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t"lr. Bnice Humphries <br />April 12.2000 <br />inflow from the main (Dam No, 1) decant pond. We have no documentation of the <br />historic closure of the system although it appears that the system is partiall%' silted. <br />Henderson proposes to close the decant system by installing a 300 ft. grout plug from <br />Decant Tower :N back towards Dam No. 2 (see Drawing 301-583 j. The length of the ? <br />plug has been recommended by Woodward-Clyde in a 1499 evaluation of the system. Z <br />W <br />Henderson will locate the 30-inch decant pipe and drill an initial hole into the concrete <br />pipe approximately 300' f 0-300) from Tower ?N. A 2-inch inside diameter grout casing = <br />V <br />will be inserted into the pipe to inject grout. A second hole will be drilled to"vards TowJ,r Q <br />5N at approximately 0+270. A grout casing will be inserted in this hole as well. A third <br />ventigrout hole will be drilled at 0-25 if necessary. Q <br />Once the vent and grout holes have been inserted. grouting will begin at the hole located <br />at 0-~-300. A low slump, low mobility grout consisting of peagravel, soil and cement will <br />be injected. Approximately 8 to 10 yards of grout will be placed at this location. This <br />will result in an initial solid grout plug of approximately 40-30 feet in length. The <br />ultimate compressive strength of this grout will be 700 - 1300 psi. Specifications for the <br />grout are attached. <br />Upon completion of the low mobility plug, a non-shrink slurry grout consisting of sand. <br />flyash and cement will then be injected at the hole located at 0-270. Groutine at this <br />location will continue until grout is observed at either the vent hole at 0_7 or at Tower <br />5N. Grout volumes will be checked to make sure that grout is not being lost,ornewhere <br />in the system. The ultimate compressive strength of this grout will be at least 3000 psi. <br />Specifications for the grout are attached. 411 grout holes will be backtilled with ,rout to <br />the surface upon completion of the grouting. <br />Henderson will have an independent testing facility collect and cast a minimum of tear <br />grout cubes from each type of grout used for compressive strength testing. The tests will <br />be conducted at 7-days, 14-days and 28-days (one cube will be a back up). A report will <br />he sent to the Division that will include grout volumes and the results of the concrete <br />compressive strength tests within 60 days of project completion. <br />Dam No, 3 Seep Collection System <br />Seepage from Dam No. 3 is currently collected from a series of toe drains in two <br />seepwater collection ponds at the toe of Darn No. 3. Seepage is then pumped back <br />tl-iough a I 0-inch diameter steel pipe that runs to the south of Dam No. 3 face and, <br />discharges the seepwater approximately 200 feet upstrearn of the crest. One 40-Itp pump. <br />rated at 500 gpm. is used to pump the average seepwater flow of 30 -,pm. <br />There are several potential problems with the current system. The location of the <br />dischar__e from the seepwater system could potentially cause some saturation ut the right <br />abutment area of Dam No. 3. In addition. the pH of the seepwater is bcmeea 21.5 and 3.5