Lake Mead Intake No. 3
In March 2008 the Southern Nevada Water Authority (SNWA) awarded a $447 million design-build contract to Vegas Tunnel Constructors (VTC) -- a joint venture of S.A. Healy Co. and Impregilo S.p.A. The Lake Mead Intake No. 3 Program, of which this project is a part, is intended to protect the existing water system capacity of the Southern Nevada Water Authority (SNWA) against potential inoperability of the existing Intake No. 1 and/or No. 2, as well as allow SNWA to draw better quality water from the deeper elevation and location in the lake. The lake has dropped 110 feet since 2000, leaving it at half capacity. The new intake is designed to accommodate a flow of 1,200 MGD.
The project is located in the Lake Mead National Recreational Area, managed by the National Park Service. Completion is planned in 2014.
The scope of the work covered by the contract awarded to VTC includes the design and construction of the following structures:
30 ft finished diameter, 611 ft deep from final cover at +1254 ft AMSL to tunnel invert +643 ft. The shaft is excavated by drill and blast methods with dowels, mesh and shotcrete for initial support with an 18-inch thick CIP concrete final lining.
The construction of the access shaft was approached in a traditional method, with excavation by drill and blast, immediately followed by the installation of temporary supports. The muck removal is first performed by a standard 200 T crawler crane, and then through a mining-type, heavy-hoisting system. The lining is provided in CIP concrete, following the excavation with a top-down approach. A double-deck work deck system is provided to support all of the operations.
Pre-excavation grouting was performed all along the length of the shaft from El. 1,090 downward, for a total length of 400 ft., as the trigger limit of 50 gpm was reached in the probe holes. Pre-excavation grouting is performed through a set of primary holes, varying from 20 to 25 in number, and for a depth of 120 ft., followed by grouting through an equivalent number of secondary holes, using ultrafine cement or Type III cement.
IPS-3 Connection Stub Tunnel
A stub tunnel from the Access Shaft at a depth of 400 ft is provided for future connection to the existing pumping and treatment facilities. It is 83.5 ft long from shaft to bulkhead, 20 x 20 ft horseshoe by drill and blast methods. Final lining is shotcrete with rock bolts.
TBM Launch Chamber
At the bottom of the shaft, a large chamber is excavated to allow assembly of the TBM. This chamber is 202 ft long centered on the Access Shaft, 47 ft wide by 38 ft tall horseshoe, excavated by top heading and bench, drill and blast methods. Final lining is shotcrete with rockbolts.
IPS-X Connection Stub Tunnel and Back-Shunt Tunnel
At the south end of the chamber, a tail tunnel is excavated. It is 85 ft long, 20 X 20 ft horseshoe, excavated by drill and blast methods at bottom of access shaft adjacent to launch chamber. This tail tunnel may facilitate future expansion of the system.
TBM Starter Tunnel
To maximize the erection of the TBM and provide space for a continuous belt magazine, a 360 ft long, 26 ft horseshoe tunnel is excavated by drill and blast and mechanical methods to the north of the launch chamber. Due to adverse geological conditions, an umbrella canopy system is employed to allow for sequential excavation as required. Final lining is steel fiber reinforced shotcrete.
15,000 foot long tunnel to be bored in rock with cover ranging from 587 feet (at launch) to about 30 ft upon entering the intake structure. Nominal groundwater heads range from 14.8 bar to 10.9 bar at the riser structure. A 23.6 ft cut diameter (7200 mm with new gage cutters), dual mode (open and pressurized mode to 17 bar) TBM will be used to excavate the tunnel and install a one-pass lining consisting of 20-foot ID, 14-inch thick bolted and gasketed precast concrete segments. The tunnel will be bored through pre-Cambrian metamorphic rock (primarily amphibolite, gneiss and schist including 190ft of detachment fault consisting of Phyllonite and crushed brecciated rock); Tertiary sedimentary rock (Muddy Creek formation primarily consisting of mudstone, siltstone, sandstone, conglomerate and conglomerate breccia); Tertiary sedimentary rock (Red Sandstone formation primarily consisting of breccia); and Tertiary volcanic rock (Callville Mesa formation primarily consisting of basalt).
For the excavation of the tunnel, a prototype Hybrid TBM has been designed and supplied by Herrenknecht of Germany. The machine is sufficiently flexible to cope in Open Mode with variable types of rocks, and capable of operating as a Slurry TBM in Closed Mode in the presence of an unstable face, high inflow of water, or water inflows at high pressure, up to a pressure of 17 bars.
The segmental lining has been designed as a universal tapered ring, consisting of five elements and one key, with a ring length of six feet, designed to comply with the stringent requirements of the project. A tight set of tolerances have been established and are verified on a continuous basis by a dedicated QC staff. Waterproofing is ensured by gasket design to meet the specifications, and was tested at a pressure of 36 bars.
The intake structure is a 14 ft ID stainless steel riser casing with a concrete base elbow and TBM reception eye. It is installed within a large lake bottom excavation 60 ft deep dug below a water depth of approximately 350 ft and then backfilled with tremie concrete. Base size of the excavation is 95 feet by 59 feet.
Special blasting techniques that are very rare in construction are used. VTC is utilizing shaped charges, an explosive charge designed to focus the effect of the explosive's energy. A remote operated frame, with the shaped charges attached, is lowered by a crane barge to the lake bottom to set the charges. The frame is then removed and the charges detonated.
The intake structure is constructed on a docking barge anchored near the shoreline. Upon completion of the underwater blasting and excavation, the intake structure is transported to the proper location by tugboat pushing the docking barge. The intake structure is lowered into place and anchored by approximately 12,000 cubic yards of underwater concrete.
After a final survey, the TBM alignment will be verified and the TBM will excavate into the reception area of the intake structure for the final connection.