Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of ConduitsFigure 59.—A box shape is not commonly used for conduits. The designerneeds to carefully consider the advantages and disadvantages of thisshape.shape is likely to result in poor watertightness. Reinforced cast-in-place boxconduits are designed with reinforcement extending across the joint andwaterstops to improve watertightness.• The economy in the box shape is typically due in part to the straight sides,making forming less expensive, and the flat bottom, making foundationexcavation easier. However, best practice for compaction requires the sides ofthe conduit to be sloped to 1H:10V or more through the im pervious zone ofthe embankment dam to allow equipment to compact the earthfill directlyagainst the conduit. The preferred shape for a box conduit is shown infigure 52.4.1.6 Cradles and beddingExternally shaped circular conduits should be constructed on concrete cradles toavoid problems with compacting beneath the haunches of the conduit. Cradles aretypically used in conjunction with precast concrete pipe. The cradle should beformed concrete that provides vertical, longitudinal, and lateral structural support tothe conduit. The cradle should extend for the full length of the conduit and shouldencase the lower half of the conduit extending up to the springline. Conduit shape Fillustrates a circular conduit with a concrete cradle extending up to the springline.The design of the conduit support through an embankment dam will depend uponthe hazard class potential associated with the dam, compressibility of the foundation,83