SAN DIEGO – U.S. Border Patrol agents, sweating under a hot Texas sun, squared off against an array of fences. They swung axes at posts, used blowtorches to melt steel, tore through sheet metal with crow bars and scaled walls with ladders.

Government engineers with the agents rammed remote-control SUVs loaded with 10,000 pounds of sand into the barricades at 40 mph.

Together, in a nine-week project called Fence Lab, they were trying to solve one of the nation’s most vexing problems – how to find fencing strong enough to protect the U.S. from one of the largest human migrations in history, but sensitive enough to the fact that Mexico and the U.S. are friendly nations.

Consider the government’s requirements: The fence must be formidable but not lethal; visually imposing but not ugly; durable but environmentally friendly; and economically built but not flimsy.

“It’s not that simple,” said Collin Sloan, whose Idaho company was among those submitting designs to Fence Lab.

The largest fence expansion in the history of the United States’ southwest border is under way, with more than 70 miles erected in 2007 and 225 miles planned for 2008.

Often lost amid the debate over how to control the border are the physical barriers themselves. Different terrains call for different approaches.

In 2007, the Department of Homeland Security began the search for new fences, inviting contractors to submit ideas.

Proposals had to meet certain specifications. The barrier had to be 15 feet to 18 feet high. It had to be able to withstand the impact of a vehicle moving at 40 mph. It had to be strong enough to keep smugglers from cutting through it in less than 15 minutes.

Sloan Security Fencing, Collin Sloan’s family-owned company in Idaho, submitted a double-layered, steel-mesh design influenced by fences in Europe.

The government tested nine barriers at Fence Lab, held at the Texas Transportation Institute at Texas A&M University in College Station. Security aimed to ensure that details about methods would not be leaked.

The dozen or so Border Patrol agents on hand came from the fence crews that roam the border to patch, weld and make other repairs to torn-up barriers. The engineers from Sandia National Laboratories, a Department of Energy research and development lab, called the agents “the red team.”

“They came in as the opposing team, the adversaries,” said Brian Damkroger, who oversees border security work for Sandia. “The team had a number of procedures and tool kits that they had developed and seen utilized in certain areas of the border, and they went through a systematic series of tests designed to judge the vulnerabilities of the fences.”

The red team attacked the array of fences with battery-powered saws, grinders, fire axes and ladders. One by one, in minutes, they reduced the fences to tatters.

“I think they were impressed by the inventiveness, and I know we opened their eyes,” said Chris Wells, the Border Patrol’s assistant chief from California’s El Centro sector. “We exposed them to a world that is normal to us but is unusual to them.”

The goal, experts say, isn’t to find a barrier that can’t be breached. The idea is to develop fences that slow down illegal crossers, agents say, allowing time to stop migrants before they disappear into border communities, known as “the melt zone.”

Innovations have come out of Fence Lab and similar testing projects.

Hollow steel tubing, easily cut by blowtorch-wielding smugglers, is now filled with concrete. Immigrants still get through it, but it takes more time. Rectangular posts have given way to harder-to-climb rounded ones. Fencing is taller.

In urban areas where the melt zone is a dash from the border, layers of barriers are erected.

In San Luis, Ariz., migrants who scale the landing mat fence now find themselves blocked by a new secondary barrier of steel mesh. Mesh is a favored material because of its tight cross-hatch pattern, which makes it difficult to find a toehold for climbing.

In rainy areas, however, mesh and solid-steel fencing won’t do because they impede water flow and can cause flooding. In the monsoon-drenched Altar Valley south of Tucson, Ariz., the government is placing tall steel tubes 4 inches apart, gaps too small for people but big enough so that water can flow freely.

Then there’s the question of aesthetics.

In the past, people found the steel-mat fencing such an eyesore that they painted it beige or covered it in murals. The federal government, sensitive to complaints from Mexico, doesn’t want new fencing to look like a wall.

“They want to make it seem like you could shake hands through the fence,” said Peter Andreas, a political science professor at Brown University who studies border security issues.

Whether the new fencing meets aesthetic standards remains an open question. One of the Fence Lab barriers that agents liked best is a double-mesh barrier of thick welded wires in a tight honeycomb design. The tiny holes between the wires make climbing difficult. Axes and crow bars are useless because the layers give under pressure. Blow torches get through, but it takes more than 15 noisy minutes to cut both layers.

Still, this summer a similar double-mesh fence went up along seven miles of the border in Naco, Ariz., and within days Mexican smugglers had found a way to defeat it. By inserting screwdrivers into the holes to use as handholds, they scaled the fence as if it were a pegboard.

“They get over in about 15 seconds,” said John Ladd, 52, whose 14,000-acre ranch abuts the border.

Even so, Border Patrol agents see progress.

Agent Sean King, based in Tucson, said only the most athletic migrants possess the strength to pull themselves over with screwdrivers, and they can’t do it en masse.

“Now, it’s one immigrant coming over at a time instead of 100.”

AP-NY-11-17-07 1359EST