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Construction Methods of the Ancient Inca Offer Sustainable Lessons

September 16, 2010

By Tudor Van Hampton
This article originally appeared on Engineering News-Record

Civil engineers and other researchers working under a $90,000 National Science Foundation grant are studying the Great Inca Road of South America for clues to help modern society build roads, bridges and other infrastructure that last longer and have a less harmful impact on the environment.

Great Inca Road across Peru
Photo © C.J. Schexnayder
The condition of the Great Inca Road across Peru is imperiled by lack of maintenance and development. In this section near the town of Castillo, a new paved road cuts through the ancient thoroughfare.
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“Where modern engineers really try to tame the environment—cut through rock, move it out of the way—the Inca tried to work with the natural terrain,” says Christine Fiori, who teaches at Virginia Polytechnic Institute and State University, Blacksburg, Va., and is the research team’s lead investigator. “It might take longer, but it lasts.”

This past July, researchers spent 10 days in the Peruvian highlands walking about 30 miles of the road northeast of Huaraz; they used ground-penetrating radar to probe the subgrade, and they used satellite equipment to report findings in real time to colleagues in the United States. The team will hold a workshop next summer in which 20 university students—10 from the U.S. and 10 from South America—will be selected for a similar expedition next summer. The team’s findings, including video of the trip, are scheduled to go on display in spring 2013 at the Smithsonian’s National Museum of the American Indian in Washington, D.C.

The roughly 600-year-old main Inca road stretches 3,500 miles from Ecuador to Chile and includes about 20,000 miles of smaller roads, all ranging in elevation from sea level to about 14,000 ft. Road widths range from just a few feet to 60 ft. Much of the road was paved with stone and built using retaining walls, culverts and bridges to accommodate gushing mountain springs and rainwater.

“That environment is so harsh that if you don’t work with nature and you don’t pay attention to nature, you don’t survive,” says Clifford J. Schexnayder, a retired professor at Arizona State University, Tempe, and one of the researchers. “I think all their engineering knowledge came from agriculture, because they had to have the agriculture first.”

The team was surprised to learn that the road—even though parts of it are still in use today—is not built up with layers of subgrades, as were Roman thoroughfares. “They were only using rock on top,” says Manuel Celaya, a research engineer and doctoral candidate at the University of Texas, El Paso. He scanned the road this summer using ground-penetrating radar that would “look” under the road without disturbing the surface. “They didn’t do any other layering,” save for the small paving boulders about 8 in. to 12 in. thick, Celaya adds.

Because the Incas did not use wheels, as the Romans did, and traveled with llamas and on foot, why was it necessary to “pave” the road? It turns out the rocks may have performed double duty, researchers say. For one, the stones may have provided a simple, lasting surface that would not break down under the constant foot traffic. Second, the rocks may also have acted as a type of permeable pavement that allowed water to drain off the mountainsides without eroding the road.

“In most places, it was paved for water,” says Schexnayder. “When the Conquistadors came, they said that the road was better than the Roman roads in Spain.” Incas also built other stone structures near or integrated into the road—culverts, ditches, sidewalls—to accommodate water flows and support side slopes. Hydraulic pressure, as with today’s solid-concrete retaining walls and levees, was likely not a problem due to the permeability of these stones. Today, some asphalt and concrete pavements now are being designed with permeability in mind.

The Incas also understood how to harness water without damaging the surrounding infrastructure, researchers say. Under a 12-ft-long bridge seen on this summer’s expedition, a stream seems to have been engineered with hydraulic “jumps,” or gradual steps in rock that sits between the bridge’s abutments and under its flat, stone slabs. “It looked like it was purposely done so it would slow the water down—so it wouldn’t wash away the embankment on the other side,” says Edward Jaselskis, who teaches at Iowa State University, Ames. Under today’s bridges, he adds, “I don’t really see us using hydraulic jumps.”

Amazingly, the Incas also standardized much of the road’s design and construction—using local labor and materials—without a written language. Though the Andean civilization employed a system of accounting, called Quipu, using knots tied in rope, much of its knowledge is thought to have been passed down orally. “Oral training and memorization was the methodology in military training,” explains Jose Barreiro, a Latin America expert at the Smithsonian. “Education in engineering technique likely followed the same method.”

Maintenance was carefully engineered, too, though not with a method that would be practical today: Inca men paid a sort of national labor tax by regularly maintaining sections of the road near their homes and villages.

This summer, the team lugged satellite equipment to the road so they could discuss findings with Smithsonian experts while in the field as well as test-drive remote, two-way broadcasting tools, which may have applications in commercial construction in the future. “It’s kind of like we had our own Internet cafe at 14,000 ft,” says Jaselskis. “We can bring the jobsite to you.”

Though the Great Inca Road research is yielding some clues to sustainable building—such as how to observe the natural world and design with it in mind—how to implement the Inca way will require further study.

“Engineers today need to gain that perspective,” Fiori says. “And I don’t know how you put that into a curriculum.”

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