Southern Nevada Water Authority (SNWA) investigated the construction of a large diameter water-supply tunnel from their existing water treatment plant into Lake Mead’s Boulder Basin. The new intake would provide critical drought protection and water quality improvements for the rapidly growing Las Vegas region. The primary purpose of the new intake was to replace the existing intake should the water elevation of Lake Mead drop below an elevation of 1050 feet above sea level. The new intake would also allow access to higher quality water within deeper levels of Lake Mead. The new intake would extend some 12,000 to 16,000 feet into Lake Mead.
Southern Nevada Water Authority, MW/HILL – a joint venture of MWH and CH2M HILL, Crux Subsurface Inc., Quanta Subsurface
Crux provided drilling and geophysical services to locate and design the new intake tunnel system, and provided information for the tunnel system construction. Exploration work was completed in multiple phases of land-based and offshore drilling.
The project required minimal environmental impact, and Crux provided a drilling program that successfully accomplished this. To do so required drill fluid and spoils containment, specialty fueling systems, and the development of Best Management Practices for land-based and offshore drilling operations within the national park.
The land-based exploration program consisted of drilling HQ3 borings to depths of 600 feet on or near Saddle Island. The offshore exploration program consisted of drilling borings within the waters of Lake Mead. At the boring locations, water depths ranged from 10 to 350 feet, and the borings were drilled to depths of 300 feet below mudline.
The project geology was variable along the proposed land-based alignments, consisting of Precambrian-age metamorphic rock, Tertiary-age alluvium, lake deposits, and extrusive and intrusive volcanic rocks. Subsurface conditions under Lake Mead consisted of Quaternary deposits underlain by intrusive and extrusive volcanic rock, and deep un-cemented sedimentary units in addition to sandstone, siltstone, shale and limestone.
In addition to obtaining high-quality samples, Crux provided a suite of in situ testing completed by Crux Oriented Borehole Logging (COBL) services. In situ testing included optical and acoustical televiewer, natural gamma, temperature profile, permeability and dilatometer testing. Offshore drills were equipped with data loggers to record penetration rates, drill pressure changes and other drilling parameters.
Severe weather, in combination with poor geologic conditions, created a challenging drilling environment, especially for accessing offshore locations. Strong easterly winds created on average 7- to 8-foot swells. In order to facilitate drilling in these water depths and conditions, Crux developed a multi-string casing system to limit buckling of drill strings and allow for drill fluid recirculation and containment.