Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 13—Replacement of Conduitsin the embankment dam, any desiccation cracks in the existing dam must beremoved, and the earthfill surface disked and moistened. This process will probablyhave to be delayed until immediately before backfill of an interval of theembankment dam is ready to commence. If backfilling of the excavation isinterrupted during hot weather, the surface of the reconstruction backfill also shouldbe closely inspected for desiccation features before placing new fill. Poorly bondedlifts can occur during interruptions of fill placement. They provide an avenue forpossible internal erosion.Designers should consider these important points:• Testing.—Soils used to rebuild the embankment dam should be evaluated by thesame tests that would be used to evaluate soils for a new embankment dam.The water content, plasticity, gradation, compaction properties, and dispersivityof clay fines are important evaluations. If the replacement fill is in a zonedembankment dam, similar zoning should be used.• Water content.—Soils used to rebuild the embankment dam should usually beplaced wet of Standard Proctor optimum water content to improve theirflexibility and resistance to cracking and arching. Compacting soils at watercontents that are 1 to 3 percent wet of optimum significantly improves theirflexibility. At the same time, the likelihood that pore pressures could begenerated in medium to high plasticity clays in fills of significant height shouldalso be evaluated. Designers must weigh the advantages of compacting soilswet of optimum against the disadvantages of this wetter compaction watercontent. The lower shear strength and potential pore pressures generated bywetter compaction water contents must be considered in the design stabilityevaluations. Many designers consider excessive pore pressures to be a lesserlong term danger to the successful performance of an embankment dam thanthe danger of arching and hydraulic fracture if the soils are placed dry.• Exposed filler.—Special care to remove desiccation cracks in exposed fill surfacesis important. This applies to the exposed excavation slopes and to layers of fillused in reconstructing the embankment dam.For additional guidance on hydraulic fracture and closure sections, see section 5.2.13.6 Construction impactsGenerally, the construction period for a complete removal and replacement of aconduit will require more time than other renovation methods. Mitigating theimpacts of a longer construction period may require consideration of the following:335