I've found it's better if you're GDI to mine the area between the tunnel and the power plant, and if you're Nod to mine the area past the rock by the power plant tunnel and also the area outside the other tunnel. That way, if people want to disarm these mines, they have to step into the lines of fire of the obelisk and turrets, or in GDI's case the AGT will WTFown anyone trying to disarm a mine in the open. This works a lot more effectively than mines in the tunnel... when I told people to use it against me as a test, I couldn't get through, period. I can get through every time if mines are in the tunnels, that's easy to avoid.