Of construction

Economic development is always integrated and multifarious and includes construction and operation: 1) of civil and industrial installations (residential, social, municipal, factory buildings and structures); 2) of linear structures (railroads and highways, pipelines, underground lines, power lines; 3) of airfields; 4) of hydrotechnical structures. The special features of construction and operation of buildings within the permafrost regions will be discussed in the next chapter. This chapter is concerned with the other kinds of construction.

Road and railway building

Considerable attention has been focused recently on roads and railways under construction within the permafrost regions. These are the BAM railroad (3145 km in extent), the Surgut-Urengoy-Yamburg railroad, the railroad and motor road being designed for the Yamal peninsula, etc. Three alternative principles are used when designing highways (Fig. 17.3). The first principle is preservation of perennially frozen ground in the base of a roadbed during the whole period of operation by means of raising the permafrost table up to an embankment base. The second principle is based

Fig. 17.3. Examples of use of principles of highway and railway design and construction within the permafrost zone: a - with creation of a permafrost core in the base of an embankment (the first principle); b - with partial permafrost thawing (the second principle); c - with prethawing of the permafrost (the third principle); 1 - fill; 2 - moss-vegetation layer; 3 and 4 - surface of permafrost under natural conditions and in the period of operation, respectively.

Fig. 17.3. Examples of use of principles of highway and railway design and construction within the permafrost zone: a - with creation of a permafrost core in the base of an embankment (the first principle); b - with partial permafrost thawing (the second principle); c - with prethawing of the permafrost (the third principle); 1 - fill; 2 - moss-vegetation layer; 3 and 4 - surface of permafrost under natural conditions and in the period of operation, respectively.

on the partial thawing of the perennially frozen ground of the base by an amount determined through calculation from the allowable deformations of a roadbed. The third principle assumes thawing of frozen ground prior to the beginning of construction with soil drainage under the highway itself and within the strip along the roadway.

Design on the first principle is conducted within areas of low-temperature perennially frozen ground prone to great subsidence, when the thawing of the ground can cause impermissible deformations and destruction of the pavement. The highway is designed with the embankments composed of uncemented clastic materials with retention of the moss-turf cover in the original undisturbed state at the base of the embankment and along the whole roadway. To retain the moss-turf cover it is recommended to build an embankment 'away from you'. Machines being used for construction move on the filled embankment in this case and the soil is placed directly on the moss cover. To reduce the embankment thickness, thermal insulation inter-layers of peat, compacted moss and slag, etc. are placed in the base of a roadbed. Using thermally insulating foam plastic shields with rather high strength give good results. These shields were used successfully in the course of construction of certain sections within the Urengoy-Yamburg and BAM railroads.

Design on the first principle can be carried out using the base soil prefreez-ing method. This method was suggested by B.I. Popov, N.F. Savko and others in the course of roadway building in Western Siberia, towards the southern limit of the permafrost. It was impossible to use water-saturated soils from swampy ground with thick peat layers as a base for motor roads. Removal of snow and vegetation cover of the alignment of a road had caused freezing of the upper layer of peat with formation of a thin permafrost interlayer within 2-3 years which was used as a base for the embankment.

Design following the second principle is usually carried out for an embankment composed of clay-rich and sandy soils with the moisture content below the plastic limit and showing only slight subsidence in the course of thawing. The moss-turf cover is not removed in this case, at the base of the embankment. Design on the third principle is carried out mainly in easily drained soils. It is used chiefly when preliminary thawing of the permafrost, drainage of roadway strip and strengthening of base soils as a result of their pre-construction settlement is possible. It is necessary in this case that the roadway strip should be cleared from forest, scrub, the moss cover should be removed within the strip completely, and the drainage ditches should be made no less than a year ahead of the beginning of work.

Design of railroads and motor roads within the permafrost zone is carried out mainly on fill. Cuts are allowed mainly within the areas with favourable geocryological conditions (hard rock debris and ice-poor gravel) and hydro-geological conditions (absence of suprapermafrost waters). If it is necessary to make a cut within the area of fine ice-rich soils the cut should be designed only with a guaranteed thermal insulation of slopes, replacement of ice-rich soils and assured drainage from the cut.

The differential heaving caused by nonuniformity of soils and of moisture and freezing conditions, presents the most severe hazard for a roadbed. The main measures to prevent and reduce the heaving are the following: replacement of soils, placement of coarse-grained layers (to raise the freezing intensity and to interrupt the paths of water migration); drainage of the areas adjacent to the roadbed; use of thermal-insulation layers (to decrease the depth of freezing); soil salinization (lowering of freezing temperature) etc.

Roadway construction is adversely affected by more intense icing processes. The main reasons are the change of natural ground runoff conditions in the course of construction of cuts, a rising of the permafrost table within embankments constructed on the first principle; inadequate arrangement of surface run-off around constructions associated with water (pipes, small bridges); and change (spreading) of water of rivers and streams in the course of bridge construction. Temporary and permanent snow and soil barriers, ties and board fences, water-diversion ditches, waterproof screens and permafrost belts the effect of which is based on interception of subsurface flows and artificial displacement of icings uphill (Fig. 17.4), serve as measures for icing control. In addition to these methods of icing control, drainage of the area and deepening of stream beds, etc. is carried out. Recently the method of prevention of icing formations with the help of groundwater extraction by pumping from wells has been introduced. When making cuts, cutting slopes

Fig. 17.4. Diagram of one of the structures for use in the permafrost belt for control of icings due to suprapermafrost water: 1-2 - soils (1 - fill; 2 - high seepage 3 - snow; 4-5 - boundaries (4 - of soil freezing from the surface; 5 - of permafrost). Arrows show the direction of flow in the soil.

or when there is inadequate arrangement of water-carrying constructions or features, an intensification of the thermal erosion processes is a possibility, with thermal erosion control being a serious problem making it necessary to do much excavation to remove gullies.

Trunk pipelines

These are now an integral part of the landscape within the permafrost regions and extend over hundreds and thousands of kilometres, passing through different geocryological zones. Pipe laying is performed by various means. At the same time the temperature regime of the product being pumped changes greatly along the pipeline route as a rule. All the above dictates specific geocryological research in the planning of the pipeline.

The construction features and temperature regime of pipelines depend on the character of the product being carried. Thus oil pipelines and water pipelines operate under positive temperature, with the minimal temperature being +5 to +10°C for oil, because under lower temperature the oil becomes thick, paraffin plugs are formed and the oil becomes unsuitable for transportation. Gas pipelines can have positive as well as negative temperatures.

The pipelines are divided with respect to their position in relation to the ground surface, into those underground, those placed inside an embankment or those without piling-up of soil (exposed) and those elevated above-ground (Fig. 17.5). When passing through a water course, underwater-laying is used. The pipelines laid under ground exert the greatest thermal

Embankment With Water Filled Pipe
Fig. 17.5. Methods of pipe laying: a - underground; b - in an embankment; c - above surface; 1 - earth infill; 2 - sandy pad; 3 - fill; 4 - collar beam; 5 - pile.

effect because the pipes (especially those large in diameter), widely used in trunk pipelines, are laid below the maximum depth of seasonal thawing inside the permafrost. The smallest heat effect on the permafrost is observed with above-ground pipelines.

There can be four main variants of combinations of mean annual temperature imean, maximal fmax and minimum imin gas temperatures characterized by rather different thermal effects on the permafrost (Fig. 17.6), the particular formation of a seasonal or perennial annulus of freezing or thawing depending on the temperature regime of the pumped product (for example, of gas, having the widest range of temperature variations in the course of its transportation). All the illustrated variants of the temperature regime of a pipeline are a possibility within one route. This situation prevents the use of a single pipeline-laying method. The gas has a constant temperature of + 30 to +40°C when leaving the compressor station. Under the interaction of a pipeline with soil or air the temperature is progressively lowered along the pipeline route approaching the temperature of the natural environment. In this connection the thawing of perennially frozen ground around the pipeline is a possibility within the initial section of the pipeline route, while at a distance 100-150 km away from the compressor station seasonal or perennial freezing is a possibility. The Medvezhye-Nadym-Punga pipeline is characterized, for example, by such a regime.

The main undesirable cryogenic processes from laying of underground pipelines with positive temperature are formation of a thawing annulus around the pipeline, ground settlements, paludification and thermokarst development. In addition to this, thermal erosion often develops along the trenches. Under negative gas temperature when a freezing annulus is formed around the pipeline, heaving is a possibility.

Laying pipe inside embankments poses a problem of ensuring the stabil i a

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