Barriers Along the U.S. Borders: Key Authorities and Requirements [January 27, 2017] [open pdf - 932KB]
"Federal law authorizes the Department of Homeland Security (DHS) to construct barriers along the U.S. borders to deter illegal crossings. DHS is also required to construct reinforced fencing along at least 700 miles of the land border with Mexico (a border that stretches 1,933 miles). Congress has not provided a deadline for DHS to meet this 700-mile requirement, and as of the date of this report, fencing would need to be deployed along nearly 50 additional miles to satisfy the 700-mile requirement. Nor has Congress provided guidelines regarding the specific characteristics of fencing or other physical barriers (e.g., their height or material composition) deployed along the border, beyond specifying that required fencing must be reinforced."
CRS Report for Congress, R43975
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html