The charge would have to be inside against the concrete liner I would think..?? The water pressure would ensure enough blast force got directed to the concrete to rupture it.
Unless you have a standoff shaped charge, a backpack bomb, even against the liner, isn't going to do anything other than make a scorch mark. These things are HUGE - and all a shaped charge would do is gouge out some of the concrete. Most of these things in critical areas (where the Corps hasn't been hindered by morons) are a hundred feet thick or more.