Most contractors don't have to worry about installing a catwalk high enough that an 8-ton Asian bull elephant on the ground can't reach a human handler walking across it, but that was one of the many challenges of building the Denver Zoo's new 10-acre, $50-million Toyota Elephant Passage exhibit.

The most complex issue faced by the project team—led by the Denver office of general contractor Kiewit Building Group—was creating one of the most sustainable animal habitats in the world. The team recently learned it had succeeded when the exhibit became the first large-animal complex in the U.S. to receive a LEED-Platinum rating.

"In the zoo community, this is a landmark project," says Tom Black, Kiewit's construction manager for the Toyota Elephant Passage (TEP). "There's nothing comparable to this exhibit anywhere else in the world."

Planning, design and fundraising for the project, which was called Asian Tropics until Denver-area Toyota dealers donated $5.4 million to complete its construction, began in 2003. It broke ground in December 2009 and opened June 1. The project employed more than 300 workers who had to work around not only the animals, but also the 2 million people a year who visit the zoo.

Structures at the Southeast Asia-themed project—the 116-year-old zoo's largest, most expensive exhibit to date—include a 20,000-sq-ft elephant facility, a 8,500-sq-ft rhinoceros and tapir building, a 5,700-sq-ft multi-use pavilion, a 1,200-sq-ft leopard building and a 920-sq-ft home for white-faced gibbons.

TEP's largest sustainable components are its underground water-recycling system for animal swimming pools, which contain 1.1 million gallons of water, and its cutting-edge biomass gasification system, which creates clean energy from human trash and animal waste.

The entire zoo already has ISO 14001 certification for all its operations, the first U.S. zoo to achieve that distinction.

"We want to be a zero waste facility by 2025," says George Pond, the zoo's vice president of planning and capital projects. "Anything we can't convert into energy and use on site will be reused or recycled."

Complex Gasses

With help from the National Renewable Energy Laboratory in Golden, Colo., the zoo took three years to figure out how to handle the waste produced at TEP. One elephant alone can produce 200 pounds of waste a day, and the exhibit can accommodate as many as 12 elephants.

"A big question was what could we do with our elephant poop," Pond says. "We looked at anaerobic digestion, but the reality is we didn't have the quantities of poop or the space for it. At that point, we started down the road of looking at waste-to-energy using gasification."

The complex system designed for the new exhibit converts 98% of its waste stream into 85% of the energy needed to power the area, according to Kiewit, which built the structure that houses the system. Waste is processed into dry pellets, which are then converted to gas that produces electrical and thermal energy.

A boiler plant with four high-efficiency, gas-fired, 500,000-Btu condensing boilers augments the gasification system. The boilers generate heated water that is pumped throughout the exhibit via 31,000 ft of large- and small-diameter underground pipe.

"There was a willingness by the zoo to go all the way on energy conservation," says Sam Mosier, marketing manager at The RMH Group Inc., Lakewood, Colo., the project's mechanical and electrical engineering consultant. "They looked past where other facilities would stop."

Waste Water

The 1.1 million gallons of recycled water used by TEP's pools, which have a maximum depth of 10 ft, and its "lazy rivers" equals the amount of water used by the rest of the zoo. The water is continuously cleaned and disinfected by an onsite, closed-loop filtration system, including 20-ft-deep settling chambers. The zoo expects the filtration system to handle 1.7 billion gallons of water a year and save 350 million gallons.

The filtration system "keeps the pools consistently clean," says Greg Dykstra, partner at CLR Design Inc. of Philadelphia, the Denver Zoo's longtime design firm. "And the zoo won't have to dump and fill the pools all the time, wasting water."

The exhibit's other green components range from daylighting and extra ventilation for improved air quality to xeriscaping. Even the 50 cottonwood trees Kiewit had to clear to create the exhibit were recycled as interior paneling for buildings, mulch and scratching posts for animals. "No tree on site has been wasted," Black says.

Some parts of the exhibit are sustainable in another way—they must handle the force of multi-ton animals. Walls, fences and gates are reinforced with steel to withstand 10,000 to 15,000 lb of pressure from elephants and rhinos leaning on them. Peter and Ella's Crossing—a 17-ft-wide, 12-ft-tall, trestle-like overhead bridge near the main entrance—can hold two 7-ton elephants.

A 100-year-old brick storm sewer unearthed during excavation had to be replaced, partly because of concerns it couldn't support the weight of huge animals walking above it.

Structures must also take into consideration an elephant's ability to reach 5 or 6 ft with a trunk that can weigh 300 to 400 lb. The soft substrate floors in the elephant building, for example, are 4 ft thick, with foundations below that and a sub-slab beneath depressed areas so that elephants can't get to the pipes under the floors.

"You have to be aware of reach distances …. Elephants have trunks, which compounds what they can do; they can reach significant distances," Dykstra says. "You have to be aware of an elephant's mass, weight and power, and the things they might do to walls and gates."

Crews even had to adapt how they work in the zoo setting, including using quieter tools because the noise and vibrations sometimes upset animals in other exhibits. At the end of construction, the zoo used a sophisticated procedural system so animals moving into TEP and crews moving out didn't interfere with each other.

The Toyota Elephant Passage is just part of the zoo's ongoing millennium master plan, which includes $125 million in long-term improvements—especially green ones.

"Zoos push conservation, so they need to demonstrate it in their own backyard," Dykstra says. "There's pressure on zoos to walk the talk."