President Trump’s promised border wall is one step closer to construction as the U.S. Army Corps of Engineers begins testing selected areas for compatibility.

The engineers began drilling and taking soil samples for testing to determine what particular type of wall would be compatible with specific geographic regions, the New York Times reported. The testing process moves forward as the Department of Homeland Security (DHS) evaluates proposals for various prototype projects.

DHS spokesman David Lapan told reporters engineers are conducting the tests in El Paso, Texas; Santa Teresa, New Mexico; Calexico and San Diego, California; and in the Rio Grande Valley. The engineers completed testing in El Paso and Calexico, Lapan disclosed.

DHS previously classified the San Diego and Rio Grande Valley Sectors as priorities for early development of President Trump’s promised border walls. The president asked for $1.6 billion for the FY 2018 budget to build walls, primarily in South Texas, Breitbart Texas reported in May.

The plan is for the Rio Grande Valley Sector to receive 32 miles of a new border wall, and 28 miles of a new levee wall. The levee in Hidalgo County is designed to address flooding. The budget refers to the new border wall as “a physical wall.”

In June, CBP Acting Deputy Commissioner Ronald Vitiello revealed some of the preliminary plans for the proposed border wall prototypes, Breitbart Texas reported. Shortly after the unveiling of the plans, President Trump also floated the idea of including solar panels to generate electricity to help defray the cost of the wall he insists will still be paid for by Mexico. “Think of it,” President Trump told supporters at an Iowa rally. “The higher it goes, the more valuable it is. Pretty good imagination, right?”

The House Appropriations Committee approved the $1.6 billion for border wall construction in a spending bill earlier this week, the Times reported.

The president has been more narrowly defining his ideas of a series of border walls where they are necessary and would be effective border management tools. “You don’t need 2,000 miles of wall because you have a lot of natural barriers,” Mr. Trump told reporters in a conversation with reporters aboard Air Force One last week. “You have mountains. You have some rivers that are violent and vicious. You have some areas that are so far away that you don’t really have people crossing. So you don’t need that. But you’ll need anywhere from 700 to 900 miles.”

The process of building the prototypes is expected to begin in the next few months. Vitiello said the CBP would issue contracts to between four and eight companies. Each company will have 30 days to build their section of the wall. The prototypes will be built in San Diego.