Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 115 shows an example of poor construction practice (improper consolidationof concrete). For an example of how poor design and construction practice can leadto the failure of a concrete conduit, see the Olufson Dam case history in appendix B.Poor design and construction practices particular to reinforced cast-in-place conduitswere discussed in the previous paragraphs. However, poor design and constructionpractices affect all types of conduits. The following paragraphs briefly discuss effectsof poor design and construction practices affecting other types of conduits, such asprecast concrete, or CMP. The appearance of these defects can lead to preferentialseepage paths and the development of potential failure modes for conduits. Some ofthese include:• Deformation.—Deformation occurs when load or force changes the shape of theconduit. Deformation is typically caused by the application of excessiveexternal load (e.g., improper selection of design loadings), loads from heavyconstruction equipment, or seismic activity. Figure 116 shows an example ofwhere heavy construction equipment likely caused deformation of a CMPconduit. CMP is flexible and is designed to deform some as it transfers loadinto the surrounding backfill. The surrounding backfill provides stiffness andload carrying capacity. Improperly designed backfill or inadequately compactedbackfill under the CMP haunches does not provide the needed lateral stiffnessto the CMP. This can result in excessive deformations and structural failure orcollapse of the CMP (Kula, Zamensky, and King, 2000, p. 3).• Differential settlement.—Differential settlement occurs when the embankmentmaterials next to the conduit are improperly or inadequately compacted orwhen the conduit is placed on a foundation of varying density. The conduitlocation and the resultant embankment loading can result in differential190Figure 115.—A rock pocket at the bottomof a conduit side wall.