 |
|
 Each month AUA will feature an outstanding Underground Project currently under construction, featuring its unique aspects in terms of technology, location, function, etc. Contact AUA to nominate projects. This month's Featured Underground:
Lake Mead Intake No. 2Las Vegas, Nevada
|
|
|
The Saddle Island location was chosen, after evaluation of many potential sites, because it met several site selection criteria: deep water, good water quality, suitable geology for underground construction, minimal environmental impact, and cost. Saddle Island, which is 2 miles long and approximately 2,000 feet wide, is seismically stable and there is no significant geologic risk from having both intakes 400 feet apart. This location also offers the lowest risk for the highly specialized construction necessary to build the intake. The new facility will be 50 feet deeper than the present intake to increase reliability in the event of lower lake levels. Lake Mead is a popular recreation area and part of the National Parks System. Great efforts have been made to maintain public safety for boaters and park visitors and to tread lightly within the Park to preserve the Park's environmental integrity.
PROJECT DESCRIPTIONSaddle Island is a block of old basement rock surrounded by a large area of geologically youthful volcanic flows and pyroclastic deposits. The bedrock of the project site is mainly dark-colored Pre-cambrian metamorphic rocks. The metamorphic rock is generally hard, contains several groups of joints, a number of shear zones and fault zones. There is a wide variation in rock strength, ranging from about 7,000 to 35,000 psi.The underground works on Saddle Island include the access shaft, surge chamber, access shaft-intake connector tunnel, intake tunnel, forebay, well shafts and underwater works for an intake shaft on the east side of Saddle Island.
Access Shaft
Most of the access shaft excavation will occur below the water table. Grouting will occur to contain the inflow of water into the shaft. The shaft is now constructed to a depth of 200 feet with completion expected in mid-October.
Surge Chamber
Intake Tunnel
The tunnel will be excavated by drill and blast using smooth wall blasting methods. TBM excavation was eliminated from consideration due to the potential for high groundwater inflows. Rock support will range from occasional rock bolts, to fiber-reinforced shotcrete with pattern rock bolting.
Forebay
Wells
Fifty-inch well casings will be set in the drilled well shafts and will have a three-inch grout annulus. To avoid excessive rock loss during the "hole through" process of the shafts, rock bolting and shotcrete will be used in the arch of the forebay. The well casing will be set close to the arch profile of the forebay and the annulus will be blocked for grout placement. The annulus grout will be placed in increments to assure filling the annulus void. With the annulus grout in place, the transition between the forebay arch and the casing will be smoothed by mortar.
Underwater Excavations Intake Shaft The upper part of the intake shaft excavation will be in soil and weathered rock. A 25-foot long starter casing extending into the unweathered rock will be required to support soil and weathered rock. The finished shaft will be a 12-foot inside diameter steel liner. The liner will be fitted with a rock deflector at bench level. The deflector will also have provisions for a retro-fit fish screen with 1-inch openings. The construction sequence includes the completion of the intake shaft structure well in advance of the tunnel excavation reaching the vicinity of the tunnel-shaft elbow. The tunnel-to-shaft elbow will be excavated by non-explosive methods. The overall project is approximately 39% complete and on schedule.
PAST FEATURES: Minnesota Library Access Center - University of Minnesota, Minneapolis, MN Addison Airport Tunnel Project - New toll road tunnel will ease Dallas traffic congestion Los Angeles Metro Red Line - Santa Monica Mountains Tunnels The Central Artery/Tunnel Project in Boston, Massachusetts. The Central Artery, a six-lane elevated road built in the 1950s to handle about 75,000 vehicles a day, is now jammed with over 190,000. A few years ago, with the specter of 14-15 hour a day traffic jams looming in our region's future, the Massachusetts Highway Department began construction of the Central Artery/Tunnel Project. |
|
Lake Mead is a man-made lake created with the construction of the Hoover Dam, near Las Vegas, Nevada, in the 1930s. Lake Mead Intake No. 2 is one phase of the Southern Nevada Water Authority's Capital Improvements Program to increase the capacity and upgrade the reliability of the Las Vegas Valley water supply and delivery systems. This specific project will benefit the community by ensuring an adequate and safe water supply.
The new intake, located on Saddle Island, includes a 12-foot-diameter intake shaft approximately 200 feet below the surface of the lake and a 1,600-foot-long tunnel leading to an underground pumping chamber. Submerged pumps in the underground chamber will pump water from Saddle Island to the mainland. The water will be conveyed through a series of pipelines, tunnels, and pumping stations to a new water treatment facility in the Las Vegas Valley. The existing Lake Mead Intake No. 1 is already located on Saddle Island, approximately 400 feet north of the selected site for Lake Mead Intake No. 2.
The access shaft for construction and maintenance of the underground facilities will be located approximately 40 feet north of the centerline of the intake tunnel, offset to reduce the risk of accidents from falling objects. The shaft will have a finished diameter of 20 to 30 feet. The shaft will be lined with reinforced cast-in-place concrete.
The 1,600-foot-long intake tunnel will connect the intake shaft, the access shaft and the forebay. The diameter will gradually increase from 14 to 25 feet. The purpose of the transition is to smooth water flows from the intake tunnel to the forebay.
An array of 22 wells will be constructed approximately 30 to 40 feet below grade. Deep well pumps will operate inside 50-inch steel well casings extending to the arch of the Forebay. The wells are located in two north-south rows of 11 wells. Pilot holes will be drilled using directional drilling from the ground surface to the forebay arch to set the location and orientation of the well.