Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsslopes of embankments are examples. Several chemicals have been used to modifydispersive clays. The most common additive is hydrated lime. The NRCS referenceon dispersive clays (NRCS, 1991) discusses chemical amendment in more detail.However, the designer is cautioned that treatment with lime will increase thematerial’s brittleness.Chimney filters that coincide with a filter diaphragm or collar around conduits arethe preferred method for preventing failures associated with dispersive clays.Embankment dams constructed of dispersive clays should use a substantial filterdiaphragm around the conduit, if a chimney filter is not included in the design of thedam.5.4 Frost susceptibility and ice lensesSome soils, especially silts, are frost susceptible. When conduits are in contact withfrost-susceptible soils, large ice lenses may form in the soil if the conduit is exposedto freezing temperatures. When these ice lenses melt, voids are left in the earthfillthat are subject to internal erosion, if they are connected to the reservoir andcontinuous in a direction transverse to the embankment dam. This is a suspectedfailure mode in the Anita Dam and Loveton Farms Dam failures (see appendix B)and Kelso Dam in southern Ontario, Canada (Milligan, 2003, pp. 786-787).Guidance concerning frost susceptibility and ice lenses includes:• Process of ice lens formation.—When wet soil freezes, most of the water in the soilpores becomes ice, and it expands about 10 percent (Sowers, 1979). However,in frost-susceptible soils, it has been found that not all of the water freezes.Capillary soil water may remain in liquid form at temperatures of 28 °F, andsome liquid water may even still exist at temperatures as low as -4 °F (Penner,1962, p. 1). When the water in the larger pores freezes, the moisture content inthe soil is reduced, and unfrozen capillary water in the surrounding soil tends tomigrate toward the frozen zone because of surface tension. The frozen waterbecomes an ice lens, which will draw water from the surrounding soil as long asunfrozen water is available and the temperature at the lens remains cold. Theresult is that the ice lens grows larger, and the total expansion of the soil ismuch larger than that which would occur with the expansion of the originalamount of water present in the soil.The growth of these ice lenses creates very large pressures in the soil, creatingfrost heave within the embankment dam. These forces can damage rigidconduits and can cause excessive deformation of flexible conduits resulting inpathways for internal erosion. When the ice lens defrosts, it can create a voidor low density zone that can initiate the internal erosion process.128