Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of ConduitsFigure 71.—Actual embankment dam settlement can differ from predicted settlement.Excessive lateral movement of the embankment/foundation system can also occurwhen thin weak layers in the foundation are loaded beyond their shear strength.These movements may result in slope instability problems but can also damage theconduit if it is located over the offending layer. Slope flattening and berms arecommonly used design measures to prevent such movements, and this will result in alonger conduit than otherwise would be needed. Foundations under conduits shouldhave relatively uniform compressibility characteristics to prevent differentialsettlement and movement of conduit joints. The Hernandez Dam case history inappendix B illustrates problems that can occur when conduits are located partly overcompressible fill and partly over nonyielding bedrock.Special precautions should be taken for joints where the conduit connects to astructure, such as an intake structure (figure 72). This location may be in an areasusceptible to differential settlement due to the differing weights of the twostructures and the foundation beneath them. An engineered fill to limit settlementmay be needed under the intake structure, when the structure and conduit cannot belocated on bedrock or a firm foundation. If the intake structure is constructed on apile foundation, special precautions are also required for the first few joints of theconduit because high stresses can develop as a result of bending stresses caused bydifferential settlement. Extending the conduit and locating the intake structurebeyond the limits affected by the embankment dam can reduce these stresses.Special precautions are also required, if the conduit operates under pressure or highvelocities, has discharges that create a surging effect, or if the conduit is constructedon a compressible foundation. This may include the use of welded steel pipe toserve as a liner within the conduit. The steel pipe liner provides ductility and awatertight seal.101