Fig. 17.10. Temperature dependence of specific cutting resistance k: 1 - sandy-silty material; 2 - sandy-silty clay; 3 - clay; 4 - sand.
by gravel-pebble deposits. Mechanical mining is difficult in the frozen state and therefore they are usually subject to prethawing.
According to G.Z. Perl'shteyn the methods of thawing frozen ground used in the course of placer mining include: 1) snow retention (snow thickness increase) during the cold period and its mechanical removal at the beginning of the warm period; 2) removal of vegetation and the upper layer of soil; 3) arrangement of shallow (20-30 cm) warming ponds in summer; 4) flooding of the area with water for a winter period and creation of an ice-air system; 5) injection thawing; 6) steam thawing; 7) electric heating; 8) ther-mochemical methods. All the methods are used also in the course of precon-structional thawing of frozen ground (see Chapter 18). In addition to the methods cited above, layer-by-layer sprinkling and 'infiltration'-drainage thawing, soil salinization and the formation of artificial 'sushenets' (drained land) are usually used.
When layer-by-layer thawing is used the soil is mechanically removed from the surface to a depth of 10-20 cm, as it thaws naturally, resulting in exposure of the frozen soil. Then the natural (radiational-heat) thawing of the next layer to the same depth occurs and it is removed also. This manipulation is repeated during the whole summer period intensifying the thawing process. It is possible to thaw a placer up to 10-15 m in thickness in such a manner during the warm period.
Sprinkling thawing is used for preparation of areas of a placer where the material is highly permeable in a thawed state. The method consists in spraying water with the help of sprinkling installations placed in a grid (Fig. 17.11a). To intensify the process, film coatings reduce the expenditure of a 1 2 b a 1 2 b
heat in evaporation and losses through turbulent heat exchange, and salt solutions allow transfer from the frozen state to the thawed under negative temperatures.
Seepage-drainage thawing of frozen ground involves transfer of heat from a horizontal seepage flow developed in the thawed layer under the effect of a difference in water levels between a sprinkler and a drain (see Fig. 17.11b). On account of the small temperature gradients of the seepage flow this method is usable only for thawing soils with seepage coefficients higher than 40m day-1.
Gravelly pebble-rich soil with small sand content can be prepared for year-round mining by dewatering. The main point of this method is that at moisture contents less than 2-3% these materials do not possess under negative temperature the particular properties of frozen ground and are similar, in mechanical mining, to thawed materials. Seasonally thawed ground artificially dewatered in the summer period is often termed 'sushenets'.
Underground workings involve shafts and tunnels. One considers the special properties of frozen ground within the permafrost zone, and as well, in regions of unfrozen ground (for subways), because the method of artificial freezing is often used in sinking or driving in weak water-saturated and fluid earth. Driving is performed under the protective enclosure of frozen ground having higher strength than before and low permeability. Once the driving is finished and the continuous support is installed, there is no need for the ground enclosure to be frozen and the ground is thawed. Thermal-technical and statistical calculations for the frozen ground around workings are performed at the design stage. Thermal-technical calculations dictate the selection of measures to cool or to freeze the ground and involve determining the temperature field around the underground structures. With the help of statistical calculations it is possible to assess stability and to determine safe dimensions. It should be taken into account that frozen ground can, with time, develop plastic creep strains many times higher than the momentary (elastic) deformations. Therefore when stability calculations for ground are performed it is necessary to start with rheology theory and to carry out the calculations for ground strength and strains with regard to the time factor.
Underground industrial structures (storages, plants, electric power stations, shelters, etc.) within the permafrost regions are, with respect to the thermal regime, divided into structures with positive and negative operational temperatures. The former are usually sited on hard rock (bedrock) although other sufficiently strong and stable ground can be used. In the narrow underground coolers of adit or vertical shaft type the temperature is usually maintained by natural cold reserves inside the permafrost and does not go below — 10°C. When the thermal-technical calculations are carried out, the heat remaining inside the structure and the heat coming in through air exchange between the structure and the atmosphere are taken into consideration. Winter ventilation or artificial ventilation and adequate thermal insulation in summer enrich the 'cold reserves'.
There are underground structures the inside temperature of which is lower than the natural temperature of frozen ground. High-powered coolers having a temperature below —20 to — 30 °C and storage of condensed gas having a temperature below — 180°C, are in this category. Structures of such type demand use of additional natural or artificial cooling. In the course of their operation development of a number of processes in the ground associated with the low temperatures is inevitable. These are cracking (increasing the gas permeability of the containing frozen ground); ice sublimation (causing falling and collapse of ground walls); ablimation and icing of walls of underground workings, etc.
Drilling of prospecting and production oil and gas wells
Within the permafrost zone, such activities cause warming of the ground adjacent to the well and its thawing. Experience shows that use of water as a flushing fluid, as is the practice outside the permafrost zone, is practically impossible within the permafrost because it causes further warming and thawing of the ground, formation of cavities around the wellhead and distortion of the wellhead. Therefore special-purpose flushing fluids which do not freeze under negative temperature have been developed. In addition, it is profitable to use air as a working medium. Fixing of casing pipes (strings) is no less complicated a problem.
Operation of oil and gas wells is also associated with heat release. Gas and crude oil in pools have temperatures varying from 20-60°C while discharge from the wells may run as high as hundreds of tons per day. If there are no countermeasures in the course of well construction, one might expect permafrost thawing around the well and ground subsidence around the wellhead (Fig. 17.12) and increase of loads on casing strings with time. Ground thawing can be prevented with the help of well bore zone cooling using circulation of special-purpose coolants (the active way) and with reliable thermal insulation of the well bore (the passive way). In the first case two concentric jigs are positioned inside the permafrost layer with the space between these jigs and the production string filled with air or nitrogen (Fig.
17.13). Thus the problems requiring geocryological investigation in well design are the following: calculation of temperature patterns around the well for the period of sinking and operation, and the assessment of the possibility of cavity formation as a result of thawing of ice-rich ground and
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