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

DESIGN AND PERFORMANCE OF WATER-RETAINING EMBANKMENTS IN PERMAFROSTF .H.SaylesCold Regions Research and Engineering LaboratoryHanover, New Hampshire, USA(Formerly with Office of Federal Inspector, Irvine, California,USA)To date, the water-retaining structures constructedand maintained on permafrost in NorthAmerica have been designed and built using a combinationof soil mechanics principles for unfrozensoils and unproven permafrost theory. In theUSSR. at least five sizeable hydroelectric and watersupply embankmnt dams as wel as severalamall water supply embankment dams have been constructedand maintained on permafrost, The largerdams are understood to have performed well, butthe smaller darns have been a mix of successes andfailures. (See Table 1 €or examples of problemsrecorded in the literature.) Specific criteriaare still lacking for design, operation, and postconstructionmonitoring of water-retaining embankmentsfounded on permafrost. The purpose of thispresentation is to review the current practice,point out how it is deficient, and note what majorproblems need attention.General ConsiderationsCURRENT PRACTICEIn current practice, the designs of water-rerainingembankments on permafrost can be dividedinto two general types, frozen and thawed. Thefrozen type of embankments and their foundationsare maintained frozen during the life of thestructure. The thawed type of embanklnents usuallyare designed assuming that the permafrost foundationwill thaw during either the construction orthe operation of the structure. In some locationswhere water is to be retained intermittently forshort periods of time, thawed embankments havebeen designed assuming the permafrost is to remainfrozen throughout the life of the embankment. Inselecting the type of design for a particularsite, many factors that are peculiar to cold regionsmet be considered, including:The anticipated type of service of the embankment;i.e. retain water continuouslyor only intermittently.The width, depth, temperature, and chemicalcomposition of the body of water tobe retained by the embankment.Regional and local climate conditions, especiallytemperature.The temperature of the existing permafrost.- The extent in area and depth of the permafros t.The availability of the type of earth materialsrequired for construction.The accessibility of the construction sitefor logistics involving man-made constructionmaterials.- The consequences to life and property inthe event of embankment failure.The effects of the construction and operationof the embankment on the environment.The orientation of the downstream face(i.e. the dry face) of the embankmentwith respect to solar radiation.* Frost action on the dry slopes and crestof the embankment.The economics of constructing a selecteddesign in the cold region.In addition to these rather general factors,each type of design has special requirements thatrmst be taken into account in making the final selectionof a particular design.Unfrozen Embankment on ThawingPermafrostThe design for an unfrozen embankment foundedon thawing permafrost is most suitable for siteswhere the foundation mterials are thaw-stable;Le. where the thawing strengths of the earth materialsprovide an adequate factor of safetyagainst shear failure, and deformations resultingfrom thawing will not endanger the integrity ofthe embankment. This requirement usually restrictsthe use of the thawing foundation designto sites where permafrost soil is ice-poor orwhere reasonably sound bedrock can serve as thefoundation. At sites where only a portion of thefoundation contains ice-rich permafrost at shallowdepths, this ice-rich portion is usually thawedbefore placing the embankment, or the frozen soilis excavated to a predetermined depth (Gluskin andZiskovich, 1973). MacPherson et al. (1970) suggesta method of estimating the depth of excavationso that thaw consolidation can be limited toa predetermined amount during the operation of theembankment .Where the permafrost is not removed and thefoundation is expected to thaw during the life ofthe structure, the embankment design is similar inmany respects to that of a water retaining embankmentlocated in a temperate climate. However,special consideration is given to certain elementsof the embankment. One such element is the imperviouszone, which rmst be constructed of selfhealingsoils (Gupta et al., 1973) so that thiszone can remin "impervious" even if cracking occursduring the settlement of the foundation.Soils that becow stiff and brittle when compactedin the impervious zone are avoided. Other designprovisions that are often included to accommodatethe anticipated settlement are: the use of flatterembankmnt slopes; overbuilding the height of theembankment by an amount equal to the anticipated31