Ground Stabilization | Geotechnical Construction
Lake Mead Intake No 3 (Shafts & Tunnel)
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Project Description
The Southern Nevada Water Authority (SNWA) planned the construction of a third water supply intake from Lake Mead to supplement the two already in operation. The reservoir currently supplies 90% of Las Vegas Valley's total water usage, and there is concern that the existing intakes will be unable to accommodate demand with continually decreasing water levels. The third intake will follow a 3-mile path under the deepest areas of Lake Mead and should be unaffected by lowered water levels.
A portion of this project consists of a 348-foot long horizontal starter tunnel that extends from a 600-foot vertical access shaft to provide an entryway for a custom-built Tunnel Boring Machine (TBM). The starter tunnel runs through both rock and soft ground, and required a combination of shotcrete, rock bolts and steel ribs for reinforcement.
Team Members
Southern Nevada Water Authority, Vegas Tunnel Constructors, Brierley Associates, Crux Subsurface Inc.
Crux’s Role
Crux provided exploration drilling services to Vegas Tunnel Constructors (VTC) on extremely short notice after the starter tunnel for Lake Mead Intake No. 3 experienced an inflow of material. Crux drilled 12 exploratory, vertical boreholes to depths of 600 feet and identified a void at a depth of 150 feet below the surface. HQ3 coring and air rotary methods were used to complete these exploratory borings. VTC contracted Crux to drill an additional 22 holes above the tunnel alignment as a means of ground stabilization. The grout holes were drilled to 600-foot depths and ranged from 4 to 8 inches in diameter. Crux installed either TAM pipes (Manchette Tube Systems) or 6-in diameter steel pipe, which VTC used as slick line tremies for grout placement. Once grouting was complete, Crux drilled confirmation holes to verify if subsurface conditions had improved.
Simultaneous to surface drilling, Crux drilled from the face of the entrance to the starter tunnel. Underground work included the installation of gravity dewatering holes and the drilling of long-range, exploratory, horizontal core holes. Access to the starter tunnel was through a 30-foot diameter access shaft, although the framework was such that equipment had to fit through a 10-foot by 10-foot opening. Crux equipment is custom-designed and built to accommodate low weight and confined space requirements on specialty projects such as this.
Inside the starter tunnel, Crux installed gravity dewatering holes at varying inclinations. The holes were 5 inches in diameter and up to 80 feet in length. Crux also completed three horizontal core holes to 400-foot depths to investigate rock quality for proposed realignments of the starter tunnel. An HQ3 triple tube coring system was employed during all horizontal drilling to minimize deviation in the soft, unstable terrain.
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To request additional information, please use our Contact Page or call 866-CRUXSUB (866-278-9782).
