct soil burial is polystyrene insulation. Two forms of expanded polystyrene are currently manufactured. The first is extruded and expanded in a single-step process (XEPS) and has a homogeneous interior appearance with somewhat more dense surface skins. The second is molded polystyrene (MEPS) "beadboard" formed by first pre-expanding the polystyrene foam in bead form and then molding and sintering the beads together to form boards or billets that are cut into boards.

Urethane insulation absorbs large amounts of water into its open cell structure. Molded polystyrene beadboard (MEPS) may absorb considerable water in the pores between the expanded beads of polystyrene depending on the molded density and the process control in the molding stage. The absorption of water increases the thermal conductivity and reduces the effectiveness of the insulation. XEPS polystyrene appears to resist the absorption of ground water best, even when submerged below the water table for long periods at a time. After 20 years of exposure and at least 5 years of submergence in the Chitna Test Road (Esch 1990), XEPS insulation had moisture contents ranging from 1.1 to 2.2% by volume. Studies of the XEPS polystyrene insulation used in the Trans Alaska Pipeline workpad were made in 1981, approximately 6 years after


installation. These studies showed that moisture content in the buried insulation at 11 sites in the Brooks Mountain range and the north coastal plain were below-0.5% by volume, except for one submerged sample that had a moisture content of 0.8%.

Words and expressions:

Foam Пена

Undesirable Нежелательный, неподходящий.

Moisture absorption Поглощение влаги

Artificially Искусственно

Peat Торф

To consolidate Затвердевать

Cell Ячейка, отсек

Installation Установка

Content Содержание, объем

Site Участок

Thermal effects and properties of selected materials

It is not feasible to directly compare the thermal effects of a layer of insulation and a layer of embankment material. The thermal effect of a layer of insulation in a gravel embankment is a function of the thermal conductivity, the density, and the specific heat. Although the insulation will reduce the flow of heat through the layer, it will not store any significant amount of heat as compared to a similar layer of soil. The thermal diffusivity of a material gives a measure of how well heat diffuses throughout the material in the absence of a phase change in the moisture. Thermal diffusivities of the insulation and the embankment material it replaces would seem to provide one way to assess the effect of the insulation. However, during periods of equilibrium when the


temperature is not changing very much, heat storage in the layer is also not changing significantly. During these periods thermal conductivity (k) will provide a better picture of the heat flow process. The thermal diffusivity (a) is:

к
a=----------

pC (6.1)

where:

к = the thermal conductivity (BTU/hr ft °F or W/m °K)

p = the density (lbm/ft3 or kg/m3)

С = the specific heat (BTU/lbm °F or cal/kg °C)

Thermal diffusivity, thermal conductivity, and moisture contents need to be used together in assessing the relative value of the insulation in protecting permafrost. Comparing the two shows that the effect of the insulation during transient temperature periods is not nearly as significant as the effect of insulation during the stable steady state condition. During steady state conditions the insulation has a decided advantage over gravel.

TABLE 1 Thermal Properties of Selected Material.

 

    Specific Thermal  
Material Density Heat Conduct Diffusivity
(Unfrozen) Obm/ft3 (BTU/lbmmm -F) (BTU/ft hr °F) (ftJ/hr)

Gravel

5% moisture 130 0.44 18 0.32

Sand

10%moisture 120 0.32 17 0.44

Silt

20% moisture 90 0.59 8 0.15

Extruded polystyrene (dry) 3.5 0.29 0.18 0.18
Table 1 shows the thermal properties of selected soils and insulation.

It should be emphasized that neither type of polystyrene insulation is


compatible with hydrocarbon solvents such as gasoline, benzene, and diesel fuel. Contact with these solvents causes polystyrene foam insulation to dissolve. Gravel pads under fuel storage tanks or other areas in which a spill is likely are not good candidates for polystyrene insulation. Foamed glass or urethane, although inferior performers otherwise, are much better choices in these cases. The lower insulating characteristics should be compensated for by using thicker layers of insulation. The designer should increase insulation thickness by two to one for urethane, and by three to one for foamed glass when they are used to replace extruded polystyrene.

Thermal storage is an important property of a road embankment. The ability of the embankment to store heat and to release it later when the temperature drops, slows down the transient effects of changing temperature. This can have the effect of preventing the surface from becoming frost covered during sudden, short below-freezing temperature excursions.

Words and expressions:

Density Плотность, удельный вес

Transient Скоротечный

Steady Постоянный

Gravel Гравий

Sand Песок

Slit Ил

Hydrocarbon Углеводород

Property Качество

To store Хранить

To release Высвобождать

Winter roads

One of the attractions to working in the winter months is that temporary roads and airfields can often be constructed inexpensively and without damage to


the environment. Permits to work in or to cross streams or wet terrain can often be obtained in the winters that are not possible during warm weather. Many frozen rivers provide access to areas reached only by air during the summer. Care must be used when traveling over ice, but with proper precautions, frozen rivers and lakes can provide inexpensive access to many remote sites. Snow roads allow travel overland without the surface damage that result from as little as a single passage when the active layer is thawed.

Many winter roads are established each year. These roads provide transportation between several villages and allow heavy freight to be brought in that cannot be transported to the site any other way. This has been the traditional way for heavy mining equipment to be delivered to remote mines.

During the era of placer mining in the north, the enormous dredge machines were moved to new locations over snow roads. Many of these machines were so large that they had to be cut into three sections with each section mounted on skids. A team of crawler tractors would be used to move the pieces to the new location. The classic example of these moves was the move of the Fairbanks Explorations Co. dredge no. 6 from Gold Hill near Fairbanks, Alaska to Sheep Creek, an 8 miles run. This large dredge was moved nearly intact, only the digging bucket arm, ladder, and stacker were removed. The crawler tractor team used 15 tractors pulingand 3 tractors in the rear to push when needed or to dig in their ripper blades to restrict the speed if required on downhill stretches.

These operations would not likely be allowed now because of the potential damage to the terrain, but it illustrates the possibilities that exist for moving heavy loads when properly prepared snow and ice roads are utilized.

Words and expressions:

Attractions Привлекательность

Damage Вред


Access Доступ

Single passage Единственная дорога

Dredge machine Экскаватор, драга

Crawler tractor Гусеничный трактор

Terrain Местность

Drainage requirements

During the spring thaw, excess water saturates the thawed soil. The frozen underlayers often prevent proper drainage, and the saturated surface layers become soft and incapable of supporting loads. This is called "thaw weakening." It occurs during every spring breakup to greater or lesser degree. The problem is considerably worse if the road embankment has accumulated ice lenses during the winter. The moisture content of the thawing soil is then much higher than it was at the beginning of spring. Saturated and supersaturated soil conditions are likely due to the poor drainage through the frozen lower layers. The ability of the embankment to support loads is severely limited under these conditions. Load restrictions are commonly placed on most major roads until the road embankments have drained and dried sufficiently to prevent permanent damage to the embankment. As the spring thaw moves north, roads become impassible or at best severely restricted. Heavy loads that are caught in the restricted zones can be stalled several weeks waiting for load restrictions to be lifted. The Alaska Department of Transportation has determined that the passage of one heavily loaded large truck and trailer rig will cause as much wear and tear on a road as the passage of 7000 passenger vehicles.

Excess moisture in the soil during the freeze-up season is equally hazardous. Frost heaving can cause buildup of thick ice layers within the embankment. Remember the requirements for frost heaving; water, wicking, and winter. If all three of these are present, water will be transported by wicking


action to the freezing front within the embankment. There it will accumulate until an ice lens has formed. The size of the lens will depend on the water supply, the conditions for transport (distance through which water must be moved and wicking action of the particular soil), and the length of time frost-heaving conditions exist.

Ice lenses can grow very large, but in a road embankment they are usually numerous small layers of ice. The total accumulation of all of the heaving forces from all of the lenses is directed against the surface which is exposed to the cold air. The heaving force will be directed perpendicular to the freezing front which is generally parallel to the exposed surface and which moves into the soil from the surface. A road surface will thus be heaved upward and a shoulder will be heaved outward perpendicular to its slope.

A vertical retaining wall will generate a freezing front parallel to the wall, and if heaving conditions of water and wicking are also present, the wall will be subjected to huge forces from behind that often cause it to fail. Good drainage is important to reduce the amount of water available for frost heaving, but another solution is to heavily insulate the back of the retaining wall. If the insulation is sufficient to limit the heat loss through the retaining wall, the progression of the freeze front from the retaining wall into the soil will be much slower than the progression of the freezing isotherm from the surface of the soil behind the wall. In this case the freezing front will be perpendicular to the retaining wall. The frost heaving force that is perpendicular to the freezing front will then be parallel to the retaining wall and will not fail it.

Good drainage in an embankment is a prime requirement for minimizing frost heave, and for avoiding soil softening conditions during wet periods such as spring breakup. Permeable embankment materials when available should be used, but when poorly draining materials (which are common throughout much of the north) must be used, other means of drainage must be provided. Lateral ditches may work in some areas, but they should be avoided in high-ice-content


permafrost areas because of the subsequent melting of the frozen soils and loss

of support for the embankment shoulders. Adequate drainage must be provided
for both surface runoff and for subsurface water movement.

Words and expressions:

To saturate Насыщать

Loads Нагрузки

Drainage Осушение

Truck Грузовик

Vehicle Автомобиль

Surface Поверхность

Requirement Условие, требование

Breakup Распутица

Subsequent Последующий

Subsurface Находящийся под поверхностью

 








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