U.S.-Mexico Border Being Patched Up : Immigration: The fence south of San Diego is being reinforced to try to stem the flow of illegal aliens.

Along the southern levee of the Tijuana River, a crew of U.S. Navy Seabee engineers and Border Patrol welders labor to reinforce the 10-foot-high fence that marks the U.S.-Mexico boundary.

“They’d have to drive a truck through this to break it,” one of the Navy men said confidently after completing the addition of heavy-duty metal planking along a section of the battered fence.

A few miles east, several dozen men and women gather at the end of a half-mile stretch of reinforced fencing, casually walking back and forth across a frontier where there is no barrier, awaiting nightfall to make their dash into San Diego.

“They can put another fence, and another fence, and another fence--still people will cross,” said Manuel Hernandez, a 40-year-old who was among the many massed at the end of the marked swath of border. “If the fence is too high, we’ll just bring a ladder.”

The U.S.-Mexico border in San Diego County, the prime entry point for illegal immigrants heading from Mexico into the United States is undergoing major renovation. The tattered barrier has been long derided as easily breached. Navy engineering, or Seabee, units have been helping bolster the seven-mile chain-link border fence since late last year.

The goal is to improve the fence’s admittedly limited enforcement capabilities by installing unsightly, but to date effective, surplus military sheets that were designed for use in building portable airstrips.

The metallic strips cover about two miles of fence in the San Diego border area. Renovating the other five miles of fencing could take the rest of this year, if not longer.

The metal strips were surplus and the Seabee labor is being paid by the Navy.

The ongoing work has rekindled an old debate about the effectiveness that even the world’s most imposing barrier might have in an area such as the U.S.-Mexico border, a 2,000-mile boundary line.

Each day, thousands of people fleeing economic distress and civil strife pass through the porous boundary almost at will, crossing a landscape that ranges from deserts and rugged mountains to deep-rutted canyons and lush river areas, seeking opportunities unavailable to them at home.

Authorities say there is less than 30 miles of fencing between Mexico and the United States, although there are some scattered portions strung with metal cables, strips of barbed wire and other primitive dividers.

Even if workers reinforce the entire seven miles of fence in the San Diego area, perhaps another five miles of fenceless border will remain essentially open frontier.

Against such a backdrop, some experts ask, what good is any barrier?

“A better fence may make it more difficult to cross--the coyotes (smugglers) will probably charge more--but it will never stop people from coming,” said Jose Luis Perez Canchola, a Tijuana social scientist who studies immigration patterns. “I think it’s a useless expenditure. . . . They’ve said before that fences were indestructible, but people have always found a way to destroy them or get around them.”

Immigration and Naturalization Service Commissioner Gene McNary has said that barriers, combined with other enforcement techniques, can be an effective weapon at the border, particularly at well-traveled crossing zones such as San Diego. Agents say fencing helps to funnel border-jumpers away from heavily populated urban areas--such as the San Diego border neighborhood of San Ysidro--where they quickly blend into a maze of residential and commercial developments and are difficult to catch.

On a broader level, authorities say, improved barriers are an important component of an overall border stratagem that includes deployment of additional agents; use of night-vision equipment, better aircraft and other improved equipment; use of high-technology to unmask document fraud; construction of more detention space, and putting heightened pressure on employers who illegally provide jobs to aliens.

The improved fencing in San Diego has affected crossing patterns of migrants, who traditionally respond rapidly to enforcement strategies.

Some analysts favor a massive barrier-building campaign at the border. The Federation for American Immigration Reform, a Washington-based advocacy group that seeks to stem illegal entries, has called for the construction of “secure” fencing--perhaps including concrete structures--to be built along about 60 miles of key illicit crossing zones near San Diego and El Paso. At an estimated cost of $3 million a mile, such a grandiose plan seems unlikely to be implemented soon.

Moreover, any plans for new barriers beyond the repair and reinforcement of existing fencing would likely strike a discordant note in the border area, where residents are extremely sensitive to any kind of Berlin Wall-type schemes from Washington. Intense opposition to a highly publicized ditch planned for the Otay Mesa area of San Diego led to the scuttling of that concept last year.

Border Patrol crews seeking to deter so-called “drive-throughs” of vehicles in the flat Otay Mesa area are repairing existing metal cables strung in the area, sometimes using railroad track to help string the heavy-duty cable.

Even the biggest supporters of the improved fencing are hesitant to take the position that the reinforced fence deters the arrival of illegal immigrants. Rather, Border Patrol officials say, the idea is to cut off some entry and escape routes, improving the odds somewhat for the limited numbers of patrol agents who are nightly charged with the near-impossible enforcement task of stemming the arrival of thousands of aliens.

“It funnels them; it doesn’t stop them,” said John C. Johnson, a supervisory Border Patrol agent who headed Seabees and immigration personnel who were reinforcing the fence last week with the military material. “It puts them where we want them to be.”

Some would-be border-jumpers who watched the patchwork effort proceed last week seem bemused by the Border Patrol’s latest efforts to keep them out.

“When a Mexican says he will cross the border, that means he will cross the border no matter what,” said Juan Olegario Martinez, a 22-year-old from the interior state of Oaxaca. “Sometimes they’ll catch us over and over again, and sometimes it may take a week or more before we cross, but we will eventually make it, always.”