Exploration Drilling and Ground Stabilization
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 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 TBM. The starter tunnel runs through both rock and soft ground, and required a combination of shotcrete, rock bolts and steel ribs for reinforcement.
Vegas Tunnel Constructors
Crux provided exploration drilling services to Vegas Tunnel Constructors (VTC) on extremely short notice after the starter tunnel 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. 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 feet and ranged from 4 to 8 inches in diameter. Crux installed either TAM pipes or 6-inch diameter steel pipe, which VTC used as slick line tremies for grout placement. Once grouting was complete, Crux drilled confirmation holes to verify the 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 long-range horizontal core holes. The starter tunnel was accessed through a 30-foot diameter shaft, although the framework was such that equipment had to fit through a 10-foot by 10-foot opening. Crux’s equipment is custom-designed to accommodate confined space requirements 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. Additionally, three horizontal core holes were drilled to 400 feet 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.