WASHINGTON — One bidder wants to build President Donald Trump’s 2,000-mile wall with impenetrable concrete made from the soil found on the Mexican border.

Another proposes a see-through wall — but only from the American side.

Tuesday they joined more than 500 other contractors submitting bids to the Department of Homeland Security for small prototype designs of the wall Trump promises will stop illegal immigration from Mexico.

The bids did not answer the biggest question around the project — the cost, estimated by Trump’s team at $12 billion and Homeland Security at $21 billion, though no one knows for certain the eventual price tag.

Interviews with several contractors planning to submit bids provided a picture of obstacles facing the wall’s construction.

Roads, for example, will have to be built in desolate areas to bring in concrete and other materials, said Kay Smith, owner of Haltom City, Texas-based Diversified Highway Products. She proposes to use border soil for concrete to save money and time.

The project also faces an assortment of environmental and other challenges, said Scott Fee, who signed up to get details for his construction management class at Minnesota State University in Mankato so they can build a mock-up.

For example, how to deal with wildlife, many of them endangered species, who could find their feeding and mating grounds disrupted. Also, the structure cannot interfere with cross-border waterways, including the Rio Grande River, under a 1947 treaty with Mexico. 

Michael Evangelista-Ysasaga, CEO of Fort-Worth general contractor The Penna Group, said he talked with border agents and they want to see through the wall into Mexico. But they don’t want bad hombres on the other side to see them.

“This is an epic project,” he said. “The last time anybody tried anything like this was in China” – a task that took centuries to complete to prevent the incursion of barbarian nomads. The Great Wall of China was built with millions of Chinese laborers and is now the country’s main tourist attraction.

The purpose of the mock-up bids for Trump’s wall is to get an idea of what it will take to build a barrier that can’t be penetrated under, through or over. It has to meet the president’s vision of an inviolable “contiguous, physical wall.”

Homeland Security in January sought models for a solid concrete wall that would be as high as 30 feet high and anchored many feet into the ground but also be scenically pleasing from the U.S. side.

Beyond that, the department is looking for ideas. It plans to choose 20 bidders to build a 30-foot long, actual-size prototype in San Diego.

Bidders interviewed said they could build the model for $200,000, the pre-solicitation price. But they declined to guess the entire cost of the entire wall, which will cut through mountains and terrain “that looks like you’re in a Mars rover,” said contractor Evangelista-Ysasaga.

To some bidders, the project’s biggest obstacle is the strong feelings by the wall’s opponents over the cost and the necessity to wall on the U.S. southern border.

Evangelista-Ysasaga said he got a taste of the deep-seated dissention from threatening phone calls once he publicly indicated interest in building the wall. He said many Hispanic-owned construction companies decided not to submit bids because of the opposition.

The Fort Worth contractor said he decided to proceed out of concern other contractors might propose dangerous security measures like electric fences. “Nobody wants to hear about a kid dying on the wall,” he said.

Evangelista-Ysasago has appeared on NPR and other news outlets, noting he is the grandson of illegal immigrants. “Not many people could channel the right message” to build the public support necessary to avoid conflict, he said.

Drew Manatt said his family’s Brooklyn, Iowa, construction company, Manatt’s Inc., decided not to submit a bid because the company has many Hispanic workers. “Their perception of the project is so negative the board was fearful of a negative impact internally,” he said.

Instead Manatt is submitting its idea through another company, STS Construction Concepts of California. Their concept creates an access road along the border of the four states next to Mexico, then uses the roadway’s bed for erecting the wall.

Manatt said his design would try to build support for the wall by letting people buy panels to etch their names and messages, though the government might object to graffiti no matter how artsy it might look.

James Carpenter, an Army veteran who helped build perimeter walls around military installations in Afghanistan, said he would mix concrete with stone masonry, a barrier he said deterred suicide bombers.

“I’ve seen suicide bombers try to get through to blow themselves up and it didn’t work out too well for them,” said Carpenter, who said he was raised on the Mexican border and now owns a construction company in Mission, Texas, near the Rio Grande.

“This (wall) hits home for me,” he said.

Contact CNHI Washington reporter Kery Murakami at kmurakami@cnhi.com.