3. Forecast of the geocryological environment in connection with use ofground amelioration methods and landscape recultivation.


3. Forecast of the geocryological environment in connection with use ofground amelioration methods and landscape recultivation.

impact is one of the main results. In this case by 'sensitivity' is meant the response of the geosystem to the impact and the degree of its change, while by 'stability' is meant the capacity of the geosystem to resist the impact without any change in its state and structure, i.e. without such changes of the components of natural complexes and interactions between them which could cause impermissible deformations of structures or irreversible deterioration of the ecological situation. In essence, sensitivity and stability are closely related concepts and are specified by the 'value' of changes in geocryological characteristics. However in the latter case some restriction on the limits of permissible changes is assumed depending on the practical problem under solution. Clearly, the stability of geosystems can be assessed from various aspects, with respect to the various kinds of structures, as they respond to changes in ground conditions, properties and in the processes underway. The same changes in geocryological conditions can be dangerous or not depending on the kind of structure. And if the stability of geosystems is defined by permissible technological impact, not causing dangerous changes in engineering-geological conditions, they can also be essentially different depending on the kind of structure (for example, permissible impacts in the course of airfield construction differ from permissible changes in the course of highway construction and, especially, in the course of industrial construction). At the same time for the same kind of structures the permissible impacts depend on the initial natural situation. Thus, the concept of'stability' is largely a matter of convention and in each particular case one should note the aspect of its application.

The sensitivity of the geosystem (of the natural complex) does not depend on technological impact - being the property of the system, the capacity to respond to the impact. Therefore this concept is more distinctive, it is always characterized by the degree (value) of changes of individual (or several) geocryological characteristics under the individual (or a set of) impacts.

Technological geocryological forecasts are subdivided with respect to impact of construction on the natural environment into: a) general forecasts, providing for the assessment of the changes in components of the natural complex without taking into consideration the thermal and mechanical impact of the structure itself; b) engineering forecasts, which include the assessment of the results of the direct impact of the structures on the geocryological situation.

When the general geocryological forecast is made the possible changes in the permafrost conditions as a result of such processes as disturbance of vegetation cover or of snow accumulation conditions, peat removal and replacement of soil, grading, change of surface and ground runoff, arrangement of artificial cover, grass plots, trees and shrubbery plantations, etc. are described. The necessity for the general geocryological forecast when the engineering problems associated with economic development of the permafrost zone are being solved, stems from the fact that heat releasing and heat absorbing structures under any kind of construction occupy only a part of the area of the disturbed terrain. Thus, according to G.V. Porkhayev's and V.K. Shchelokov's data the density of buildings in northern cities of the former USSR accounts for 13-40%, the rest of the area of the cities is occupied by roads, streets, squares, public gardens, etc. According to [AllUnion Research Institute of Hydrogeology and Engineering Geology] VSEGINGEO'S data heat releasing structures occupy only 9% of the area of Western Siberian gas fields; within the rest of the area changes in natural conditions in the course of construction take place. In addition there are numerous cases in practice when there exists a time lag between the begin ning of the construction and putting the structure into operation. In such cases the results of the general forecast serve as initial data for making the engineering geocryological forecast.

When the engineering forecast is made the particular features of the thermal and mechanical impact of structures on the permafrost are assessed.

Various methods are used to assess the technological impact on the geocryological situation. Among these methods those of mathematical modelling have received wide recognition. However experience shows that efficiency and reliability of the forecasts (as far as the whole range of problems is concerned) increases when several methods are used, such as physical and mathematical simulation methods, methods of analogues, analysis of construction experience and extrapolations and classifications.

When making the engineering-geocryological forecast using one method or a series of methods, one must use the universal method consisting of successive studies of the geocryological conditions with the assessment of the role and effect of each natural environmental factor and typification of technological impact, on the basis of the requirements of the projected engineering structure and with the background of experience. The forecast is made as a result of solving a set of problems to assess the possible changes in engineering-geocryological characteristics and the efficiency of amelioration measures intended to guarantee the reliability of the structure and to control the dangerous engineering-geological processes.

Depending on the duration of time for which the geocryological conditions are predicted, the forecast is subdivided into short-, long- and ultra-long-term. The short-term forecast is made for a period of up to 10 years and characterizes changes in geocryological conditions under the effect of short-term (3-11 years) climatic fluctuations, excavation, construction work and operation of structures during the first years, when changes of the state and characteristics of the ground, especially near the surface (in the layer of seasonal thawing and in the layer of annual ground heat storage), proceed most intensively and can be dangerous for the structures. The long-term forecast is made for the period from 10 to 100 years and characterizes changes in geocryological and engineering-geological characteristics corresponding to the new steady-state temperature and moisture regime of the ground. The ultralong-term forecast is made for especially important structures, the period being longer than 100 years. It is used mainly to assess changes in geocryological conditions under the effect of natural environment dynamics or of regional and global transformations causing changes in ground thermal conditions over large areas, as well as to assess the long-term effect of structures on the geocryological situation.

It was shown in Chapter 17 that the various kinds of construction have different effects on the environment. Therefore the range of problems of forecasting the ground temperature regime will have particular features for each kind of construction.

In the course of urban construction the ground temperature regime is formed under the effect of many factors which are arbitrarily divided by G. V. Porkhayev and V. K. Shchelokov into three groups: common, local and particular. Among the common factors are components of external heat and mass exchange within the construction area such as radiation balance, turbulent heat exchange, heat expenditure on evaporation and condensation of moisture at the ground surface. Among the local factors causing change in ground temperature regime within a comparatively small area are the thermal effects of buildings, structures and service lines. Among the particular factors are those typical of particular places only. For one region these may be hydrogeological features, for another these may be atmospheric circulation conditions, etc. Forecast of changes in geocryological conditions within the locality where construction is taking place must be made taking into consideration variations in the enumerated factors: common and particular factors are taken into consideration in general forecasts, local ones in engineering forecasts.

The variability in heat sources and flows within the ground causes complex effects on the ground temperature regime. Thus, for example, in parallel with formation of thawing zones under heat-releasing buildings and structures, lowering of mean annual ground temperature and freezing can occur within an area where taliks existed before construction.

The problems of technological geocryological forecasting in the course of linear construction include assessment of changes in ground temperature regime within the route, beyond the area under the direct effect of the linear structure (general forecast) and within the area under heat and mechanical effects of the structure (engineering forecast). In the latter case the necessary parameters of embankments, excavations and artificial structures (such as heights and material for the embankment, the necessity for thermal insulation layers, the depths of excavations, etc.) for rail roads and highways are established for the purpose of selecting the appropriate principle of using ground as the roadbed base. For pipelines the thickness and dynamics of thawing or freezing annuli are assessed, providing the means for selecting the appropriate manner of laying, the technological regime of operation and particular construction features.

The main problems of forecasting in the course of hydrotechnical construction are to facilitate the selection of the design of a dam body (in the frozen or in the thawed state); the design of cooling systems required for keeping the core of a dam frozen; and the estimation of the dynamics of the many-year ground thawing under water reservoirs. Assessing the intensity of abrasion of banks and of changes in natural conditions (including geo-cryological) within the territory adjacent to the reservoir is of great importance when the hydrotechnical construction is carried out, because very large water masses cause fundamental changes in microclimatic conditions and their effect can extend a considerable distance from the water storage body.

When subsurface construction is carried out the most important problem for forecasting is assessment of the ground temperature regime to select the safest and most economical way of mining, sinking of shafts, tunnelling or pile-driving.

Technological geocryological forecasting for the purposes of agrobiological development includes the problem of providing the optimal soil climate, and the assessment of ground temperature regime and of depths of seasonal freezing and thawing, as well as the assessment of the duration of positive and negative temperatures at the soil surface and at various depths.

Forecasts of changes in the permafrost properties as well as in other geocryological characteristics are based on the knowledge of their nature as discussed in Chapter 8.

Formulation of the problems in the forecast of changes of properties depends first of all on the forecast of changes of ground temperature regime. Two cases are considered here: 1) when the mean annual ground temperature changes (increases or decreases) remaining negative, when changes of the proportions of unfrozen water and ice in the soil occur; 2) when the sign of the mean annual temperature changes and changes in state of soil aggregation, composition and structure occur. Forecasts of changes in soil properties can be made in the first case from the results of field and laboratory tests conducted in the negative temperature range at various densities and moisture contents, and with various soil structures, taking into consideration possible changes in conditions in the course of construction and operation of buildings. In the second case when change of the sign of the mean annual temperature at the soil surface causes perennial thawing there is a need to assess the change of thermal-physical properties, to determine thaw settlement, shearing strength and other properties of thawing soils. If new permafrost formation takes place the problem arises of determining changes in the thermal-physical and mechanical properties of thawing soils depending on the new steady-state regime. This concerns possible changes in density, the total moisture of the frozen soil being assessed based on its composition and initial density (that is, existing at the moment of freezing) and moisture content.

Forecasts of cryogenic geological processes are based on the establishment of relationships between the nature of each process and the factors responsible for the process. Methods for forecasting quantitative cryogenic processes as a whole are developed in less detail than those for forecasts of temperature regime and soil properties; this is associated with the complexity of the physical models of the processes - with the necessity to take into consideration when the forecast is made, the great number of parameters responsible, and with the complexity of mathematical simulation of the cryogenic processes.

The greatest amount of work is devoted to methods for assessment of the soil frost susceptibility index, the quantitative characteristic of the soil's heaving value, and of nonuniformity of the heaving processes over the area. These are obtained using calculation methods put forward by N.A. Puzakov, I.A. Zolotarev, V.O. Orlov, G.M. Fel'dman, E.D. Yershov and others. The problems of quantitative forecasting of natural and technological thermokarst are considered in papers by G.M. Fel'dman, Yu.L. Shur, V.V. Lovchuk and M.S. Krass among others, in various detail. The forecasting of the possibility of solifluction processes developing can be made using the approximate formulae put forward by L.A. Zhigarev and V.S. Savel'yev. Methods for the forecasting of thermal erosion are considered in papers by E.D. Yershov, D.V. Malinovskiy and V.K. Dan'ko.

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