Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams9.5.2.1 Exterior inspectionExterior inspection of the areas above and surrounding the conduit can providemany clues concerning the condition of the conduit. Items to look for include:206• Look for signs of infiltration of soil into the conduit. Depressions, sinkholes(figures 119 and 120), or cavities that exit onto the surface of the embankmentdam along the centerline conduit alignment are usually an indication thatinternal erosion or backward erosion piping is occurring. These features oftenappear as holes that line up with one another. Such features should be markedwith reference points and monitored to determine whether they are expandingwith time. Sinkholes should be probed to determine the extent of the void,which may be dome shaped and enlarge with depth. The seepage and flowconditions on the downstream slope and through the conduit, should beexamined for evidence of association with the sinkhole. Sinkholes are a causefor immediate concern and further investigation. Beware that some animalsmay take over these areas, and they may not be recognizable as sinkholes orcavities.• Look for signs of seepage or indications that seepage is sometimes present.The best time to look for seepage may be when the conduit is operating in apressurized condition or at full discharge capacity. Evaluate the following:1. If an area on the surface of the embankment dam is wet, the area shouldbe marked or staked, and photographed, to see if it is expanding overtime. If the seepage is flowing, measures should be taken, such as theinstallation of a weir, to collect and measure the quantity of flow. Aseepage rate that is increasing faster than expected, relative to the reservoirlevel, may be an indication of internal erosion or backward erosion piping.Seepage in these areas may be characterized by increased vegetativegrowth or the presence of plants that thrive in wet areas. Ifinstrumentation is available, measurements of seepage should becompared to previous measurements to reveal changes in flow rates.Piezometers should also be monitored.2. The quantity of seepage along the conduit or through the conduit’sbackfill may indicate that adequate compaction around the conduit wasnot achieved or internal erosion or backward erosion piping is occurring.The area where water outlets from a seepage diaphragm should be closelymonitored. Seepage areas may be indicated by changes in vegetation orcolor. The limits of a newly wet area should be marked to determinewhether the area is increasing in size. When possible, the seepage shouldbe channeled away from the embankment dam and directed through apipe, weir, or other device that will allow the quantity to be measured.