When the University of Minnesota, USA, built a new underground library, the job required moving 9,000m2 of precast concrete panels into caverns little more than 7.5m high. Precast lining was used because in situ casting would have meant installing heavy steel forms, placing concrete against the cavern walls and cool underground temperatures delaying setting. But moving and placing the 400 precast panels was also a problem. They each weighed 11.2t and stood 7.5m high – and had to pass through a 4.5m-high cavern entrance. Construction consultant Len Middleton of Middleton Engineering Associates modified a Caterpillar 325 excavator by removing the digging boom. He attached a mechanism to carry the panels, tilt them up and set them in place with only 50mm of vertical clearance. A three man crew was able to transport and place five panels in four hours. The ability to set fully cured concrete panels near the cavern face speeded the excavation, reduced costs and gave a better quality wall on the $42m project, Middleton said.
