When done properly, conduit is laid that fiber is then run through. You don't re-dig to lay new media, you simply drag it through the existing conduit. That's why "dig once" is such a big deal.

http://www.muninetworks.org/tags/tags/dig-once

http://www.sfexaminer.com/sanfrancisco/san-francisco-seeks-t...

http://www.whitehouse.gov/the-press-office/2012/06/14/execut...

You don't redig for more fiber unless you really screwed up the initial dig. Always leave lots of room in the tubes to blow more glass down later.