Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 5—Foundation and Embankment Dam• Frost-susceptible soils.—Silts and silty soils are the most frost susceptible, becausethe voids can be of capillary size, and the permeability of the soil is sufficientfor migration of pore water. Fine grained clays are not conducive to formationof large ice lenses, because they are too impermeable to allow substantialmigration of the soil water. Sands and gravels with less than 3 percent finescontent (material finer than 0.075 mm, No. 200 sieve) are not generally frostsusceptible (Reclamation, 1998a, p. 54).Thousands of embankment dams constructed of frost-susceptible materialshave experienced no apparent problems. For both case histories in appendix Bwhere ice lenses are suspected as contributing factors in dam failure, theconduits were relatively large metal pipes. Such large pipes may allow sufficientcold air flow through them to cause the adjacent embankment soils to freeze,while the heat in flowing water in smaller conduits may minimize the problem.Also, the metal pipes are relatively thin, so cold temperatures are transmitted tothe surrounding soil relatively quickly. Cold temperatures would take longer toaffect the soils surrounding conduits with thicker concrete walls.• Design considerations.—Backfilling adjacent to conduits with clayey materialswould minimize the potential for formation of ice lenses in the embankmentdam. Also, construction of a properly designed filter diaphragm or collar nearthe downstream end of the conduit would control seepage along the outside ofthe conduit and minimize the potential for failure by internal erosion.129