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

D.C.EschAlaska Departmentof Transportation and Public FacilitiesFairbanks, Alaska, USAThis review presents a summary of currentpractice, research efforts, and major researchneeds relating to road and railway embankments overpermafrost, with emphasis on North Americanwork. The stages of preliminary investigations,design, construction, and operation and maintenanceof road and rail embankraents are discussed insequence. Embankments for roads and railroadsnecessarily encounter a wide variety of terraintypes and permafrost conditions, and routings canseldom be selected to avoid all potential permafrostproblem areas, due to origin and destinationconstraints. Because of the great distances coveredby these facilities, subsurface informationon any single permafrost unit is normally inadequateto predict accurately embankment performance.Embankments are therefore usually constructedwithout use of any intensive permafrostrelateddesign analysis. Most engineering analysesare made only in the course of repair or reconstructionof the most severe post-constructionproblem areas. Road and railway embankment movementproblem resulting from underlying permafrostare almost always of the chronic, long-term type,so the true economic consequences of inadequatedesign and construction practices are seldom ifever known.CURRENT PRACTICEPreliminary InvestigationsAirphoto and geologic interpretations of subsurfaceconditions from surface features, withborings at intervals, are heavily relied upon inthe route selection and subsurface investigationplanning stages for roads and railways, but geophysicaldetection methods based on electromagneticand radar soundings are currently under development.Theory and practice in geophysical detectionof subsurface conditions are wel developedin the mining and petroleum industries, but thesetechnologies have not been readily adaptable toapplication by transportation engineers and geologists.There is considerable interest and hope,however, that these wthods will provide the geologistwith a new low-cost source of Subsurface informationon which to base his field boring andsampling program and even the preliminary routeselection.At the Fourth International Conference onPermafrost, 11 papers in the areas of remote sensingand geophysical sounding for the detection,mapping, and evaluation of permafrost terrain werepresented. In view of the extreme variability ofsubsurface permafrost conditions, such as ice contentsand active layer thicknesses, research isbadly needed to speed the development of new wthodsof remotely detecting subsurface conditionsthat affect embankment performance. Informationon the variability of soils and permafrost isneeded on a continuous basis along a proposed roador rail route to predict the thermal and physicalstability of alternative embankmnt designs.Design ofEmbankmentsCurrent design practice is not developedenough to predict distortions, ride roughness,travel speed, or maintenance costs related to alternativeembankment designs. Engineers have generallyadopted a negative view of such design featuresas insulation layers or passive heat-exchangesystems, although the much higher initialcosts will result in improved long-term performance.Unfortunately, researchers have not yet definedthe overall life-cycle economics of suchfeatures. Insulation is not commonly used or evenconsidered except under extreme arctic conditions,where shortages of gravel result in a direct savingsin construction costs, such as the Alaska oilpipeline workpad (Wellman et al., 1976) of whichabout 112 km (70 mi) were designed and constructedas insulated embankments.Simple and functional two- and three-dimensionalthermal. analysis methods, which would permitdesigners to analyze the long-term stabilityof alternative embankment designs, are not yetreadily available, although there is considerableresearch activity in the area of thermal modeling.The "modified Berggren" calculation method is mostcommonly used for simple, one-dimensional, singleyearthermal predictions.Heat exchange of the embankment surface ishandled by use of the simplified "n-factor" approachin design calculations. This factor, whichis the ratio of surface to air temperature freezingor thawing indices, appears to be quite usefulfor analysis of paved surfaces where evaporativeor latent heat exchange is a minor factor. N-factors€or many different surfaces have been determined(Lunardini, 1978). Surface albedo, solarexposure, and wind effects are primary conslderationsin selection of an appropriate n-factor fora given site. The importance of traffic-generatedwind effects and surface abrasion on the n-factorhave been demonstrated (Berg and Esch, 1983).Embankment slopes present a much more difficultsurface temperature simulation problem thanlevel paved surfaces. Ideally the effects of sunand slope angles, wind, snow cover, vegetationcover, evapotranspiration, and shading by adjacenttrees and brush should all be taken into accountin selection of a proper n-factor. A full energybalance approach should perhaps be attempted.25