Construction crews have already replaced most of one layer of fencing along 22 kilometres of the San Diego-Tijuana border and started work on the secondary fence this week.

The stretch of old secondary barrier being replaced is a little more than a 2.5km shorter than the primary fence.

The replacement secondary barrier will run the full length of the primary one.

The eight prototypes, which Trump visited about a year ago, stand between the end of the old secondary fence and where construction crews have put up the first panels of nine metre tall bollards - concrete-filled steel posts placed close together - where the additional 2.5km or so of barrier will go.

The agency said it doesn't yet know when the prototypes will come down.

"That's always been the plan," the CBP spokesman said. "They just don't serve a purpose anymore."

The bits of wall, four made of concrete and four of other materials, cost between $US300,000 ($A420,871) and $US500,000 each.

They taught the agency a lot through the tests conducted, the CBP spokesman said.

The replacement barrier construction project is expected to last until early 2020.

Australian Associated Press