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<br /> <br /> <br /> <br />23 <br /> <br />Figure 5. Abandonment of a conduit by cement grout <br />through holes drilled from the surface of the embankment <br />dam to depths of up to 60 feet. [1] <br />Completely filling the existing conduit is <br />recommended. Partial filling of an abandoned conduit <br />would need to be evaluated and consider long term <br />safety (failure mode) concerns. The indicated grouting <br />and backfill procedures in this section may require <br />modification to adapt to given site conditions. The <br />designer is cautioned that grout from the surface, <br />unless carefully controlled, has the potential for <br />causing hydraulic fracture within the embankment <br />dam. Drilling from the surface of the embankment dam <br />is not advisable for situations where the reservoir <br />water surface cannot be lowered. <br />Drilling Into Existing Embankment Dam <br />Drilling into an embankment dam can cause serious <br />damage and the need to do so should be carefully <br />considered. If drilling into an embankment dam has <br />been determined to be necessary, drilling through any <br />portion of an embankment dam should be performed <br />with extreme caution. Improper drilling procedures <br />increase the potential for hydraulic fracture. Drilling <br />fluids, such as water or bentonite, are sometimes used <br />during drilling to enhance removal of drill cuttings, but <br />these fluids should be avoided when drilling in <br />embankment dams. <br />Auguring is the preferred method for drilling in the <br />core of embankment dams. Auguring uses no drilling <br />fluid and is inherently benign with respect to hydraulic <br />fracturing. A hollow-stem auger permits sampling in <br />the embankment and foundation through the auger’s <br />hollow stem, which acts as casing. If fluids must be <br />used, the risks must be understood and specific <br />procedures should be employed to minimize the <br />chance for hydraulic fracturing. <br />For more information on drilling into existing <br />embankment dams, see the previous Western Dam <br />Engineering Technical Note article Volume 2 Issue 1: <br />"Poking the Bear: Drilling and Sampling for <br />Embankment Dams." <br />Inspection <br />A thorough inspection of the existing conduit is <br />required prior to beginning any abandonment <br />activities. Depending on the diameter of the conduit, <br />man-entry or CCTV inspection methods should be <br />used. The condition of the existing conduit, existence <br />of any defects, protrusions or obstructions, joint <br />offsets, amount of deflection, and evidence of leakage <br />or internal erosion should be determined. <br />A detailed discussion about inspection of conduits was <br />presented in Volume 2: Issue 2 of Western Dam <br />Engineering Technical Notes: "You Con-du-it; How to <br />Fix a Leaky." <br />Preparation <br />The existing conduit surfaces against which grout will <br />be placed should be free of roots, sediments, mineral <br />deposits, dust, laitance, loose or defective concrete, <br />curing compound(s), coatings, and other foreign <br />materials. Any sediment or debris should be removed <br />from the invert of the existing conduit. Where <br />possible, any bolts or other projections should be cut <br />flush and ground smooth with the interior surface of <br />the existing conduit. <br />Abandonment of the existing conduit may need to be <br />scheduled to allow grouting operations when the <br />reservoir is at its lowest annual elevation. Siphons or <br />pumps can be used to further reduce reservoir <br />elevations. In some cases, the construction of a <br />cofferdam may be more applicable if the reservoir <br />water level needs to remain at a constant elevation. If <br />a new conduit is being constructed, grouting of the <br />existing conduit can be delayed until the new conduit <br />can be used for diversion.