Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 5—Foundation and Embankment Dam• The flowable fill will develop interior cracks (shrinkage) that should beintercepted with filter diaphragms to ensure no movement of soil particles.Flowable fill may be applicable for low hazard embankment dam applications, ifused in conjunction with a filter diaphragm or collar. Use of a filter diaphragm orcollar is a valuable defensive design measure, even for low hazard classification siteswith favorable conditions. The use of a lean concrete in lieu of flowable fill mayallow for elimination of the filter diaphragm or collar, but eliminating this valuablefeature should be carefully considered and justified based on extremely favorable soilconditions, good conduit construction materials and methods, reliable constructionpractices, and favorable foundation conditions. Conditions where flowable fill maybe applicable for low hazard embankment dams include:• Backfilling trenches dug in relatively nonyielding materials, such as bedrock orglacial till in which a conduit is installed. Flowable fill provides a uniformmaterial surrounding the conduit, which has strain properties similar to those ofthe adjacent foundation. This would allow somewhat steeper side slopes forthe excavation.• Design of the flowable fill provides similar deformation characteristics in thecured fill as in the adjacent foundation materials.5.3.2 Compaction of backfill material against conduitRecommendations for compaction of soils and rock zones against the conduit are asfollows:• Minimum strength.—Prior to placing embankment adjacent to the conduit, theconcrete must have attained minimum strength. Minimum strength should bebased on anticipated/estimated loading conditions (i.e., construction surcharge,embankment load, etc.). As a rule of thumb, placing embankment should notbegin until curing of the concrete is completed (typically 7 to 14 days afterconcrete placement) and the concrete has achieved its design strength.• Average moisture content.—The average moisture content during compactionshould be in the range of 1 percent dry to 3 percent wet of optimum content,where optimum water content is defined by a Standard Proctor energy (ASTMD 698) compaction test. The compacted unit weight of the backfill around theconduit should be equivalent to that required for the surrounding soil.• Angular particles.—Earthfill placed within 2 feet of the conduit should notcontain large angular particles that could damage the conduit from compactiveeffort used in compacting soils near the conduit. In rockfill zones, a cushion121