The Department of Homeland Security announced Friday that Secretary Kirstjen Nielsen has given U.S. Customs and Border Protection permission to ignore environmental and land regulations to expedite construction of a secondary barrier, or wall, that runs parallel to the primary one between Tijuana and San Diego.

The secondary wall is meant to run for 12 miles and is expected to be 18 feet tall, a sector spokesman told the Washington Examiner during a visit to the region last year.

The Army Corps of Engineers awarded the project in December.

The San Diego Sector is unique as a southwest border sector in that it has had a double wall for two decades, but it has still seen active smuggling efforts and increased illegal immigration in recent years. Despite the double wall, people were easily getting over or through both of them, prompting the sector to step up its infrastructure.

In fiscal year 2018, the Border Patrol's San Diego Sector, one of nine on the southern border, arrested 38,000 people. In the first four months of 2019, the region has hit nearly half the total number apprehended last year.

"Tactical infrastructure, when combined with the appropriate technology and personnel, significantly reduces the amount of illegal border entries and enhances the Border Patrol’s ability to secure the border," DHS said in a statement.

The "primary" wall, or that which is closest to Mexico, stretches 14 miles from Imperial Beach on the Pacific Ocean past the Otay Mesa port of entry and into the mountains, where it is difficult to travel on foot or vehicle.

[Also read: 'A WALL is a WALL': Trump mocks Congress for talking about fences, barriers]

The main wall was installed in 1991. It consists of thousands of 8- to 10-foot-tall corrugated steel "landing mats." The mats were surplus material from the Vietnam War that had been meant to serve as helicopter landing pads in rice paddies.

In the early 1990s, agents were making more apprehensions of illegal entrants in their 60-mile stretch of the border than all apprehensions seen across the border last year. Prior to 1991, barbed wire fencing was all there was to stop people from entering the country outside of a port, and it wasn't installed continuously across the border. In 1997, San Diego began installing a secondary wall.

In 2016, San Diego learned it would receive federal funding to replace the landing mats with bollard fencing, which consists of vertical steel posts that have been planted in the ground.

President Trump campaigned in 2016 to build a "beautiful" wall between the U.S. and Mexico. When he took office in January 2017, the barrier between both countries' 1,954-mile border covered approximately one-third of that space.

Last April, CBP announced several projects that would replace and build new barriers on 100 miles.

Friday's decision to skirt environmental review processes is the third since the fall. Late last year, Nielsen waived regulations for border projects in the Rio Grande Valley of Texas.

Nielsen stated in the Federal Register memo that she had the authority to make the exceptions under the Illegal Immigration Reform and Immigrant Responsibility Act of 1996, which states a chief can waive all legal requirements if a wall, road, or other infrastructure is immediately needed.