Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 1—Generalfacilitate future inspection and renovation (i.e., sliplining). This will alsoprovide improved compaction near the conduit and eliminate any stressconcentrations resulting from the bend.Some techniques that have been found to be applicable for designing conduits oncompressible foundations include:• Excavate and replace compressible foundation soils.—To reduce differential settlementor to reduce total settlement, excavate compressible foundation soils andreplace with less compressible compacted soil.• Properly locate controls.—Position the control gates and valves upstream of impervious zone in the embankment dam.• Avoid pressurizing the conduit.—Avoiding pressurized conduits through imperviousembankment dams, unless the pressure conduit is placed within a largerconduit. To prevent pressurizing of the conduit, a free standing welded steelpipe supported by cradles can be placed within a larger reinforced concreteconduit. Access is provided along the side of the steel pipe. The steel pipe isconsidered to be ductile and will deform and still maintain a watertight conduit.When possible, field weld the steel pipe joints after the initial foundationsettlement of the conduit has occurred.• Bridge over weak areas.—Longitudinal reinforcement extending across the jointsof the reinforced concrete encasement surrounding the welded steel pipe linercan provide a rigid beam effect and bridge over weak foundation areas tominimize locally concentrated deflections.• Utilize longitudinal reinforcement across joints.—Large horizontal movements oftenoccur at randomly selected conduit joints, rather than uniformly along theconduit length. This type of concentrated movement can open gasketedconduit joints that are not designed for large horizontal movements. The useof longitudinal reinforcement across the joints and continuous welded steel pipeliners are effective in reducing concentrated openings within conduits.• Provide camber.—A conduit that is not located on bedrock must be designed sothat the amount of predicted foundation settlement does not damage theconduit or its function. A conduit constructed on a compressible foundationshould be cambered to accommodate the predicted foundation settlement, toachieve a proper final grade.In lieu of constructing tunnels or conduit penetrations through embankment dams,siphons can often provide alternative reservoir drawdown capability for low hazarddams. Siphons are particularly useful for recreation reservoirs that do not make39