Limestone Mine, Iowa

When Skyline Construction, Inc. concluded that traditional excavation methods were not feasible to reach a deposit of high-quality limestone, they assembled a team of experts to design a New Austrian Tunneling Method (NATM) underground passage. A team of experts, headed by Peter Kolitch & Associates, was assembled to design the adit, specify the required construction products and construct it using NATM techniques. Kristian Loevlie of Shotcrete Technologies, Inc. was chosen to be a member of this team because of his extensive experience with NATM projects, shotcrete applications, and the formulation of shotcrete design mixes. In addition, Shotcrete Technologies, Inc. supplied the accelerators, superplasticisers, and retarders that were used in the project.

The adit was excavated using an Alpine Miner AM 50-P roadheader supplied by AMS Technologies. This unit, propelled by a crawler-system, utilizes a cutter boom outfitted with a rotating cutting system that features replaceable cutter teeth for aggressively digging into the ground at the heading. As the tunnel is excavated, it is immediately supported with a system of lattice girders, welded wire fabric, rock bolts, and 2 inches of shotcrete.

For shotcreting, a Shotcrete Technologies robotic arm unit was fitted onto a Belarus front-wheel-assist farm tractor equipped with a 59 horsepower engine. The three-sectioned telescopic boom is hydraulically actuated. With a remote control unit, the nozzle operator can precisely direct the nozzle anywhere towards the back or sides of the decline for applying the shotcrete.

The use of the NATM method combined with the products and experience of Shotcrete Technologies allowed a high quality limestone deposit to be reached in a safe and cost-effective manner while avoiding cost-prohibitive conventional methods.