It's called a ramp:
John Byrne
Published: Wednesday February 4, 2009
The planned 670-mile fence along the US border with Mexico has proven remarkably ineffective at deterring or stopping illegal crossers to the United States, advocates and critics of the fence admit.
Moreover, the fence's construction remains in limbo in numerous areas, where legal, political and engineering obstacles have brought its implementation to a halt.
And, where the 600 miles of fencing are already up, along the borders of California, Arizona and New Mexico, "smugglers and illegal immigrants continue to breach the fencing that is up, forcing Border Patrol agents and contractors to return again and again for repairs," the Wall Street Journal noted Wednesday. "The smugglers build ramps to drive over fencing, dig tunnels under it, or use blow torches to slice through. They cut down metal posts used as vehicle barriers and replace them with dummy posts, made from cardboard."
The "rough" measure by which the Border Patrol keeps track of illegal crossings was down 18% for 2008. Crossings appear down in parts of Arizona. But a May report by Congress' research service found a "strong indication" that crossers had simply found new routes. Homeland Security Secretary Janet Napolitano once famously quipped that a 50-foot barrier would simply spur the invention of a 51-foot ladder.
http://rawstory.com/news/2008/Drug_smugglers_successfully_build_ramps_to_0204.html