Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 70.—Water seeping through a joint in an outlet works conduit. Thejoint has experienced differential settlement. This joint had nolongitudinal reinforcement extending across the joint. The mortar jointfilling has cracked and deteriorated.Based on observations of conduits through 20 selected NRCS dams constructedduring the 1960s on compressible foundations (Casagrande Volume, 1973, p. 235):• 70 percent of the joint opening occurred during construction of theembankment dam.• Additional measurement of joint openings several years after constructionshowed negligible increases.Conduits constructed on compressible foundations are more likely to experiencejoint spreading problems. Special attention should be given to evaluating thecompressibility and shear strength of these soil types. Performing field vane sheartests and similar evaluations are appropriate to evaluate the undrained strength ofthese types of foundations. Available references and procedures for predicting theamount of conduit spreading based on foundation consolidation and shear strengthparameters are not well known and can be inappropriately used by inexperienceddesigners. The NRCS’s Computation of Joint Extensibility Requirements (1969) uses thepredicted vertical strain beneath the conduit, the shear strength of foundation soils,and the geometry of the embankment and foundation in an attempt to predict thehorizontal strain at the conduit. Additional information on conduit extension can befound in the Casagrande Volume (1973, pp. 209-237). For an example of a projectwhere spreading of conduit joints occurred, see the case history in appendix B forLittle Chippewa Creek Dam.100