According to V.A. Kudryavtsev (18) geocryological forecast refers to the scientific prediction of changes of geocryological conditions which will occur in the future, either in the context of natural circumstances or in the course of economic development.

In line with this definition two types of forecast, the evolutional (often designated natural-historical) and the technological are distinguished. The first includes forecasting the changes in permafrost characteristics and of the processes within it under the effect of the natural dynamics of climate, of neotectonics, of world ocean levels, of denudation and sedimentation processes, of ice covers and of hydrogeological, hydrological and geobotanical conditions. It also includes forecasting of the reverse effect of the geo-cryological characteristics on the components of the geosystem.

Technological forecasting includes assessment of changes of geocryological conditions under the effect of various technological disruptions of the natural system (beginning with changes of the local landscape and geo-cryological conditions and ending with global natural transformations such as creation of water reservoirs, industrial air, surface and groundwater pollution, changes in the temperature regime, etc.). Engineering-geo-cryological forecasting is one of the most important divisions of the technological forecast. The engineering-geocryological forecast is made to solve such practical problems as selection of construction sites and routes for linear structures, assessment of arrangement of structures and the selection of principles for their construction, selection of methods for highway location and of mining methods and a number of other questions necessary for design, construction and operation of various structures. On the basis of geocryological forecasting measures are recommended for eliminating or limiting the consequences of disruption of the balance of natural geosys-tems, dangerous to structures and the environment, and methods for permafrost control are developed. At the same time in line with the character of technological impact a forecast is made of changes in ground temperatures, of the amount of permafrost freezing and thawing and of changes in its composition, structure and properties and the possibility for the activation and initiation of new geocryological processes, and the time for their stabilization, and of conditions causing their progressive development. In all cases the engineering-geocryological forecast must take into account the results of the evolutional one. The character and formulation of the problems depend on regional features of the terrain and on the detail of the investigation (geocryological survey scale) and its aims.

The main objective, problems and methods of the forecast at various stages of engineering geocryological investigations are presented in Table 19.1. We can see that when the forecast is made at small and medium scales the assessment of sensitivity and stability of the permafrost to technological

Table 19.1. Stages, problems and methods of geocryological forecast

Stages of engineering The aim of The main tasks for geological cryological the cryological The methods of investigations forecasting forecast forecasting

Stage I - Master plans for development of national economy on the basis of engineering-geological investigations of natural regions and zones at general scales (1:2 500 000-1:1 000 000)

Stage II - The diversified development of individual regions on the basis of national engineering-geological survey at the 1:500 000(1:100 000) scale

The assessment of the influence of the geocryological situation on natural resources

To guarantee the rational use (optimizing, arrangement and type of construction included in the industrial complex)

1. Forecast of the natural dynamics of the geocryological conditions.

2. Assessment of the sensitivity of the geocryological situation to the technological impact.

3. Assessment of the possibility of controlling the 'cryogenic processes' for effective utilisation of natural resources.

1. Assessment of the stability of the geocryological situation and of permissible technological impacts.

2. Assessment of the reverse effect of changes of the geocryological environment on landscape development.

3. Forecast of the efficiency of measures to ensure the reliability of the construction and environmental protection.

1. The method of analogies on the basis of experience of development.

2. Logical and mathematical simulation.

3. Expert assessment.

1. The method of analogies on the basis of the study of construction experience.

2. Mathematical and physical simulation (including observations of regime.

3. Extrapolation.

4. Classification of permafrost and cryogenic processes.

Table 19.1. (Continued)

Stages of

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