Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsmost concern. Once a crack forms and water enters the crack, three possibilities canresult (ASDSO, 2003):1. Water penetrates soil adjacent to the crack and the soil begins swelling. If thecrack is small and not continuous through the embankment dam, it can swellshut and not develop into a problem.2. Water runs through the crack, but the crack is small, so the velocities are low,and the soil is resistant to erosion. A wet spot may appear, but no internalerosion takes place.3. The soil in the embankment dam, such as dispersive clay, is erosive. Internalerosion begins, and a concentrated leak develops. More and more internalerosion occurs, and the embankment dam fails from the breach that is formed.Cracks in embankment dams caused by hydraulic fracture may provide a pathway forinternal erosion. If soils in the pathway of the crack are highly erodible, the crackwill enlarge quickly, leading to a breaching type of failure. Hydraulic fracture iscommon near conduits, because the conduits create differential strains insurrounding embankment soils.Hydraulic fracture of embankment dams can occur when the piezometric head ofwater within the dam is greater than the lateral effective stress on the earthfill.Sherard (1986, pp. 905-927) discusses hydraulic fracturing in detail. Figure 81illustrates how on first filling, a wetting front moves through the embankment dam.Figure 82 shows an example of hydraulic fracture in an embankment dam. For anexample of a project that experienced hydraulic fracture near a conduit, see the casehistory for Piketburg Dam in appendix B.Problems often occur on first filling of the reservoir. About 42 percent of allembankment dam failures due to internal erosion or backward erosion piping occuron first filling (Foster, Fell, and Spannagle, 2000, p. 1025). As discussed in section9.1, filling the embankment dam’s reservoir for the first time requires caution. Slowfilling of the reservoir is important to allow the wetting front to slowly penetrate intothe embankment dam. This will allow the soils to swell and deform, which helpsprevent hydraulic fracture. Typical filling rates are in the range of 0.5 to 2 feet perday. The designer should consider the rate of reservoir rise when determining thehydraulic capacity of the conduit. For guidance on the hydraulic design of conduits,see chapter 3.Excavations for conduits increase the potential for differential settlement, andspecial care is recommended for any excavations used near and under conduits. Theexcavation should be wide enough to accommodate motorized compaction116