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<br />Las Vegas Area Taps Lake Mead to Satisfy A Growing Thirst (4/24/2000) <br /> <br />Page 5 of7 <br /> <br />000785 <br /> <br />For Calo's crew, the biggest challenge came last December while cutting <br />into southern Nevada's water lifeline, a 10-ft-dia concrete pipe that delivers <br />lake water. Shutting off its flow for 20-hour stretches, subcontractor TDW <br />Services Inc., Signal Hill, Calif., cut two holes into the 11-in.-thick pipe. "It's <br />the largest 'hot' tap we're aware of," Calo says. The 8.5-ft-dia holes divert <br />water to ozone contactors; a 600-ft-Iong cut-and-cover tunnel leads to an <br />existing aeration basin. Next October, for the first time, crews plan to draw <br />water into the basin using the new lake intake. <br /> <br />INTAKE The intake's controversial siting takes advantage of good geologic <br />conditions, acceptable water quality, minimal environmental impacts and <br />optimal operational logistics, SNW A officials say. Surrounded by <br />geologically youthful volcanics, the site overlies a weathered block of <br />Precambrian metamorphic rock. "To me, it was probably the best selection," <br />agrees Rick Kimball, project manager of the Kiewit team that built the intake. <br />But because the rock is highly jointed in places, it complicated grouting. <br />SNW A granted a time extension. Kiewit used just two of the extra three <br />months and earned a $ I-million bonus, after starting in 1998 with a $69- <br />million contract that ended up totalling $80 million when completed this <br />week. The contract covered the intake, an adjoining water tunnel, well shafts <br />intersecting the tunnel's forebay, an aqueduct from the wells and more. <br /> <br />With procedures similar to those used by a different contractor at <br />Arizona's Roosevelt Lake (ENR 4/1/91 p. 24), Kiewit used barge-mounted <br />clamshells, hammers and drills to gouge out a 76-ft-deep column ofrock to <br />make room for the 12-ft-dia steel intake. Divers and a remotely operated <br />vehicle also worked in the 240-ft-deep water near Saddle Island. Less jointed <br />than expected, the rock required more drilling and redrilling than anticipated. <br /> <br />Many more fractures lurked along the 1,600-ft-Iong tunnel that ties into <br />the intake. Tunneling under the island to create a 7-ft-radius semicircular arch <br />and 9-ft-high walls, Kiewit crews encountered faults with clay in the <br />Precambrian gneiss. Before blasting ahead 80 ft, they drilled 100 ft ahead to <br />grout. To control seepage for easier constructibility, they tried to reach even <br />small cracks with micro fine cement that costs 10 times as much as the <br />conventional. <br /> <br />"It should have helped," Kiewit's Kimball says. "We used twice as much <br />grout." But starting in February 1999, Kiewit lost much of the next 40 days <br />staunching seepage of as much as 750 gallons per minute. "We were outfoxed <br />by Mother Nature," explains Stephen J. Navin, Parson's area construction <br />manager. <br /> <br />Rock fissures forced a big cost increase at one of North America's largest <br />recent drilling programs, for one of the world's greatest concentrations of <br />wells. At a mere 20x150-ft plot on Saddle Island, 22 shafts descend 330 ft. <br />Initial plans called for "raised boring" each 6-ft-dia shaft, by drilling a pilot <br />hole down to the tunnel's forebay and reaming out supposedly solid material. <br />Subsequent tests indicated more fractures and groundwater than suspected. <br />Engineers considered extensive grouting, versus a relatively rare but more <br />reliable and expensive technique-"blind boring"-with successively wider <br />concentric drilled holes and a handling system to separate solids from water. <br />Kiewit's team received a $7-million change order to blind bore. Since 3,000- <br />hp turbines must fit inside the 22 holes, subcontractor Zeni Drilling Co. <br /> <br />http://www.enr.comlnew/c42400.asp <br /> <br />05/15/2000 <br />