Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of ConduitsConstruction Specification 42 is for other types of conduits, including nonreinforcedconduits, such as culverts and drainage pipe. Specification 42 refers to severaldifferent material specifications, depending on the specific application beingconstructed. Material Specification 541 is referenced for reinforced concretepressure pipe, Material Specification 542 (2001d) is for concrete culvert pipe, andMaterial Specification 543 (2001e) is for nonreinforced concrete pipe. A few of theearly embankment dams constructed by NRCS used nonreinforced conduits, but themajority of the embankment dams constructed by NRCS have used reinforcedconduits.The NRCS process for designing PCCP in embankment dams uses NRCS TR5, TheStructural Design of Underground Conduits, (1958) to determine required three-edgebearing strength for a conduit considering embankment, foundation, and cradle orbedding conditions. Such three-edge bearing strength is specified on theconstruction plans. Pipe supplied to the construction site is either tested asdescribed previously, or documentation is submitted indicating pipe componentparameters, which can be checked against the NRCS Specification Note No. 5design curves to ensure adequate three-edge bearing strength.As with reinforced cast-in-place concrete, individual precast pipe sections may alsoneed to be longitudinally designed for tensile stresses due to spreading of theembankment dam along the conduit. For PCCP, the strength of the steel cylinderresists these tensile stresses. Sample calculations can be found in NRCS’s Use ofAWWA C302 Pipe for Principal Spillway Conduit (1970).Figures 66 through 69 show the different arrangements of reinforcing steel and/orprestressing wire used in the various types of precast concrete pipe.4.2.2 PlasticCurrently, the primary source of design information for plastic pipe is frommanufacturers. However, most of this information is targeted to sewer and waterdistribution pipes and does not address the unique factors involved in using plasticpipe within embankment dams. FEMA is sponsoring the development of asupplemental “best practices” guidance document pertaining solely to plastic pipeused in embankment dams. This document will contain detailed procedures andguidelines for design, inspection, maintenance, and repair of plastic pipe. Theguidance document will be based on experience provided from experts in the fieldsof civil and geotechnical engineering and construction. The expected publicationdate is 2006. This document will be made widely available for use by the dam safetycommunity. Interim guidance can be found in NRCS’s Structural Design of FlexibleConduits (2005). This reference provides design guidance for flexible pipe materials,including metal and plastic.95