Sorry, but mobile reserve does not mean, that the guys are standing on the street, waiting for the order to run to the fights.
I am quite sure, that units in reserve will have their own foxhole to seek cover. It will just be some km behind the frontlines. [;)]
Mobile reserve means that force is concentrated near roads/highways so it can quickly deploy, it can hide in woods if there is any. They are not dug in and spread because that increases response time.
They usually sit in some kind of barracks and it can be rotated with units on front lines for them to recuperate.
If you let your forces sit in muddy and cold trenches for more than few weeks half of them will be sick and you will loose more soldiers of sickness than of bullets.










