Bear Swamp: Crane in a Hole
An underground powerhouse like the Bear Swamp Project is a very large excavation. Once excavated, the powerhouse is built very similar to all large structures. A normal building has access all around the outside perimeter for construction. The only access to an underground powerhouse is normally an access tunnel into the top of the powerhouse. The access tunnel is mined to the powerhouse and used to excavate the crown of the wide excavation. When the powerhouse is complete the only access will be through the access tunnel. While the access tunnel and crown of the powerhouse were being mined a pair of tailrace tunnels were mined from the surface into the deepest section of the powerhouse called the draft tubes. Raisebore drills reamed four foot diameter holes from the draft tubes up 150 feet to the floor of the access tunnel in the powerhouse. As the powerhouse was mined from the top to the bottom all the shot rock would be dumped into the four foot holes and hauled out through the tailrace tunnels in the bottom. When the powerhouse was excavated 20 feet lower than the access tunnel, the equipment used for excavation was trapped in the powerhouse. Supplies were lowered to the mining crews from the end of the access tunnel above them with a small crane. Stairways provided personnel access.
My crew was assigned to take a Manitowoc 4000 crane into the bottom of the completed powerhouse excavation, get it rigged up, and operational for erection of structural steel. The moving of the crane and set-ups are a normal heavy construction activity. Transporting, arranging components in proper sequence, and rigging up in an underground powerhouse is much different. All of the components from the top of the boom back to the house of the crane must be positioned exactly where needed for erection. The last piece to come underground is the house (main body) of the crane. The house is 40 feet long ,21 feet wide,18 feet high, and weighs 200,000 pounds. It propels itself on a set of large tracks with flat pads. Turns and re-aligning the travel direction are slow cumbersome operations. The house filled the 24 foot tailrace tunnel as it crawled to the powerhouse.
When the crane was completely operational in the powerhouse it had 165 feet of boom. There was room to swing the crane 180 degrees near each end of the power house. A small crane was used to position the structural steel where the Manitowoc 4000 could make the picks. The structural steel columns were installed vertically along each side of the powerhouse and attached to 2 inch rock bolts installed 40 feet into the rock during excavation. Four foot beams were installed on top of the columns high in the powerhouse. The beams had a rail on top. They would support the permanent 880 ton overhead crane which would service the powerhouse. The Manitowoc 4000 installed some of the structural steel for the various floor elevations in the powerhouse before breaking it down and removing it from underground.
Large cranes with 165 foot booms working underground aren’t common………………………..they are a result of leading edge design and engineering.