When I was doing some tests on a map I started a long time ago and just now got to giving it the finishing touches (will upload later), I discovered that workers cannot mine minerals under dweb. You can have a min wall only mineable from one side! And, because of dwebs it takes longer ranged units to hit a worker mining the wall, and with 2 layers of dwebs I don't think anything (maybe a tank) can hit it. I think there's a lot of ways you could use this for interesting gameplay.
if you simply place some unbuildable terrain on the side where the workers shouldn't harvest, nobody will harvest from there. Only in very rare circumstances.
Also it's not obvious that you can't harvest there - I heard that for the first time, too - so I'd rather not use this.
I don't remember which map it was on anymore, so I can't find it and check, lol. But it definitely worked out the way I explained above =/ The mineral block I think was not under the block, and the worker was. When told to right click on the block he just walked up to it and stood there, just how units trying to attack eachother under dweb do. I might have replay =/
Sorry but I make a test map, place some minerals under lots of dwebs and my workers (and the comp's) can mine just fine, they can't attack, but they can mine. Maybe it is a special situation?