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[–] NOT_TALLEST_SKIL ago 

I dont know but I do know they pump em full

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[–] NACHTJAGD ago 

Yes, as part of an agreement with Mexico. A vibration cable is placed into the tunnel as well as a copper wire that will act as a sensor when ground pen radar goes over it to see if the concrete is still intact. They will pump in very watery concrete from the US side with just Mexican officials watching their side to not let it overflow. The vibration wire is pulled back to the US side as the tunnel is filled to get rid of air pockets. The copper wire remains. Usual a few wheelbarrels are needed on the Mexican side to fill in the vertical shaft.

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[–] techieguyjames ago 

What is the reason for the copper wire?

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[–] NACHTJAGD ago 

As i said, the copper wire will help in finding the tunnel by ground penetrating radar to locate and make sure no new tunnel is created. It can also be used to bounce radio signals off to detect changes in the structure of the wire, which would indicate they re-dug some of the tunnel.