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