2000-04-12_REVISION - M1977342 (2)

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Mr. Bruce Humphries • • April 12, 2000 inflow from the main (Dam No. 1) decant pond. We have no documentation of [he historic closure of the system although it appeazs that the system is partially silted. Henderson proposes to close the decant system by installing a 300 ft. grout plug from Decant Tower SN back towards Dam No. 2 (see Drawing SO1-583). The length of the plug has been recommended by Woodward-Clyde in a 1999 evaluation of the system. Henderson will locate the 30-inch decant pipe and drill an initial hole into the concrete pipe approximately 300' (0+300) from Tower SN. A 2-inch inside diameter grout casing will be inserted into the pipe to inject grout. A second hole will be drilled towards Tower SN at approximately 0+270. A grout casing will be inserted in this hole as well. A third vent/grout hole will be drilled at 0+25 if necessary. Once the vent and grout holes have been inserted, grouting will begin at the hole located at 0+300. A low slump, low mobility grout consisting of peagravel, soil and cement will be injected. Approximately 8 to 10 yards of grout will be placed at this location. This will result in an initial solid grout plug of approximately 40-50 feet in length. The ultimate compressive strength of this grout will be 700 - 1200 psi. Specifications for the grout aze attached. Upon completion of the low mobility plug, anon-shrink slurry grout consisting of sand, flyash and cement will then be injected at the hole located at 0+270. Grouting at this location will continue until grout is observed at either the vent hole at 0+25 or at Tower SN. Grout volumes will be checked to make sure that grout is not being lost somewhere in the system. The ultimate compressive strength of this grout will be at leas[ 3000 psi. Specifications for the grout are attached. All grout holes will be backfilled with grout to the surface upon completion of the grouting. Henderson will have an independent testing facility collect and cast a minimum of four grout cubes from each type of grout used for compressive strength testing. The tests will be conducted at 7-days, 14-days and 28-days (one cube will be a back up). A report will be sent to the Division that will include grout volumes and the results of the concrete compressive strength tests within 60 days of project completion. Dam No. 3 Seep Collection System Seepage from Dam No. 3 is currently collected from a series of toe drains in two seepwater collection ponds at the toe of Dam No. 3. Seepage is then pumped back through a ] 0-inch diameter steel pipe that runs to the south of Dam No. 3 face and dischazges the seepwater approximately 200 feet upstream of the crest. One 40-hp pump, rated at 500 gpm, is used to pump the average seepwater flow of 30 gpm. There aze several potential problems with the current system. The location of the discharge from the seepwater system could potentially cause some saturation of the right abutment area of Dam No. 3. In addition, the pH of the seepwater is between 2.5 and 3.5