USE OF COMPUTERS IN GEOCRYOLOGICAL ENGINEERING

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1 USE OF COMPUTERS IN GEOCRYOLOGICAL ENGINEERING L.N. Khroustalev Department of Geocryology, Faculty of Geology, Moscow State University, Vorobyovoy Gory, Moscow, Russia Abstract In this paper, we introduce four computer programs (WARM, TEM, PFL, NAST) created at Moscow State University for use in geocryological The program ÒWARMÓ simulates conductive heat transfer with phase changes to forecast the thermal and phase state of permafrost, and thawing and freezing soils when interacting with engineering structures and the environment. The software ÒTEMÓ is designed for processing bore-hole temperature measurements. The purpose of this program is to reveal the zones inside permafrost in which the difference between the actual temperature and the projected temperature is dangerous for structures. The software ÒPFLÓ represents a reference and teaching system. The program ÒNASTÓ calculates the reliability of the system Òstructure - foundationó and can reduce total costs by including the estimated cost of construction and the price of the risk (cost equivalent of the reliability). Prediction of temperatures in the foundations of engineering structures Forecasting the heat and mechanical interaction of structures with foundations and frozen, freezing, and thawing ground is a major goal of geocryological Thermal and mechanical processes during freezing and thawing of ground result from the complex interaction between various elements of the geological environment and structures. Mathematical models for this interaction are considered to be the most complicated of mathematical physics. Their numerical realization is a significant problem that can be solved partially by using computers. - simulation of mobile phase-change interfaces; - simulation of calculated domains of any configuration; - simulation of internal heat sources and sinks, for example, electric cables, thermosiphons, etc. One principal advantage of the program is that the phase-change temperature and latent heat may be specified in the space. This allows the simulation of saline ground. The first problem is the conductive heat transfer with phase changes in heterogeneous ground, the Stefan Problem. To solve this problem we developed the program ÒWARMÓ (Emelianov et al., 1994). This program predicts the thermal regime of permafrost in natural systems (i.e., rivers, lakes, thermokarst bodies, etc.), and in engineered systems (i.e., buildings, roads, pipelines, reservoirs, tunnels, boreholes, shafts, frozen dams, etc.). The program operation requires little skill and is accessible to any geologist or engineer. The program ÒWARMÓ simulates nonstationary conductive heat transfer in two-dimensional and three-dimensional axially symmetrical domains under any boundary conditions that are constant or variable with time. This program allows: Figure 1. Ground temperature around a shaft after 20 years of its operation. L.N. Khroustalev 563

2 The program operates in a friendly dialogue mode. During program operation a scheme of the calculated domain and of the temperature field is graphically displayed. During calculations a colored temperature field that changes with time is continuously displayed. It may be printed as black and white at any point of time (Figure 1). The program operates on IBM PC computers or compatibles. The program ÒWARMÓ is widely used for teaching purposes at Moscow State University. Stability control of engineering structures on permafrost Another vitally important problem of geocryological engineering is the stability control of engineering structures constructed on frozen, freezing, and thawing grounds. It is solved using the software ÒTEMÓ (Khroustalev et al., 1995). It was developed by the present author for processing temperature measurements in boreholes. The purpose is to reveal the zones in permafrost, in which the difference between the actual temperature and the projected temperature is dangerous for an engineering structure stability. Using the measurements in boreholes, the computer calculates the actual temperature field in the foundation of the structure. Then, using the actual temperature fields at various points of time the computer calculates the future temperature fields and compares them with the projected temperature field and warns about dangerous deviations. As an example, we shall consider determination of the future ground temperatures at a point A, located in the foundation of a building and underground pipeline. The results of mathematical modeling will be conventionally taken as the measurement data in boreholes. The geometrical shape of the model and disposition of the point A are shown in Figure 2. The indoor temperature is set at 20¼C, the thermal resistance of the floor at 1.7 m 2 deg/w, the temperature of the outer pipeline surface is 30¼C. The third boundary condition is the outdoor ground surface, simulating seasonal change in air temperature and snow depth. These correspond to the mean perennial values of meteorological observations in the town of Urengoi (north-western Siberia). The foundation is frozen sand with moisture content of 0.15 and temperature of Ð3¼C. The ground temperature at point A was measured once a year at the end of summer. Figure 2 shows the data of modeling of the temperature change at point A (curve 1) and the forecast results Figure 2. Design diagram and the results of temperature calculations at a point A (see text for explanation of lines and boundary conditions). Figure 3. The field of the difference in the design value of ground temperature and the value obtained by extrapolating borehole measurement data to time (see text). 564 The 7th International Permafrost Conference

3 of this change (curves 2-4). To construct curves 2 Ð 4, we extrapolated the observation data for periods of 3, 4 and 5 years of structure operation (curves 2, 3, and 4, respectively). The comparison of these curves shows that the forecast accuracy increases with the duration of the observational period and decreases with an increase in the duration of the forecasting period, remaining rather high. The software operates in a friendly dialogue with the user and gives an opportunity to see dangerous zones in the foundation of the structure on display some years before their formation in nature. Figure 3 shows zones of permafrost warming in the foundation of an inhabited building, located at the address: ul. Begicheva 2, Norilsk, Russia. These will appear in 2000 and will result in deformation of the building. These data were obtained using the program ÒTEMÓ with temperatures that were measured in boreholes throughout the period of (Emelianova, 1997) The software ÒTEMÓ operates on IBM PC computers or compatibles. It is used also for monitoring at some gas fields in West Siberia. Estimate of the reliability of engineered system foundations A new scientific school of forecast and a control of geotechnical system reliability using computers, has developed at the Department of Geocryology, Faculty of Geology, Moscow State University, in the last fifteen years. The system represents the engineering structures and the geologic environment that interacts with them. In the context of this school, heat and mechanical interaction between structures and the environment is a random process that develops in the system quality space. A set of the systemõs states that are permissible from the standpoint of quality forms the domain in quality space. If the process crosses the boundary of the domain, the quality of the system is lost, i.e., its failure occurs. The probability of quality preservation throughout the period of system operation is designated as the geotechnical system reliability. The sequence of this probability with time is designated as the reliability function, determined by environmental, climatic, and technological factors. The elements of a structure have a pronounced effect on the reliability function. We can directly act on them, thus changing the reliability function, unlike the rest of the input data. Therefore, a forecast for the working capacity of the system becomes interactive in character. If we can calculate the reliability function, we can not only state that the design is acceptable but also improve it to reach either the predetermined, or the best system reliability under given conditions. If we can change the reliability function by changing some parameters, these parameters are designated as control parameters. Using a particular combination of control parameters, we can attain any predetermined system reliability. This raises the question of what it should be. It is known that all the man-made systems have the reliability less than 1. As a consequence, a system failure inevitably occurs. Structures are deformed and damaged as a result. The higher the reliability, the less the damage. On the other hand, high reliability is expensive. Clearly the system must be designed so that the integrated costs are minimal. They include the cost of construction and ground preparation (initial cost), maintenance costs, and expenses associated with structure failure, the cost associated with the risk, entirely determined by the system reliability function. This optimizing problem can be solved using the program ÒNASTÓ that was developed by the present author (Khroustalev et al.,1993). This program allows the selection on a scientific basis of the following: - optimal arrangement of buildings and optimal routes of linear structures; - principles of using the permafrost ground as foundations; - the foundation construction and the routes of the linear structure; - type and depth of the foundation; - the depth of the preconstruction ground preparation. The program calculates the reliability of the geotechnical system and its integrated costs, as well as its initial cost and the cost associated with risk, i.e., the cost equivalent of the reliability. Three groups of input data are used in the program. They are: 1. Table of properties of the ground samples. Each sample has 11 properties. Each ground layer recognized must be characterized by not less than 6 samples. The number of the ground layers must not exceed Table of parameters. It includes determined and stochastic parameters. L.N. Khroustalev 565

4 3. Numerical values of the parameters that control the system. Any parameter of the structure can be accepted as a control parameter. This may be, for example, the depth of pile driving, thickness of the pipe wall, thickness of the coarse fill in the road base, etc. The optimal decision that corresponds to the minimum of the integrated costs is found by varying the control parameters. The data are prepared, the parameters are specified, and the calculations are made during the dialogue of the user with the program. The program operates on IBM PC computers and compatibles. The program ÒNASTÓ was used for designing two residential neighborhood units in Vorkuta Industrial Region, as well as for geological and economic regionalization of the territory around the Tiksi Atomic Power Station, and for designing villages for workers along the Berkakit-Yakutsk Railway. The potential of the program is most clearly seen when we have to decide a question: must global climatic warming be taken into account in the project or not? Let us assume that we know the probability of a warming scenario taking place. Then we can give a positive answer to the question if the rise in the initial cost of the geotechnical system associated with a consideration of warming is less than the damage associated with warming multiplied by the probability of realization of the warming scenario (Khroustalev, 1997): Coi - Coo < Vi Cdi Cdi = Cri -Croq where Coi is the initial cost of a geotechnical system, projected with a consideration of the i-th warming; Coo is the initial cost with no consideration of warming; Cdi is the damage from the i-th warming that is not considered in the project; Cri is the cost associated with risk in the case of the i-th warming scenario; Cro is the cost associated with risk with no consideration of warming; q is the optimal set of the control parameters with no consideration of warming. We shall show, how to calculate the parameters equation (1). As noted above, to find an optimal solution, we shall achieve the minimum integrated cost by varying the control parameters. This is performed for the case when warming is absent (case Mo) and for the case when it is present (case M1). The damage from the neglect of warming is determined as a difference in [1] [2] Figure 4. The plot for calculating the parameters of equation (1) (see text). costs associated with risk in cases Mo and M1, given the control parameters, corresponding to case Mo (2). The left member of equation (1) is equal to the difference in the initial costs at the optimal points M1 and Mo. This is best illustrated by a specific example. Figure 4 presents the plot of initial cost and cost associated with risk versus depth of preliminary thawing of permafrost under a building, located in Vorkuta. Two cases are considered. In the first case the gradient of the mean annual air temperature is equal to 0¼C (warming is absent). In the second case it is equal to 0.075¼C/year, corresponding to an average scenario of global warming. From Figure 4, it follows that in a case of warming the optimal point is displaced to the right along the axis of depth while the integrated cost grows by a factor of 1.4. The difference in the initial cost at points M1 and Mo is equal to 46% of the building cost while the difference in the costs associated with risk at the point Mo is equal to 60%. According to equation (1), the warming scenario considered should be taken into account, if the probability of its realization is more than With an increase in the building cost, this ÒthresholdÓ probability decreases. Clearly even the most improbable scenarios must be taken into account for expensive structures. Geocryological engineering education To understand the courses ÒFundamentals of geotechnics in the permafrost zoneó and ÒHeat and mechanical interaction between structures and frozen rocksó better, at the Faculty of Geology, Moscow State University, we created a computer reference and teaching system ÒBuildings on permafrostó (ÒPFLÓ) (Khroustalev and Emelianov, 1995). The system is created for: - the design of building foundations and methods of preparing the ground; - study of geocryological engineering and introduction to its methods; 566 The 7th International Permafrost Conference

5 - obtaining the reference information in geocryological The system is developed on the basis of standard (norms) references and text books of geocryological The system is useful for researchers, designers, and surveyors who deal with problems of permafrost development, as well as for teachers, postgraduate and undergraduate students at construction and geological institutes of higher education. During the friendly dialogue with the user, the system allows: - solution of principal problems of geocryological engineering; - display of reference and teaching material on computer display; - direct route for calculations and rapid return to the starting point in the text during reading; - view of illustrations during reading; - view of cartoons that illustrate the stages of the problem solution on display; - printing of text fragments, illustrations, and calculated data in black and white mode. The system includes 47 programs and operates on IBM PC computers and compatibles. In the near future we propose to extend the capabilities of the system by adding chapters on ÒGas and oil pipelines in the permafrost zoneó and ÒRailways and highways in the permafrost zoneó. All programs are available to interested users outside Moscow State University. - rapid search for necessary information; References Emelianova, L.V. (1997). Computer technology of data processing of temperature in the base of structures on permafrost. Ph.D. thesis, Production and Research Institute for Engineering of Construction, Moscow (139 pp.). Emelianov, N.V., Pustovoit, G.P., Khroustalev, L.N. and Yakovlev, S.V. (1994). Program ÒWARMÓ for calculation of heat interaction of engineering structures with permafrost grounds. Certificate no RosAPO. Khroustalev, L.N. (1997). The application of the theory of reliability to problems of engineering geocryology. Cryosphere of the Earth, 2, Khroustalev, L.N. and Emelianov, N.V. (1995). Software ÒBuildings on PermafrostÓ (PFL). Certificate no , RosAPO. Khroustalev, L.N., Emelianov, N.V., Pustovoit, G.P. and Emelianova, L.V. (1995). Software ÒTemperature MonitoringÓ (TEM). Certificate no , RosAPO. Khroustalev, L.N., Pustovoit, G.P. and Emelianova, L.V. (1994). Prediction of permafrost temperature in the base of structure through the data of natural observation. Soil Mechanics and Foundation Engineering, 6, Khroustalev, L.N., Pustovoit, G.P. and Yakovlev, S.V. (1993). Program for calculation of the base reliability and of the integrated reduced cost of structures on permafrost. (NAST) Algorithms and Programs. Inform. Bull. VNITIcentre, 5, L.N. Khroustalev 567