Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams122zone of smaller granular particles should be used to prevent damage to theconduit from the point loading of rocks in the earthfill.• Permeabilities.—Earthfill immediately adjacent to conduits should be compacted,so that no layers of material with permeabilities higher than in the adjacentearthfill extend in an upstream and downstream direction along the conduit.• Ramping of earth material.—The earthfill should be ramped against the conduit ona slope of 6H:1V (figure 83) to help force the earthfill against the conduit andto avoid contacting the conduit with the frame of the pneumatic roller used forcompaction. Pneumatic rollers should be operated in a direction parallel to theconduit. The pneumatic roller may form rutting and a smooth surface on theearthfill layer that will need scarification before new layers are placed.• Lateral movement.—Earthfill should be maintained at approximately the sameelevation on both sides of the conduit during backfilling. This will help toprevent lateral movement of the conduit caused by unequal compaction energyapplied to the sides of the conduit.• Disking.—The area adjacent to a conduit is normally a highly trafficked area,due to activities involved with the installation of the conduit. Disking, as wellas being sure to eliminate drying cracks and moistening of surfaces beforeadding subsequent lifts are required to prevent smooth surfaces between lifts.• Compaction.—Compacting soil next to large conduits requires differentapproaches than for compacting soil next to smaller conduits. A singlerecommended approach is not possible for a wide range of conditions.Generally, compacting soil surrounding larger conduits may employ pneumatictiredrollers or similar equipment, which is operated parallel to the conduit. Onsmaller conduits, operating large equipment near the conduit can damage theconduit, and hand compaction may be required. Compacting soil within 2 feetof a conduit with heavy equipment, such as tamping rollers or vibratory steelwheeledrollers is usually inadvisable.Hand controlled mechanical compactors (also known as tampers or wackers)have been used frequently in the past. Handheld compactors should not weighless than 100 pounds. A much thinner lift thickness than the rest of theembankment dam is required when using handheld compactors. Handcompaction is often slow, labor intensive, and tends to lag surroundingembankment dam placement. Hand compaction requires more effort to obtainproper moisture and density, may require special gradation of soil particles, andrequires intense inspection and at times is a source of irritation to bothcontractor and owner. This results in a tendency to concentrate more onprogress than good construction techniques. Operators of handheld