Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of Conduitsto be a combination of the weight of the fill directly above the conduit and thefrictional forces, acting either upward or downward, from the adjacent fill. Thisloading is also known as the “projection” condition. When the adjacent fill settlesmore than the overlying fill, downward frictional forces are induced, which canincrease the resultant load on the conduit by as much as 50 percent of the weight ofthe fill above the conduit (figure 61). Conversely, a greater settlement immediatelyabove the conduit results in an arching condition, which reduces the load on theconduit by as much as 50 percent of the weight of the fill above the conduit(figure 62). This loading is also known as the “trench” condition. This conditionmay occur for conduits placed in a trench. Some publications indicate that higherincreases in load may be applicable (by as much as 200% over the prism load basedon the Marston theory); see NRCS’s The Structural Design of Underground Conduits(1958, p. 1-7). The practice of constructing conduits within trenches with verticalside walls in embankment dams is not recommended. Loss of positive contact ofthe fill next to the conduit is possible due to the effects of arching. For guidance onthe selection of proper excavation for side slopes, see section 5.1.The designer should use caution in designing conduits through embankment damswhen the overburden is greater than about 100 feet. This is especially true forconduits that are not founded upon firm rock foundations and not constructed ofreinforced cast-in-place concrete. For large embankment dams, the fill height forwhich a conduit can be economically designed is limited. Conduits designed forembankment dams with fill heights greater than 100 feet should only be attemptedby very experienced designers. Greater fill heights result in extremely high stresses,excessive conduit wall thicknesses, and/or reinforcement requirements. For high fillapplications, designers may want to consider a tunnel rather than a conduit.The Marston theory is typically considered as a very conservative approach toquantifying loads upon a conduit for a fully projecting condition (Reclamation,2001a, p. 8). More detailed tools are available that allow for two- andthree-dimensional and time-dependent analysis. This type of analysis involves theuse of soil interaction models. Soil interaction models can accommodate largedisplacements, strains, and nonlinear material behavior. Programs, such as FastLagrangian Analysis of Continua (FLAC) and Plaxis are ideally suited for modelingthe stages of construction for the conduit, namely excavation, construction of theconduit, and then construction of the embankment over the conduit. Modeling andanalyzing the stages of construction enables the program to accurately calculatestresses within the conduit after the embankment has been placed. These stressesare then used in the design of the conduit. Sensitivity studies should always be runto account for possible variations in material properties, foundation settlements, andconstruction conditions. The designer may find it prudent to compare conduitloadings developed using soil interaction models with the results obtained usingclassic loadings from references, such as Design of Small Dams (Reclamation, 1987a)and Culverts, Conduits, and Pipes (USACE, 1998a).87