Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment DamsThe important factors that determine the timing and severity of problems related tosoil movement associated with conduit defects include:158• Type of material used in construction of the conduit.—Some materials, such ascorrugated metal, can corrode and develop defects much sooner than conduitsconstructed of more durable materials, such as concrete. Conduits overlain byhigh earthfills are more likely to be stressed beyond their strength, resulting inthe development of cracks.• Dimensions of the crack or hole, in relation to the gradation of the surrounding backfillsoil.—Even small defects in conduits can result in movement of finely gradedsurrounding soils into the conduit.• Resistance of the surrounding backfill to internal erosion and backward erosionpiping.—Very fine sands and silts are extremely prone to particle movementfrom intergranular flow of water into defects in conduits. All soils will erode, ifsubjected to sufficient concentrated flow, such as might occur in cracks in theearthfill, but plastic clays and clayey coarse-grained soils that are not dispersiveresist erosive forces better than silts and cohesionless coarse-grained soils.• Cracks in surrounding soil connected to water sources.—If cracks in surrounding soilconnect to water sources, erosion of the crack walls can increase dramaticallyand lead to catastrophic failure of the embankment dam. This can occur forerosion of materials into the conduit or along the conduit.• Existence of differential head.—The potential for internal erosion or backwarderosion piping is directly related to the differential head causing the flow ofwater, whether the flow is intergranular seepage or flow through cracks in thesoil. High gradients increase the likelihood of internal erosion or backwarderosion piping. Even if the head in the reservoir is not high, continued flowthrough cracks in the soil surrounding the conduit is likely to result in excessiveerosion of the soil.• Type of flow.—Conduits flowing under pressure are more likely to developproblems associated with conduit defects than nonpressurized conduits. Theconsequences of the problems that develop related to defects will be greater inpressurized conduits than those associated with defects in conduits that are notpressurized.• Backfill able to support a tunnel.—Water escaping from defects in a conduit mayerode surrounding soils. The ability of the soils to support a tunnel willdetermine the type of problem that develops. Backward erosion piping requiressoil to be present that can support a tunnel feature. Otherwise, sinkholes orother types of features may be more likely to be the expression of the erosion.