Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsand the use of the conduit. As noted previously, precast concrete pipe should not beused in pressurized applications within significant and high hazard embankmentdams, because failure of a single pipe joint or joint gasket could allow pressurizedwater to come in direct contact with the embankment. Conduits should beconstructed on rock or firm foundations whenever possible. When a conduit isfounded on a compressible foundation, the designer must exercise care in design ofthe conduit because of the large settlements that can occur. These settlements canopen joints and cause pipe joints to fail.Different approaches have been used to design conduits on compressiblefoundations. NRCS uses a joint design for the cradle that allows the articulation andspreading of the conduit and its support system. To allow for joint articulation, thejoint is placed at the location of the pipe joint and cradle reinforcement is notallowed to pass through the joint. In addition, the spaces between joints are filledwith a compressible material, such as high-density sponge rubber or bituminousfiberboard to allow for articulation of the cradle joints. The NRCS design guidelinesfor the configuration of pipe cradles are provided in The Structural Design ofUnderground Conduits (1958). Reclamation does not allow precast concrete pipe to beused for conduits within embankment dams and therefore would not use cradles.The concrete cradle should bond to the conduit. The sides of the concrete cradleshould always be sloped at 1H:10V or flatter through the im pervious zone to allowequipment to compact earthfill directly against the cradle. There should be no sharpor protruding corners associated with the cradle that could cause undesirable stressconcentrations in the fill. Blocks and wedges are required to support the conduit ongrade until the concrete cradle has been placed and cured.Designers of conduits through low hazard embankment dams often use concretebedding beneath fully circular conduits. Bedding generally comes up to about25 percent of the conduit height to provide support and facilitate compaction underthe haunches. Bedding often has joints located at the circular pipe joints so as to notinterfere with pipe movement. Figure 60 shows a precast circular conduit usingbedding for support. For guidance on the use of bedding in conjunction with fullycircular conduits, see NRCS’s The Structural Design of Underground Conduits (1958) andUSACE’s Culverts, Conduits, and Pipes (1998a). The selection of whether to use cradlesor bedding is typically a function of the height of the embankment dam. Cradles areoften used for higher dams, where more lateral support is required. Regardless ofwhether a cradle or bedding is used, the use of a filter diaphragm or collar is avaluable defensive design measure that should be employed, even for low hazardclassification sites with favorable conditions.The designers of low hazard embankment dams have sometimes considered the useof flexible conduits (i.e., HDPE). Cradles and bedding should not be used with84