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
Techniques for locations without proper materials
Although granular materials (clean well graded sand and gravel) are the road embankment materials of choice, there are many sites in the north in which they are not available. In many cases the nearest source for desirable materials is too far away to be economically feasible to use. In such circumstances, in -situ or locally available materials must be utilized. When frost susceptible materials must be utilized in a road or airport embankment, it is imperative to provide the best drainage possible.
Barriers to restrict capillary moisture flow into the embankment from below may also be considered. These may be layers of coarse grained material or geotextile layers. The purpose is to break the capillary action of fine grained soil in the embankment so that moisture cannot "wick" to the freezing front from a shallow water table or high moisture content soil below. This will help to prevent the formation of ice lenses in the embankment and reduce the amount of excess
moisture during the following spring thaw. When layers of coarse-grained material are used for this purpose, they must be protected from infiltration of fines that would destroy their function. Filter fabrics separating the layers of coarse-grained material from silty soil above are under study for this purpose.
Freezing just the moisture normally in the embankment does not cause large amounts of heaving. A soil with 20% moisture by volume will expand 9% of the 20% or only 1.8%.
Finnish, Swedish, and Russian road designers often use a lightweight expanded clay aggregate (LECA) or "lightweight gravel" when natural gravel is not readily available, or when they want the specific properties that this material provides. The Finnish call their product "Kevytsora" which translates into "lightweight gravel"; the Russians call their product "ceramsite." The Russian material is reportedly heavier (380 to 650 kg/m3) and less uniform in size than the Finnish product (280 to 500 kg/m3).
The lightweight gravel is formed by firing plastic clay materials until they sinter into hard but very porous nodules. Their porosity makes the nodules very lightweight yet they are strong enough to be used in place of gravel without crushing. The material is available in several grades with different densities and size gradations. Crushing strength is reported to be between 180 and 700 kPa (25 and 100 psi) depending on the density of the material. The allowable load in Finland, however, is 200 kPa (28 psi). The friction angle of compacted LECA is 37°. Uncompacted, its friction angle is 33°.
The price of the Finnish product is between 140 and 180 mk/m3 (27 and 35 US$/yd3 in 1990 dollars). The Russian product is reported to be 100 mk/m3 (19 US$/yd3 in 1990 dollars) in rail cars at the Russian border (Eerola 1990).
The low density of the material makes it less expensive to transport, keeping it competitive with natural gravel materials if they must also be shipped in from considerable distance. The low density of the material gives it good insulating properties so that it restricts the flow of heat more than conventional
coarse grained materials. It is used extensively in high-way construction and for
frost heave, abatement design. Its light weight also makes it attractive when
embankment weight needs to be minimized to reduce settlement over poor
subgrade materials, such as over permafrost or muskeg. It is also used as an
aggregate to make light weight concrete structures such as sidewalks, utility
poles etc.
Feasible Осуществимый
To prevent Предохранить
Coarse-grained Крупногравийный
Volume Объем
To expand Увеличиваться
Lightweight Легкий
Porosity Пористость
Friction angle Угол трения
Concrete Бетон
Sidewalk Тротуар