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2013SprShotcreteEMag

Soudan Mine Shaft Rehabilitation By Shane McFadden t First built in 1882, the Soudan Mine was approximately every 4 ft (1.2 m) along the shaft.off the footwall of the shaft. The rails rideon structural steel sets that are spacedhe Soudan Mine is a historic taconite andiron ore mine in Northeastern Minnesota. one of Minnesota’s only and deepest under- For the cages to ride smoothly along the length ground mines. Mining continued at the facility of the 0.44 mile (0.71 km) deep shaft, the rails until the US Steel Corporation closed the and the corresponding supporting steel sets facility in 1962, when production became too needed to be properly aligned. To accomplish costly to sustain the operation. Over 80 years this, the sets were supported against the irreg- of mining production, more than 50 miles ular shaft walls with rough timbers harvested (80 km) of underground drifts, adits, levels, and from the surrounding forests (refer to Fig. 1, 3, raises were constructed, with the deepest being and 4). Approximately every 300 vertical ft Level 27 at 2341 ft (714 m) below the sur- (90 m) in the shaft, a concrete collar was placed face—689 ft (210 m) below sea level. The mine tight to the rock to provide additional structural was donated to the state of Minnesota in 1965 support to the shaft system. and is now operated as a state park by the Min- In March of 2011, the supporting timbers of nesota Department of Natural Resources the shaft caught fire approximately 100 ft (30 m) (MnDNR). In the early 2000s, the National above Level 27. This fire consumed the shaft and Science Foundation funded the construction of caused extensive damage from Level 27 up the a major research laboratory on Level 27 to shaft approximately 350 ft (110 m), including the conduct physics and other scientific experi- burning of the shaft support timbers, pump and ments. With the addition of the lab facility, the water supply lines, communications, power site serves the dual purpose of public recreation supply, warping of many of the structural steel and education along with the advancement of supports, and destabilized portions of the sur- cutting-edge science. rounding shaft geology. Access to the mine is through the main shaft The MnDNR chose to execute the repairs to and hoist system (Shaft 8). The hoist system is a the shaft on an emergency fast-track design- dual-cage assembly, meaning that there are two build contract and selected the team of Engi- cages attached to the hoist cables at all times. As neering & Construction Innovations, Inc. (ECI); one cage is lowered into the shaft, the other cage Engineering Partners International, LLC (EPI); is concurrently raised. Along with personnel, and CNA Consulting Engineers, LLC (CNA). equipment, and material conveyance, the shaft is After defining the lower and upper limits of the the main conduit for all of the utilities to the damage to the shaft, the team’s first task was to underground portions of the mine, including the temporarily stabilize the shaft, re-establish the supply of air, power, and water, and the removal shaft utilities, and assess specific damage to the of groundwater. The shaft is split into three bays: steel structure and the impact to the shaft two cage bays and one utility chase bay (refer to geology. At that point, the repair options were Fig. 1). evaluated. The following performance criteria When the mine was first developed, modern were established: drilling and mining techniques did not exist, • All shaft structural steel support timber making blasting rock very difficult and labor- materials needed to be removed; intensive. Therefore, it was desirable to have • The shaft structural steel support structure the shaft closely follow the ore body to mini- needed to be positively braced to the rock; mize the need to construct drift tunnels. In the • The rock surface needed to be stabilized; and case of the Soudan Mine, the shaft follows the • The shaft needed to be protected from future edge of the main ore body, which lies at a steep rock falls or spalls. angle (78 degrees) (refer to Fig. 2). Therefore, Many repair methods were evaluated that met the shaft cages are hoisted vertically but are the design criteria. After thorough evaluation of also supported horizontally by rails installed the repair method options, the team determined 18 Shotcrete • Spring 2013


2013SprShotcreteEMag
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