Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsThe reinforced concrete design of some types and sizes of precast concrete pipes hasbeen standardized by manufacturers. ASTM C 361 contains tables of reinforcedconcrete design proportions for various sizes and classes of pipe. Sizes range from12 to 108 inches in inside diameter. Classes range from A-25 to D-125 where A, B,C, D represent fill heights over the pipe of 5, 10, 15, 20 feet respectively, and 25, 50,75, 100, 125 represent internal water pressure in pounds per square inch. Thedesigner should use these standardized designs cautiously since the standard assumesa simple soil prism load instead of a positive projecting condition typical of mostembankment dams. Unique designs can be accommodated in the StandardSpecification for higher external loads.An alternative to a theoretical reinforced concrete design procedure is the indirectdesign procedure based on product testing. Most concrete pipe plants have theequipment to do a three-edge bearing load test on full size pipe specimens. TheNRCS requirements for precast concrete pipe tested in accordance with ASTMC 497 shall demonstrate the following bearing loads:• For RCP or RCCP manufactured according to ASTM C 361, AWWA C300(2004a), or AWWA C302 (2004b), the load required to produce a 0.01-inchcrack, 1 foot in length• For PCCP manufactured according to AWWA C301 (1999b), the load requiredto produce a 0.001-inch crack, 1 foot in length, or the load 10 percent greaterthan the specified three-edge bearing strength, whichever occurs firstThe NRCS has commonly used PCCP in most of their high and significant hazardand larger low hazard embankment dams over the past 50 years. NRCS worked withthe American Concrete Pressure Pipe Association to develop design curves as a basisfor proof of strength of AWWA C301 (1999b) PCCP. The curves, based on testdata, show conservative relationships between the resultant concrete core stress andthe three-edge bearing strength for various pipe sizes. Resultant concrete core stresscan be calculated from the concrete thickness, cylinder thickness, prestress wireamount, and wire wrapping stress. NRCS’s Certification of Prestressed Concrete CylinderPipe, (1982) contains these design curves and procedure.The NRCS uses two construction specifications for concrete pipe, ConstructionSpecification 41 (2001a) and Construction Specification 42 (2001b). ConstructionSpecification 41 describes the materials and acceptable construction procedures forreinforced concrete pressure pipe conduits. This specification is commonly used forcontracts involving principal spillway conduits on embankment projects designed byNRCS. Construction Specification 41 refers to Material Specification 541 (2001c)which describes the minimum material requirements for reinforced concrete pressurepipe.94