Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Dams• Eliminates compaction requirements.—A tunnel eliminates the need for specialcompaction requirements around the conduit.• Allows for independent construction of tunnel.—Tunnel construction can beperformed independently of the embankment dam construction. Typically, theconstruction of the conduit through an embankment dam is a critical pathfeature for construction of the dam.• Eliminates the need for special filters.—A tunnel eliminates the need for special filterplacement and drainage requirements, which can typically slow the progress ofembankment dam construction.However, there are disadvantages associated with a tunnel, such as:• Increased cost.—A tunnel is often more expensive than a conduit through anembankment dam. This is especially true for smaller diameter conduits.However, for larger diameter conduits or where pressurized systems arerequired, the relative cost differences can be reduced. The reduction in costdifference is due to a lesser need for redundant safety features, such as steelpipe liners, special filter and drainage requirements, and more efficientembankment dam construction.• Soft ground concerns.—A tunnel may be problematic in soft ground conditions.This could result in higher design and construction costs. Also, the portalconditions must be able to accommodate the entrance and terminal structures.• Potential for overruns.—A tunnel typically involves more risk for cost and scheduleoverruns than a conduit through an embankment dam.• Requires additional engineering experience.—Fewer engineering firms maintain aqualified staff for planning, design, and construction services for tunnels thanfor conduits through embankment dams.• Construction data lacking.—Since tunnels are not very often constructed, up-todateconstruction cost data are not always readily available for comparison ofcosts to conduits through embankment dams.Tunnels are seldom used for small embankment dams and may be a more costlyoption for some larger dams. In those embankment dams, a conduit penetrating thedam may be preferred. Conduits have typically been located at about theembankment/foundation interface. They are often located so as to align the conduitdischarges with the original watercourse, bypassing streamflow during construction,and potentially emptying the reservoir by gravity. This means that for many sites, theconduit is located on alluvial soils that can be deep and compressible. This also36