: One question that comes to mind is, where did they put the excavated rock? In The Black Gate Opens, Tolkien comments upon the two large 'slag mounds' piled by the Orcs through endless years of labor before the Gates of the Morannon. If the Dwarves excavated such large amounts of material, why were there no 'slag mounds' near the East or West Gates of Moria?

: I suppose its possible that the excavated rock from the Western side could have been used in the construction of Ost-in-Edhil (doubtful, but possible), but there were no stone fortresses anywhere near the Eastern entrance. Unless, of course, the Dwarves simply dumped the extra stone down the naturally occuring shafts within the mountain.

Well, since (IMHO) they were not just building a kingdom but also mining for precious minerals, I would assume that they had a mill located within the mountain as well. The tailings then would be near the mill as opposed to being poured out the front as most old mines would.

-RobRoy