I have a load bearing cmu block wall (roof ties into it) that a plumber had to access inside to get to a water line to abandon. To do this he knocked out approximately a 12" tall by 8" wide hole in the wall near the bottom with a hammer (wall is a 10' tall and 8x4x16 open cell block not filled or rebar). See image. [IMG]
Also, what can be done to cover back up the hole?