Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsFigure 6.—Embankment dam failure caused by internal erosion of earthfillnear the conduit. The initial failure was tunnel shaped, but the collapse ofthe roof of the tunnel resulted in the observed shape of breach. Hydraulicfracture in highly dispersive clay embankment soils caused the failure. Theembankment dam design included antiseep collars, but no filter diaphragm.• Figure 7 illustrates the internal erosion process as a result of a hydraulic fracturethrough the embankment dam.• Figure 8 illustrates the internal erosion process as a result of low densityembankment materials under the haunches of a pipe due to poor compaction.• Figure 9 illustrates the internal erosion process associated with the creation of avoid caused by excessive compactive energy used to compact embankmentmaterials against the conduit.• Figure 10 shows the backward erosion piping process associated withintergranular seepage and the subsequent backward erosion of soil particles.All four of these mechanisms can lead to partial or full failures of the embankmentdam.Internal erosion and backward erosion piping can occur suddenly and with littlewarning. In these cases, little may be done to address the problem quickly enough toavert a failure. Recognizing conditions likely to result in these failure mechanisms isessential to design of conduits and embankment dams that are resistant to failures.In other cases, the failure mechanisms may develop slowly and go unrecognized untilthe subterranean erosion develops cavities in the embankment dam large enough to6