Principles and methods of the control of cryogenic processes

When development is undertaken in the permafrost regions one is repeatedly forced to solve the problems associated with the necessity of changing the geocryological conditions in the direction favourable for the national economy. Defining the problems of the control of the cryogenic process on its own requires knowledge of the natural conditions and of the current geocryological situation which will be subject to artificial change at a later time.

A combination of work carried out to control the cryogenic processes is performed in a certain order. Study of the natural (including geocryological) conditions of the region under development is fundamental and obligatory. The study of geocryological conditions is based on revealing the particular and general principles of formation and development of seasonally and perennially frozen ground and its characteristics depending on the combination of natural conditions and on each element of the geological-geographical environment. This problem represents the immediate object of geocryologi-

cal survey and is solved successfully in the course of surveying. Therefore clearly the geocryological survey must be a component part of the integrated investigations for the control of cryogenic processes and must precede all other kinds of investigations. This stage ends with the compilation of geocryological maps.

The next stage of work includes description and typification of the projected technological loads and impacts in the course of the development, and the revealing of the associated changes in natural conditions. This stage of investigations is carried out to get the starting data for the geocryological forecast.

Then, the geocryological forecast is made on the basis of the revealed characteristics in the formation of the geocryological conditions and consideration of technological impacts. Possible changes of mean annual ground temperature, of soil composition and properties, of the depths of seasonal and perennial thawing and freezing, and of the character of cryogenic processes are assessed in the geocryological forecast.

The problems of the next stage of investigations include revealing the areas within which geocryological conditions can develop which do not meet the requirements that might be imposed on them. In the design of specific buildings and constructions these can be areas within which there is no possibility of implementing the accepted principle of construction. For example, in the course of commercial mineral development there are localities within which the practical way of extraction cannot be applied. Within these areas desired changes of the geocryological conditions (of permafrost distribution and conditions of occurrence, of temperature regime, of seasonal thawing and freezing depths, of composition, structure and properties of seasonally and perennially freezing ground, and of cryogenic processes) are made clear by the results of the forecast.

Such a desired change of individual characteristics of frozen ground can be carried out using various methods of amelioration. Therefore it is necessary to select the most acceptable, rational ways in each case. For this purpose it is necessary first of all to study the methods of water-thermal amelioration and to justify the possibility of use of one or another method for directed change of the natural situation in the given region to create the desired permafrost conditions. It is necessary that the estimate of the economic efficiency of the measures used should take into consideration the time allotted for them to be carried out. It should be noted that each method, in parallel with its main (direct) effect on the individual elements of natural surroundings, causes changes of other elements of the geological surroundings to a greater or lesser extent. In some cases this can cause undesirable changes in the geocryological conditions. The results of this stage are elaboration of the measures aimed at purposeful change of the permafrost conditions, and regionally, separation of the areas characterized by similar permafrost conditions within which the particular methods (or set of methods) for the control of cryogenic process must be used.

At the final stage of investigations, each of the distinct areas of the region under amelioration is characterized with respect to the artificially created geological-geographical and geocryological conditions which will occur after amelioration. The post-amelioration geocryological maps are compiled.

A set of measures aimed at the purposeful change of geocryological conditions is performed in the following order.

1. Study of the geocryological conditions in the course of the geocryological survey.

2. Characterization and typification of technological impacts and loads.

3. Forecast of changes in geocryological conditions.

4. Substantiation of the need for designed changes of geocryological conditions.

5. Elaboration of the project measures aimed at designed changes of geocryological conditions.

6. Compilation of post-amelioration maps.

There exists now a great number of methods for control of cryogenic processes. Some of the methods of frozen ground amelioration widely used in the course of economic development in the permafrost regions were considered above. V. A. Kudryavtsev and E. D. Yershov have developed a classification of methods for changing geocryological conditions, in an effort to straighten out the existing methods. This classification allows selection of the most efficient measures as far as any kind of economic development of the permafrost region is concerned. The classification is based on consideration of the thermal-physical aspects as well as of the geological-geographical aspects of the cryogenic process to be controlled. All the existing complex of methods is subdivided into three groups.

The first group of methods changes the cryogenic process directionally by changing the parameters of the external heat exchange (Table 19.2). It includes two systematic methods; one of them regulates the relation between the radiation balance components and the other regulates the relation between the heat balance components. Each method consists of three sections, changing the elements of the external heat exchange: the integrated and the reflected shortwave radiation, the effective surface radiation of the Earth, the turbulent heat exchange with the near-surface air, the heat of moisture phase transitions (evaporation - condensation) and the heat exchange with the ground below. The main measures for changing the external heat exchange are the following: arrangement of sheds or awnings, surface covering by various diathermal films, artificial colouring of the surface, removal and planting of vegetation cover (shrubbery and trees), a modification of snow accumulation conditions, etc.

The second group combines the measures aimed at control of the processes of heat- and mass-exchange in ground by changing composition and properties of the ground (see Table 19.2). It is subdivided into two systematic methods: the first, involving changing the composition, properties and thermal state; the second, involving changing the properties and thermal state of the ground under amelioration. The first system of methods includes two sections. One section includes the measures facilitating the change of the composition of the organic-mineral portion of the soil, the other section is aimed at changes of ground moisture regime. The measurements for the second system allow regulation of heat- and mass-exchange in the ground by changing the properties and the thermal state of the ground without any essential transformation of its material composition.

The methods of the third group are used to change the temperature regime and the heat state of the ground under amelioration by using additional heat sources and sinks (see Table 19.2). It is subdivided into two systems using natural and artificial sources and sinks of heat. The first system includes two sections recognized with respect to the kind of heat carrier (water, air) being used to intensify the mass-exchange process. The second system of methods consists of three sections. Each of them uses either electric power, or thermochemical mixtures, or steam, fire, or 'thermal probes' as artificial heat sources.

The selection of control methods is fully individual in each particular case and depends on the conditions of the geological-geographical environment, on the particular features of the economic development and on the degree of efficiency of use.

When planning the amelioration measures one should bear in mind that any method, any measures being used to change a particular parameter of the environment, will inevitably result in the change of the whole complex of characteristics of the ground under amelioration to some extent. Therefore the decision to use a particular measure aimed at changing the permafrost situation, must be taken on the basis of knowing the effect of a given method on all the factors of the geological-geographical environment.

Table 19.2. A classification of the methods of control of cryogenic processes

Classes of methods Sub-classes of methods

Parameters through which the natural elements being changed can be transformed

Methods of control

A group of methods regulating external heat exchange _ The integrated shortwave radiation, Q + <

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