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10 recommended for use. Considerable care should be used when selecting HDPE due to the considerations listed in Table 1. Table 1. Summary of common conduit materials Conduit Pros Cons HDPE - Resistant to corrosion - Flexible - Inexpensive at small diameters - Lower strength, susceptible to collapse - Thermal expansion and contraction - Does not bond well with other materials Steel - Cost-effective at small diameters - Easy to attach valves, vents, flowmeters, etc. - Relatively expensive at large diameters - Susceptible to corrosion Concrete Pipe - Cost-effective at large diameters - Can be cast in a variety of shapes - Can be reinforced for higher strengths - Relatively expensive at small diameters - Susceptible to leaking at joints It is important to consider the forces that will be acting on the conduit, internal and external. Pipes are often encased in concrete to resist external embankment forces, protect the pipe during placement of embankment material, allow for more efficient compaction and distribution of soil stresses and minimize vibration under transient conditions. Non- pressurized conduits must withstand external pressure from the dam embankment, and should be water tight to prevent any leakage. Pressurized conduits must withstand internal water pressures in addition to external embankment pressure and should be air tight. Potential problems to consider include: • Differential settlement of the embankment which can deform the conduit • Separation of joints • Corrosion and deterioration • Erosion or abrasion of internal surfaces • Misalignment Factors to consider when selecting a material include strength, durability, resistance to corrosion, ease of maintenance, and cost. Some relative pros and cons associated with various conduit materials are presented in Table 1. See Reference [3] for more information regarding design of conduits. Filter Diaphragm – Dam failures often occur in the vicinity of the outlet conduit due to defects in the conduit, separated joints, uneven compaction of material around the conduit, or concentrated seepage along the interface between the conduit and the embankment, eventually leading to internal erosion. Historically, cutoff collars were installed along outlet conduits as an attempt to disrupt seepage paths and prevent internal erosion. However, these have been found to be ineffective and have been replaced by sand filter diaphragms as the preferred method for reducing the risk of internal erosion and piping. A filter diaphragm is a zone of filter material surrounding the outlet conduit. The diaphragm is designed to prevent erosion of material caused by concentrated seepage paths that may develop along the conduit due to poor compaction or differential settlement. A detailed discussion about filter diaphragm design and construction considerations is presented in Filter Design and Construction (Vol.1 Issue 1). See Reference [4] for more information regarding design of filter diaphragms. Photo 4: Piping failure along outlet conduit Energy Dissipation – Flow from the outlet, whether in a pressurized or non-pressurized system, will have a high velocity. If the downstream channel consists of bedrock, this flow can be released directly. However, if erodible materials are present, some form of energy dissipation is required to prevent erosion at the dam toe. Common energy dissipation devices associated with outlet works are stilling wells, impact basins,