Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 9—Inspection and Assessment of Conduit-Related ProblemsThis movement could cause cracks to develop or joints to open up in the conduitsimilarly as discussed for structural deformations of conduits. Conduits in sandy andsilty soils also could be susceptible to damage, if the soils are liquefiable.If this condition is suspected along the alignment of a conduit, instrumentation, suchas observation wells, could be placed near the conduit alignment. Installation ofinstruments to measure uplift pressures cannot be relied upon as the sole means ofdetection of these problems. Rather, instruments to measure pore pressure shouldonly be placed as a means of providing information on the general water pressureconditions at the location of interest. If piezometers are installed after the conduitand embankment dam have been constructed, caution should be used in consideringdrilling close to a conduit, as low stress zones with the potential to hydraulicallyfracture often exist as a result of the structure. In a zoned embankment dam,locating the instrument in a zone other than the core should be considered.Installation of instruments to measure pore pressure resulting from internal erosionor backward erosion piping cannot be relied upon as the sole means of detection ofthese problems. Rather, instruments to measure pore pressure should only be placedas a means of providing information on the general water pressure conditions withinthe embankment dam.Designers should note that a trend is growing in the industry to eliminate theinstallation of instrumentation within the cores of embankment dams duringconstruction. The performance of embankment materials is well understood, sothere is little need to repeat past research. Also, it is very unlikely that the instrumentwill be placed in the correct place to detect a chance problem. Furthermore, it isrecognized that the mere act of installing the instrument can adversely affect thequality of the embankment dam. Vertical risers associated with cables and tubingcan disrupt the proper flow of compaction equipment. Instrumentation trenches canpotentially introduce flaws that could lead to concentrated leakage.Installing instruments in the cores of existing embankment dams to detect particularproblems should still be considered. The instrument can be placed within theembankment dam by drilling techniques, but specific techniques that limit thepotential for fracturing the embankment dam should be employed. Drilling into theembankment dam with techniques that use water or air to remove cuttings should beavoided, because blockages within the drill holes have been known to cause thebuildup of high fluid pressures leading to fractures in the earthfill. For guidance ondrilling within embankment dams, see section 14.3.1.9.8.3 Seepage quantitySeepage along a conduit or through an embankment dam is a valuable indicator ofthe condition and continuing level of performance of an embankment dam.237