Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 1—Generalmeans the conduit is often located near the maximum section of the embankmentdam, which contributes to greater structural loading on the conduit. The designershould consider the following guidance in locating the conduit (Reclamation, 1987c,p. 3):• Avoid differential settlement.—Whenever possible, the conduit should be locatedwhere the profile is entirely on bedrock, or entirely on soil. Differentialsettlements can occur where the overburden soil thickness is extremely variableor foundation properties differ. The bedrock profile underlying the conduitlocation should not have abrupt changes in a short horizontal distance.• Locate the conduit in a trench.—Locate the conduit in a trench section in firm rockwhen the rock is at or near the ground surface (figure 19). For this option, theconstruction specifications should include provisions for rock excavation to beperformed to eliminate or minimize open fractures or other damage to the rockbeyond the limits of the excavation. Concrete should extend to an upper limitof the top of the conduit or to the original rock surface, if lower than the top ofthe conduit.• Locate the conduit on a bench.—Locate the conduit on a bench excavated along thebase of an abutment when geological conditions and topography are favorable.Placing concrete on the abutment side or placing the conduit against theexcavated rock reduces or eliminates requirements for earthfill compactionagainst one side of the conduit (figure 20).• Consider the potential for nonuniform settlement.—Foundation conditions along thelength of the conduit are often nonuniform, and concentrated settlement iscommon in some areas. As the height of the embankment dam is raised duringthe construction of the dam, periodic inspection of the interior of the conduitshould be performed. The frequency of such inspections should be determinedbased on anticipated foundation conditions as well as any uncertainties. Someconduits have experienced distress during and after construction as a result ofunidentified foundation conditions. If distress is observed in the form ofcracking or separation of joints, prompt remedial action is required.Reclamation has monitored and recorded these concentrated settlements at anumber of their embankment dams after construction was completed. Theresults of monitoring show that nonuniform settlement along the conduit iscommon after completion of construction.• Flatten slopes where conduits span a cutoff trench.—Where a conduit spans a cutofftrench, the side slopes of the cutoff trench may require flattening to reducedifferential settlement between the compacted backfill in the cutoff trench andthe foundation soils adjacent to it.37