Construction for the Minnesota Library Access Center (MLAC), on the University of Minnesota West Bank Campus in Minneapolis, Minnesota, started in June 1997 and will be completed in July, 1999. The MLAC has two components - Archives Collection and Overflow. The Archives Collection will house precious, one-of-a-kind documents from collections around the country. Overflow storage will compliment the existing University Library system by providing space for infrequently accessed but important materials.

This University of Minnesota Archive project consists of two storage caverns, and a six level conventional surface building. The caverns will house special books for libraries through the state as well as the University Archive collections and will be off-limits except for center employees.

The mined caverns and tunnels have a footprint of 106,000 square feet and an excavated volume of 95,000 cubic yards. Each cavern is 600-feet long, 25-feet high, and 70-feet wide and is constructed in easily excavated St. Peter Sandstone and Glenwood shale with an overhead Platteville limestone roof. Two shafts will provide vertical circulation between the surface and mined space and emergency egress. The 50 foot high curved portal provides a 42 foot wide drive-in entrance for vehicles as large as semi-trailer trucks.

Because protection of the collections from water leakage is critical and careful climate control must be maintained, a thin precast concrete structure (walls and roof) with a high quality waterproof membrane was constructed inside of the rock caverns. It is suspended from the rock roof by rockbolts.

Geologic conditions required that the concrete wall construction closely follow the mined excavation of the cavern face, in order to provide the required support to the sandstone at the perimeter of the excavation. Curved concrete panels were pre-cast to the designed thickness of 8 inches and to a 30 foot radius. The panels were built to a nominal width of 10 feet, and weighed up to 25,000 lbs each. A panel handling machine was devised to transport the panels horizontally through the 15 foot cavern entrance, and then tilted up and set in place. The ability to set fully cured concrete panels near the cavern face allowed the excavation to proceed at 10 feet per shift.

The top and bottom edges of the panel were dry packed at the footing and against the limestone ceiling and a 4 inch thickness of high slump pea rock grout was placed behind the wall panels after they were set in place. With the top and bottom ends of the panel restrained, the concave panel acted like an arch to withstand the grouting pressures.

CNA Consulting Engineers, Minneapolis, MN, provided the following services for the MLAC mined space, shafts and portal structure: feasibility study, preliminary and final design, and construction administration.

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