Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 5—Foundation and Embankment Damconditions favorable to hydraulic fracture in the overlying embankment.Two examples are:a. Soft foundation alluvial horizons.—Soils compacted into the excavationwill be significantly less compressible than the soft soils in thefoundation. This will result in a sharp difference in settlement in theexcavation backfill than in the soft foundation soils adjacent to it. Avariation of this situation is one where the excavation is made in lowdensity, collapsible foundation soils that are sometimes encounteredin western United States. These soil types would probably havemuch higher strain potential than the excavation backfill, creatingconditions favorable to hydraulic fracture in the overlyingembankment.b. Extremely dense foundation materials.—If an excavation is made inweathered shale, glacial till, overconsolidated clays, or other materialswith very low compressibility, the soil used to fill the excavation maybe significantly more compressible than the adjacent foundationmaterials. The result can be differential settlement that can createconditions favorable to hydraulic fracture in the overlyingembankment.2. If soils used to construct the embankment are extremely susceptible tointernal erosion, excavations transverse to the embankment dam thatcreate conditions favorable to hydraulic fracture should have specialattention. Flattening the side slopes of excavations is stronglyrecommended. Commonly, for problematic conditions, slopes transverseto the centerline of the embankment dam are made to be 4H:1V or flatter.Examples of soils that are highly susceptible to internal erosion are lowplasticity, clayey silts, and dispersive clays.• Closure sections in embankment dams may also contribute to differentialsettlement. Closure sections should be avoided, if possible. The Wister Dam case history (see appendix B) illustrates the dangers inherent withclosure sections in embankment dams. Many conduit rehabilitation projects involvemaking a transverse excavation to the embankment dam and removing the conduitto replace it. The arching effect of soils in the closure section can result in hydraulicfracturing. Conduits should not be installed in closure sections unless no otheralternatives are available. The USACE (2004a, p. 9-3) discusses factors related toclosure sections as follows:Because closure sections of earth dams are usually short in length and are rapidlybrought to grade, two problems are inherent in their construction. First, the119