Tunnels and hydro power
The Missouri River flows through four diversion tunnels running under the east abutment of the Fort Peck Dam. How they got there is quite a story. Gangs of workers took turns cutting into the shale with coal saws that would pivot about an axis to make a 15-foot cut. Then the material was blasted out of the tunnel, scooped into railcars and removed while more digging commenced. This happened day in and day out. Three shifts totaling 4,000 men worked on the tunnels day and night, removing about 5 million cubic yards of material to make way for the tunnels. Residents grew used to the constant noise of the blasting. Serious landslides occurred during the excavation, due to bentonite fault seams in the bedrock. The bedrock itself, known as bearpaw shale, was extremely high in water volume and some 300 yards thick.
Bag of marbles
When this unique, ultra-moist shale was dried by the arid Montana air, it began to crumble. If wetted again, it took on a slick, muddy consistency with all the stability of a bag of marbles. To keep the shale in its original state, a subsurface humidity level of at least 90 percent had to be maintained by using atomizing sprays. The diversion tunnels are all more than a mile in length and are more than 24 feet in diameter. All the steel tubing used for the tunnels was bolted and welded together and a 3-foot concrete lining placed outside of them. Each tunnel has an intake tower and is capable of carrying the normal flow of the river without help from the other three. Each tunnel has an emergency control shaft and a main control shaft. Control gates are located near the axis of the dam, housed in reinforced concrete shafts that extend upwards to the ground surface. Concrete structures house the electrically operated control machinery.