Monumental expansion programme to meet the future water needs of Las Vegas includes a lake tap shaft to depth 76 m beneath the lake surface to be bored from a barge; an intake tunnel in jointed and faulted metamorphic rock; 22 x 1.82 m-diameter pumping shafts bored from surface to the 100 m level; and a 3.65 m-diameter subaqueous concrete pipeline. Kiewit Construction has completed the 7.6 m-diameter access shaft to the 115 m level and the 792 m East tunnel has broken through using drill/blast supported by rockbolts and shotcrete at a rate of 6 m/day. Work progressing on the 502 m intake tunnel. Zeni Drilling has completed the first shaft. March 1999.
Atlas Copco 322 twin-boom and basket drilling 3.8 m holes for 2.5 m advance in intake tunnel. Mucking out by Wagner 3.5 Scooptrams. Support by fully-grouted Ingersoll Rand C-tube rockbolts of lengths to 3.8 m and 10 cm of fibre reinforced shotctrete. Forward grouting commenced after 250 m after water inflows of 600 gal/min were encountered. 25 m probe holes have been necessary. The well shafts are being pilot drilled blind to full depth using a reverse circulation rotary drillrig and then reamed to the 1.82 m final diameter to accommodate the 1.3 m diameter pumping columns. June 1999.
Zeni reported completion of all 22 well shafts, including casing and grouting. The bases of the well shafts have been exposed by a top heading through the forebay, and the bench is now underway to complete. Breakthrough of intake tunnel into shaft scheduled for late-November, 1999 and flooding of tunnel in January, 2000. November 1999.The Southern Nevada Water Authority has awarded a USD447 million contract to Impregilo and its US subsidiary S.A. Healy to design and build the so-called third straw to draw water from the Lake Mead reservoir. This is a concrete-lined tunnel 6 m in diameter and approx. 4.8 km long underneath the bed of Lake Mead. Construction should begin before the end of the year and eventually involve a massive tunnel boring machine that will be manufactured in Germany and shipped to the United States. The third straw is slated to go on line by early 2013. Visit www.impregilo.it and www.sahealy.comThe Lake Mead reservoir on the Colorado River behind Hoover Dam supplies about 90% of the water used in and around Las Vegas, but the lake level continues to drop in the face of an eight-year drought. The new intake will allow the authority to continue drawing water even if Lake Mead shrinks below the level of the two existing straws. One or both of the existing inlet pipes would be forced to shut down if the lake level falls below current intakes. Lake Mead is currently at a surface level of about 340 metres above sea level, or about 20 metres above the 320 m level of the original intake. A second intake draws water from 305 metres above sea level. It was completed in 2002, at a cost of about USD80 million. The third intake is expected to draw water from below a 275 m elevation. 13/08.The Southern Nevada Water Authority is also soliciting bids, deadline 21st April, 2008 for the Lake Mead intake No. 2 connection and modifications project (contract No. 070F 05 C1). The proposed underground works include connection to the existing intake tunnel and modifications to the existing intake structure, including but not limited to a 6.7 m-diameter 116 m-deep shaft, a ventilated building over the top of the shaft, a 4.3 m-wide by 4.9 m-high by 82 m-long modified horse shoe tunnel from shaft to existing IPS-2 tunnel, 4.3 m-wide by 4.9 m-high by 76 m-long modified horse shoe tunnel connection to the future tunnel from intake No. 3, and existing IPS-2 intake modifications. Visit www.snwa.com/nonpvcs/construction_ops/inv/070F05C1.pdf 13/08.Further technical details are available regarding the Lake Mead intake No. 3 shafts and tunnel project, recently awarded to Vegas Tunnel Constructors, a joint venture of Impregilo and S.A. Healy. The tunnel will primarily be excavated in late Tertiary sedimentary and volcanic bedrock. Of the 4,666 m-long intake tunnel, about 300 metres will be excavated in the Saddle Island Lower Plate (amphibolites, schist, and gneiss) and about 370 metres in the Saddle Island Upper Plate (schist, amphibolites, gneiss, pegmatite, dacite intrusives). The majority of the tunnel drive (about 3,540 metres) is located in the Muddy Creek formation (conglomerate, breccias, sandstone, siltstone and gypsiferous mudstone). As the TBM approaches the intake riser, it will pass through about 370 metres of red sandstone, and the final 90 metres will be in the Callville Mesa basalt.Except for an erection chamber and starter tunnel that will be excavated by drill and blast, the tunnel will be excavated by a Herrenknecht convertible hybrid tunnel boring machine. The TBM, in open mode, will evacuate the spoils by screw conveyor feeding a continuous tunnel conveyor. The continuous tunnel conveyor will feed two muck skips that are hoisted up the intake access shaft and then moved to a stockpile on site. In closed mode, the screw conveyor is retracted from the cutterhead and the TBM functions in fully slurry mixshield mode, utilizing a separation plant. The tunnel will be supported by a precast, bolted, gasketed tunnel lining with five plus a key segments. Inside diameter will be 6.1 m, and the segment ring will be 1.83 m long. Visit www.herrenknecht.comOther underground structures include the 180 m-deep intake access shaft with cast-in-place concrete liner with inside diameter of 9 m, excavated by drill and blast. A stub tunnel approximately 26 m long will be excavated by drill and blast at depth of about 110 metres to provide a connection to the future pump station. At the bottom of the shaft will be a large TBM erection chamber, starter tunnel, and tail tunnel that will provide the Southern Nevada Water Authority (SNWA) with an option for a future tie-in and water transmission. The intake riser will be constructed in Lake Mead at a depth of 90-107 m below the lake surface. Lake-bottom excavation will be by underwater drilling and blasting. The intake riser will be constructed by sunken tube technology and will provide a docking station for the TBM.The construction manager is Parsons Water and Infrastructure. SNWA’s design engineer is MW/Hill, a joint venture of Montgomery Watson and CH2M Hill. The contractor’s design engineer is Arup USA, with Brierley Associates. Visit www.snwa.com, www.parsons.com/about/bus_unit/gbu/water, www.mwhglobal.com, www.ch2m.com, www.arup.com and www.brierleyassociates.com 15/08.
