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5 Low-Level Conduits – Rehab or Replace? Introduction Deterioration of low-level outlet conduits is a common problem, especially for older embankment dams. This article presents alternatives designers should consider prior to beginning a conduit replacement or rehabilitation project and a brief discussion of the potential pitfalls sometimes seen during the design and construction phases. Rehabilitation or Replacement? The first question the designer needs to ask is whether the conduit should be replaced or is a good candidate for rehabilitation. Conduit replacement is likely the approach providing the greatest reliability, but that approach will most likely require draining of the reservoir and be the highest cost option. In some cases, rehabilitation provides a reasonable alternative. Rehabilitation is typically accomplished by one of two methods, sliplining or cured-in-place pipe (CIPP) liners. Sliplining is completed by installing a smaller, "carrier pipe" into a larger "host pipe", grouting the annular space between the two pipes, and sealing the ends. A CIPP liner is a resin-saturated felt tube made of polyester, which produces a jointless, seamless, pipe- within-a-pipe. A CIPP liner is either inverted or pulled into the host pipe, cured-in-place using pressurized steam or hot water and serves as the new carrier pipe. Although these rehabilitation methods may also require draining of the reservoir, they are typically lower cost alternatives to full replacement. FEMA (2005) provides a detailed list of advantages and disadvantages of replacement and rehabilitation, which are summarized below. Advantages of conduit replacement: • Visual embankment/foundation evaluation after conduit removal. • Allows repair of surrounding embankment that may have been damaged because of deteriorated condition of existing conduit. • Allows for easy incorporation of filters designed according to the current state-of-practice. • Potential for increasing the hydraulic capacity of the conduit. • Conduit Replacement does not require specialty contractors, equipment or personnel. Disadvantages of conduit replacement: • Typically the highest cost alternative. • Requires large open cut excavation through the embankment, which may put downstream areas at risk during construction. • Potential for developing seepage paths at the contact between the unexcavated existing embankment and the replaced earthfill. Advantages of conduit rehabilitation: • Limited or no excavation required. • Installation during weather conditions not suitable for replacement. • It may be possible to maintain a full reservoir in some cases (i.e., conduit has upstream control and is accessible from downstream). • Shortened construction schedule and reduced cost when compared to replacement. Disadvantages to conduit rehabilitation: • Not applicable for severely deteriorated conduits (i.e., conduits with severely compromised structural integrity, open joints or holes, pipe deformities, or conduits believed to have voids along the outside of the pipe). See Photo 1. • Limitations for conduits with slight bends, deformities or non-uniform diameters. • Most likely will require specialized contractors, and equipment for installation. • May adversely affect seepage paths around the exterior of the existing conduit. Additional advantages and disadvantages of the alternatives are also presented in “Technical Manual: Conduits through Embankment Dams,” produced by the Federal Emergency Management Agency (FEMA, 2005). The designer should consider both replacement and rehabilitation alternatives carefully and understand that each project site may have specific challenges that need to be considered.