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Western Dam Engineering <br /> <br /> <br /> <br />22 <br />primary injection ports that are drilled and installed <br />along the invert, spring line, and crown of the voided <br />encasement to ensure that the grout can travel and <br />vent as necessary. Secondary grout ports can be <br />added at intermediate locations if communication <br />between the primary grout ports is not confirmed (air <br />or grout return). Grout pressures should be monitored <br />during injection and each port grouted until project <br />criteria for grout refusal is met or grout return is <br />achieved at the next grout port location. <br />Communication between the voids in the encasement <br />and the surrounding embankment could exist; <br />therefore, pressures should be limited to prevent <br />fracturing the embankment behind the encasement. <br />In the past, many small and intermediate sized <br />embankment dams were constructed with conduits <br />made of Corrugated Metal Pipe (CMP). CMP has a <br />typical service life of about 25 to 50 years, but <br />depending on the metal’s reaction with certain soils <br />and water conditions, cases have been documented <br />where CMP has deteriorated in less than 7 years after <br />construction. The current state of practice is not to <br />repair severely deteriorated CMP but to replace it with <br />another conduit system. Describing methods for <br />replacing CMP conduits is outside of the scope of this <br />article but the March 2013 issue of the Western Dam <br />Engineering Technical Note can be referenced for <br />information on slip lining existing conduits. <br />Photo 9: Injection grouting voided encasement behind <br />steel liner. <br />Plastic <br />Historically, when compared with steel or concrete, <br />plastic pipe has not been commonly used as the <br />primary material in outlet conduits. FEMA (2007) <br />describes the uses of plastic pipe in embankment <br />dams. Plastic pipe is more typically used in small- <br />diameter toe drain systems. Plastic pipe, however, has <br />been used in lining rehabilitation of existing conduits. <br />Plastic conduits are generally considered to have a <br />shorter service life than RCPP conduits (approximately <br />50 to 100 years). However, in environments where the <br />water or soil may cause premature degradation of <br />concrete and steel, plastic conduits may be a favorable <br />alternative. Lining rehabilitation with plastic pipe is <br />typically accomplished by one of two methods, slip <br />lining (typically using HDPE) or cured-in-place pipe <br />(CIPP) liners. Slip lining is completed by installing a <br />smaller, "carrier pipe" into a larger "host pipe," <br />grouting the annular space between the two pipes, <br />and sealing the ends. Preventing collapse of the <br />interior carrier pipe during grouting of the annulus is <br />critical to the success of a lining rehabilitation project. <br />Pressures should be monitored during grouting and, in <br />some cases, the carrier pipe filled with water to <br />provide additional resistance to collapse. <br /> <br />Photo 10: Grouting HDPE liner pipe.3 <br />A 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 cut and cover methods for <br />full replacement. Renovation of existing conduits by