Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsLoading conditions typically analyzed include:• Usual.—This loading condition includes normal operating conditions with thereservoir at or near the normal water surface, involving combinations of verticalsoil load (due to the weight of the fill column above the conduit); horizontalsoil load; external and internal hydrostatic pressure loads; and the verticalfoundation reaction (generally assumed to be equal to the vertical soil load plusweight of the conduit).• Unusual.—This loading condition includes loads associated with a high reservoirwater surface and high discharges due to flood conditions. However, sincefloods are typically short lived, the conduit may not come under increasedexternal hydrostatic pressure. The difference in loading conditions betweenusual and unusual may be limited to increased internal hydrostatic pressure.• Extreme.—This loading condition is associated with usual loading conditionsplus earthquake loading. Depending on the “criticality” of the conduit (i.e.,consequences due to failure or severe damage of the conduit under seismicloading), a range of earthquakes should be considered, including seismic loadingto the maximum credible earthquake (MCE). Conduits are “low profile”structures and tend to have a high fundamental frequency. Unless the conduitis founded on deep layers of soil where peak ground accelerations could bemagnified, peak ground accelerations are typically assumed to act on theconduit. If the fundamental frequency of the conduit is greater than 33 hertz, apseudostatic approach generally gives reasonable results; otherwise a moredetailed seismic analysis may be required (i.e., response spectrum method ortime-history method). Additional factors that may affect loading conditions arethe type of foundation, method of bedding, flexibility of the conduit, and soilcharacteristics (internal angle of friction, unit weight, homogeneity,consolidation properties, cohesiveness, and moisture content).• Construction.—This loading condition pertains to loads resulting fromconstruction activities. These activities may include construction vehicles orequipment moving or working adjacent to or on the conduit and are consideredshort term loadings.In soils, a variety of factors should be considered when designing a conduit,including angle of internal friction, density, homogeneity and water content of thesoil. Various combinations of conditions and loadings will need to be evaluated bythe designer to ensure a long service life for the conduit.The Marston theory of embankment pressures has typically been adopted forcalculating loads on a conduit that is partially or fully projecting above the originalground surface. Using the Marston theory, vertical load on the conduit is considered86