[Rules] Can non-personnel units dig in?

[Greyfox]

I feel bad going down the rabbit whole again, once it looks like you extricated yourself from it, but the rules for digging in aren't all that great.
     Beginning dug in makes sense for many scenario's.  But I do have two major objections.

     1) Time -  According to rules it takes only 30 minutes to dig in provide one is not interupted. 
         a) No one digs in that fast - certainly not a foxhole.  As an infantrymen I can tell you that you, that soil and root system dependent, one can spend most of a night digging a foxhole.  Granted that is you and your battle buddy rotating in 15-30 minute intervals (one digging and one providing security).  It is exhausting.  God help you if you hit the water table before you can get at least 4 feet down.  
         b) The best you can do is Hasty Fighting Position (aka Hasty Grave) -https://www.google.com/search?client=safari&rls=en&q=hasty+fighting+position&ie=UTF-8&oe=UTF-8.  Does provide some protection from both direct and indirect fire, and better than nothing, but it is not what most of you probably envision when you think about digging in.
         c) Mortars, Guns and Tanks would take a hell-of-alot of effort to dig in.  Even with a larger crew it would take time to do it and do it right. Best thing to do if use the terrain (some natural low ground or berm) and quickly reinforce it with some extra digging.

    2) Once fox holes or hasty fighting positions are dug - they don't go away unless you fill them in.  Why do they disappear when you leave the area.

Mike