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WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 1, ISSUE 1, MARCH 2013
SIPHONING, LOW LEVEL CONDUITS, FILTER DESIGN, CONSTRUCTION CONSIDERATIONS
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<br /> <br /> <br /> <br />6 <br /> Photo 1: Severely deteriorated CMP.1 <br />Rehabilitation Alternatives <br />Of the two rehabilitation alternatives noted above, the <br />more common option is sliplining (see Photo 2). High <br />density polyethylene (HDPE) and properly coated steel <br />pipe are the two most common pipes selected for <br />sliplining rehabilitation and have similar design <br />parameters. With little maintenance the service life of <br />HDPE pipe can typically range between 50 and 100 <br />years. Steel pipe requires provisions for adequate <br />coating to provide similar levels of design life. <br />Fiberglass reinforced pipe (FRP) and polyvinyl chloride <br />(PVC) have also been used, but have drawbacks with <br />regards to jointing (i.e., bell and spigot) and brittleness. <br />Design considerations along with a comparison of <br />HDPE pipe and coated steel pipe are summarized <br />below. <br /> Photo 2: Installing HDPE Liner pipe.2 <br /> <br />1 Photo courtesy of www.cleanculverts.com <br />Existing Conduit Inspections: It is important to <br />complete a thorough cleaning and inspection of the <br />existing conduit before moving forward with the <br />design. For large diameter pipes the inspection can be <br />completed visually by entering the pipe from either the <br />downstream or upstream end, although confined <br />space entry procedures should be followed. For small <br />diameter pipes the inspection should be completed <br />using a remote operated vehicle (ROV). The alignment <br />(straightness) of the pipe, severity of deterioration, <br />and location and dimensions of protrusions should be <br />noted during the inspection. <br />Size Selection: When selecting the size and wall <br />thickness of the carrier pipe, the designer needs to <br />consider the hydraulic capacity, clearance from the <br />existing pipe (annular space) including consideration of <br />irregularities and protrusions, and the internal and <br />external loadings. For large internal and external <br />loadings, steel sliplining pipe may be required. The <br />reduced diameter of the carrier pipe may not result in <br />a reduced hydraulic capacity due to better hydraulic <br />efficiency (lower friction losses) of the new carrier <br />pipe. However, hydraulic capacity of the new carrier <br />pipes needs to be checked against requirements. <br />Seepage Paths: After sliplining is complete the existing <br />(host) pipe is essentially sealed. Depending on the <br />severity of deterioration, the existing pipe may have <br />been acting as a large drain for the embankment due <br />to excessive seepage through the pipe/joints. Once <br />the pipe is sealed, it is possible that phreatic levels in <br />the embankment may increase, possibly increasing the <br />potential for internal erosion (piping) along the <br />conduit. To address this concern, the rehabilitation <br />design should always consider adding a filter <br />diaphragm near the downstream end of the pipe. <br />Thermal Expansion: In general, the sliplining pipe will <br />be buried deep in the embankment and will experience <br />limited temperature changes during the service life; <br />however, the designer needs to understand the <br />expansion and contraction limits of the selected pipe. <br />Thermal expansion is not typically a large concern for <br />steel pipe. However, if installed in very hot or very cold <br />ambient air conditions, is it necessary to let the pipe <br />reach equilibrium temperature before annular <br />grouting. <br /> <br />2 Photo courtesy of www.hydroworld.com
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