Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsdevelopment of high excess porewater pressures in the foundation and/orembankment is accentuated, and second, transverse cracks may develop at thejuncture of the closure section with the adjacent already constructed embankment asa result of differential settlement. . . . Cracking because of differential settlementmay be minimized by making the end slopes of previously completed embankmentsections no steeper than 1 vertical on 4 horizontal. The soil on the end slopes ofpreviously completed embankment sections should be cut back to well-compactedmaterial that has not been affected by wetting, drying, or frost action. It may bedesirable to place core material at higher water contents than elsewhere to ensure amore plastic material which can adjust without cracking, but the closure sectiondesign must then consider the effects of increased porewater pressures within the fill.5.3 Selection and compaction of backfillProper selection and compaction of backfill material against the conduit willminimize the potential for differential settlement.5.3.1 Selection of backfill material to be placed against conduitIf the conduit is being placed in a zoned earthfill embankment dam, backfill for theconduit should usually have properties that are compatible with the adjacentembankment zones. Core zone backfill should only be used around the conduitthrough the core section, with shell backfill soils used outside the core. Where theconduit passes through the core of an embankment dam often material with higherplasticity is used near conduits. Plastic materials can be placed at a water content wetof optimum (between 1 percent and 3 percent wet of optimum) to increase plasticbehavior. An exception is where rock shell zones include large angular rocks thatcould impose point loads on the conduit. For that condition, encircling the conduitwith a cushioning soil zone of smaller sand and gravel should prevent this problem.Ideally, the earth material adjacent to conduits in the im pervious zone of fill shouldbe reasonably well graded, have a maximum particle size no greater than 1½ inches,including earth clods, a minimum of 50 percent by weight passing a No. 200 sieve,and a plasticity index between 10 and 30 percent. The water content of the materialas previously discussed should be between 1 percent and 3 percent wet of optimum.Dispersive clay and treatments are discussed in section 5.3.3.Flowable fill (also known as controlled low strength materials) is not recommendedfor backfilling around conduits in significant and high hazard embankment dams,due to the following reasons:• Flowable fill does not bond to either the conduit material or the adjacentfoundation in which it is in contact. Measures are required to intercept flowalong the interface between the flowabable fill and foundation or conduit.120