Going the way of the Gophers - Underground
The gopher is a critter known for its skill at digging holes. Holding to the tradition of its mascot-the Golden Gopher-the University of Minnesota applied an underground construction technique developed on campus to create space for archives and special collections as part of a new $41.5 million library.
The west bank of the Mississippi River, the location of the Elmer L. Andersen Library, is said to be ideal for such an underground structure. The geology of the site consists of 40 feet of soil, 28 feet of Platteville limestone, 2 feet of shale and 150 feet of St. Peter sandstone.
General contractor C.S. McCrossan dug through the soil and limestone on the bank's bluff to create a 42-ft.-wide by 50-ft.-high portal. The underlying sandstone was then excavated to create two 600-ft.-long by 25-ft.-high by 70-ft.-wide caverns totaling 106,000 square feet beneath a four-story aboveground library.
The caverns resemble the vertical lines of the letter H, according to Charles Nelson, project principal for Minneapolis-based CNA Consulting Engineering, which was responsible for design and construction management of the portal, caverns, associated tunnels and approach road. Two cross-passage tunnels cut across, with a third connecting the ends of the H.
Portal construction was the major challenge of the underground phase of the project, says Nelson. "The portal was probably 20 percent of the effort," he says. "It's a unique structure, curved, using very thin concrete. It gets its strength from the curvature."
The portal acts as an entry point and as a loading dock, and is large enough to accommodate tractor-trailers and heavy construction equipment on three lanes of concrete roadway that run the length of the caverns. "The portal works well because you don't have to rely on vertical conveyance," says Nelson.
The sandstone lent itself well to excavation, while the limestone provided a strong ceiling for the caverns. CNA designed a system of sandstone pillars left in place to support the limestone roof. The cavern walls were lined with 405 curved, precast concrete panels designed to bolster the pillar supports. According to Nelson, 10-ft.-long rock bolts placed every 50 square feet were used to "clamp" together the limestone layers with 69,000 pounds of force, creating one monolithic layer. The underground construction method used on the project is homegrown. "Years ago, the university's Department of Civil and Mineral Engineering saw this as a developable area and conducted research on mined construction," says Donald Kelsey, library facilities planning officer.
Collections call for caverns
Why store the archives and collections underground? Limited space on campus and the desire to keep archives and collections nearby were principal factors. Much of the material had been stored in several substandard facilities off campus.
"To build this type of storage facility above ground would have required 5 or 6 acres of land," says Kelsey. "And we could not have put it any closer than a mile and a half from campus. By putting it underground, we were able to have it on campus."
Archives from eight university disciplines were scattered around campus and Minneapolis, according to Kelsey. "Two sets of archives were housed in an old coffee warehouse," he says. "Another was housed in the subbasement of the old main library building, which was subject to periodic flooding and home to cockroaches."
"There are some very rare and valuable collections in the facility," says James Scott, library project architect with Minneapolis-based Stageberg Beyer Sachs Inc.
Natural climate control
The facility has enabled the university to relocate 250,000 volumes from its libraries. In all, the facility will house 1.5 million volumes for the university and state libraries.
In addition to satisfying spatial requirements, the caverns offer a climate beneficial to the preservation of the stored materials, which are susceptible to changes in temperature and humidity. "The natural conditions inside the cavern are almost perfect for the preservation of paper," says Kelsey. "It's a big space with negligible heat gain or loss," says Nelson.
Moisture content in the air is particularly important in the university's northern climate. "In the winter in Minnesota, it's almost impossible to maintain a higher moisture level," says Kelsey. "The air gets so dry in the winter."
To ensure temperature and humidity control and to protect against fire and water damage, two single-story precast concrete structures occupying 43,000 square feet each were built inside the caverns. "The structures are operated at 62 F and 50-percent relative humidity," says Kelsey. Electrical and HVAC feeds were run from the subbasement of the main aboveground building through a shaft and into the caverns and precast concrete structures.
The free-span structures consist of 181/2-ft.-long by 101/2-ft.-wide by 6-in.-thick load-bearing wall panels with 30-by-4-by-6-in. gabled, precast single-slab roofs. The gables come together in the center of each roof, where they are supported by rock bolts drilled into the limestone layer above. The buildings sit on foundation slabs poured over moisture-resistant neoprene membranes. A final layer of moisture protection was the application of a 60-mil butyl rubber coating over the buildings.
Fire and life safety
As with any underground structure, the threat of fire always is a concern because of egress and smoke issues. "For a structure such as this you are required to have life safety that is equal to a high-rise building," says Nelson.
There are three exits from the caverns: the portal; a shaft from the main aboveground building that contains elevators, book lifts, stairs and mechanical ducts and an emergency egress at the rear of the cavern.
In case of emergency, 4 feet of space is provided between the sides of the concrete structures and the cavern walls, as well as between the structures' roofs and the limestone ceiling of the caverns.
"Because the space between the walls and the caverns is space used for exiting, we had to cover the waterproofing material with a fire-resistant material to keep it from burning," says Scott. An aluminum sheathing wrap with a butyl adhesive was selected to cover the butyl rubber.
The university may only be scratching the surface of what is to come with the construction of the two storage caverns, which were completed last November. According to Nelson, there is room to build 18 such caverns on university property. "The caverns are part of a larger plan involving construction of more portals and tunnels that would allow for more structures," says Nelson.
Instead of burying their collective heads in the ground, these Minnesota Gophers appear to be burrowing their way into a future founded on mined construction.