Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 4—Structural Design of ConduitsMonolithic section(no construction joints)Figure 65.—Concrete placement for a reinforced castin-placeconduit.conduits (less than 3 feet in diameter) is by continuous placement, to ensuremonolithic integrity (figure 65). In larger conduits, horizontal construction jointstypically located at springline have been used to facilitate concrete placement.Horizontal construction joints are also useful in preventing flotation of steel linersduring concrete placement. For guidance on the use of control and constructionjoints and waterstops, see section 4.3. For guidance on construction practices for theplacement of concrete, see Reclamation’s Design of Small Dams (1987a, p. 659), andthe USACE’s Standard Practice for Concrete for Civil Works Structures (2001b).4.2.1.2 Precast concretePrecast concrete pipes (RCP, RCCP, PCCP) are designed as rigid structural elementsin the same fashion as reinforced cast-in-place concrete conduits. Internal andexternal loads are computed, and various load combinations are considered as actingon a unit length of pipe. Thrusts and moments at various points around theperimeter of the pipe are calculated. Required reinforced concrete proportions,including concrete thickness, reinforcing steel amount, steel cylinder thickness, andprestress tension, are determined as required for component concrete, reinforcingsteel, and prestressing wire strengths, respectively.Reinforced concrete design procedures and extensive examples specifically for RCPand RCCP are contained in AWWA M9, (1995). Prestressed concrete designprocedures for PCCP are standardized in AWWA C304, (1999a). The designershould use these procedures carefully since they are basically targeted towardpipelines where internal pressures are high, but external loads are low relative tomost embankment dams. Also, for prestressed pipes, the procedures assume thatpipelines are usually full of water over their service life. Some embankment dams,particularly common NRCS flood control dams, are seldom full of water, and lesserrelative humidity may allow concrete shrinkage and loss of prestress. The reader isdirected to the Introduction for examples of how design standards have been misused.93