2 Volumes Final Proceedings - Washington 1984.pdf - IARC Research

33spillways and other outlet structures usually arenot located within or adjacent to ice-rich permafrostwithout special refrigeration or other measuresto preserve the earth in a frozen state.Where possible, the outlet facilities are locatedat a remote site and preferably in competent bedrock.For small dams the use of siphons and pumpshave been used to control reservoir levels (Gluskinand Ziskovich, 1973). A chute spillway elevatedabove the embankmnt surface (Bogoslovskiiet al., 1966) is one possible solution, but themost common practice is to locate the spillway inone of the abutments on bedrock and to use refrigerationor grout to control the seepage. The importanceof a positive seepage cutoff must be emphasizedbecause of the possible formation and enlargementof a talik that can grow laterally fromthe outlet works into and beneath the adjacent embankment.As a result, the entire facility can beendangered if it is a frozen-type design.Thermal AnalysisEssential to the design of water retainingembankments on permafrost is the determination ofthe thermal regime throughout the life of thestructure (Bogoslovskii, 1958; Teytovich et al..1972). A number of methods have been developedand others are being developed €or estimating thetherm1 regimes of these embankments and theirfoundations. Some of the methods currently in useinclude :a. One-dimensional analyses that are appliedat critical locations within thecross-sections of the embankment andfoundation (Tsytovich et al., 1972). Inareas where water does not change phase,simple heat-conduction equations areused in the analyses. The Stefan-Boltzmannequation or the Neumann solution tothe Fourier equation Is used to establishthe position of the freezing andthawing front. This one-dimensionalmethod can be used to analyze the transientheat flow condition.b. Hydraulic analog computers to analyzetwo-dimensional simple heat flow conditionsand to establish the location ofthe freezing or thawing front (Tsytovichet al., 1972).C. Numerical techniques, such as the finitedifferences and finite elements methods,to analyze heat flow, including changeof phase for two dimensions (Bogoslovdmskii, 1958).Physical models constructed of soil inthe laboratory to validate the analyticalmethods as wel as to simulate theprototypes for heat flaw.e. The three-dimensional finite elementmethod of analysis, which accounts forthe heat transferred by the seepage water;this is still in the developmentstage. It is especially useful €or analyzingthe special conditions at damabutments, areas adjacent to water wtletfacilities, and short dams (Bogoslovskii,1970).Of these methods, the one-dimensional analysisis now used most often, although the two-dimensionalfinite element method, which accountsfor the latent heat of fusion, is also frequentlyused.Meaningful thermal analysis requires realisticinformation for defining the thermal propertiesof soil, the initial temperature distributionwithin the soil, the geometrical and thermalboundary conditions, and, where appropriate, informationabout the amount of heat carried by theseepage water. The analysis mst also account forthe change in the thermal properties of the soildue to a change in phase (i.e. frozen or unfrozenproperties) and the change in density due to consolidationof the soil upon thawing. At a givensite, the initial soil temperature distributionoften is determined by in situ temperature measurements.Where accurate meteorological data is availablefor several years, reasonable estimates ofthe initial temperature distribution of undisturbedsites have been =de using heat balanceequations that take into account, directly or indirectly,such heat sources and losses as solarradiation, air temperature, wind velocity, evapotranspiration,geothermal gradient, and flowingground and surface water. ‘Ihe upper boundariesfor thermal analyses are usually the interfaces ofthe atmosphere with the surfaces of the embankmentand the natural ground. Some analyses considerthe surface of the snow, if it exists, as the upperboundary. In such cases, the depth and thermalproperties of the snow are required for theanalysis. The lower thermal boundary often isconsidered to be isothermal, or a constant geothermalgradient is assumed to exist at this surface.In two-dimensional analyses, the lateralboundary surfaces are usually assumed to be adiabatic.Although few details are published on thethermal analyses for artificially freezing waterretainingembankments on permafrost, the necessaryinformation is available on methods of analysisdeveloped for freezing shafts, tunnels, and wallsto exclude water and soil from excavations duringconstruction in nonpermafrost areas (Khakimov,1963; Sanger and Sayles, 1978).The efficiency with which heat passes betweenthe atmosphere and earth materials is computed byusing heat-transfer coefficients or factors. Thevalues of the heat coefficients used at the upperboundaries depend upon the color and type of surfaceexposed to the atmosphere and sun; that is,whether the surface is light or dark and whetherit is covered with gravel, atone, snow, vegetation,or other types of materials.Methods for calculating heat transfer due toseepage and the convection of water are stillquite crude, but numerical methods are being formulatedthat should eventually improve this computation(Brown and Johnston, 1970). The convectionof air within a rockfill embankment can transferlarge amounts of heat under certain conditions(Mukhetdinov, 1969; Melnikov and Olovln, 1983).Methods for analyzing convection in embankmentshave been developed and are still under development.