We assumed a two man/dwarf crew in good physical shape. Tools were two pick axes, tweo square mouthed shovels, and an ore cart (I know that ore carts weren't mentioned, but we assumed that the dwarves would be smart enough to use even free ore carts, meaning no tracks laid) the cart would be standard meaning capable of moving one yard of material (that's about 3,000 pounds). We also assumed a work day of six to eight hours which was a typical shift for hardrock miners (working longer with hand tools wasn't practicle since you were more likely to serious injure or even kill you miners).

Of course, since dwarves are significanly shorter then men, a yard may have varied for dwarves, but I got the impression that none of the taller member of the fellowship had to duck, so I let that part go.

Now given that the dwarves were mining for precious minerals such as gold and silver they would most likely have been dealing with a quartzite ore base, which is particularly hard. Mithril is a tougher ore to get a grasp on what it would be found in, since diamonds are the hardest naturally occuring mineral, and it is mostly found in looser volcanic rock. It is very conceivable that there was a nice mix of ore since the Khazad-dum was a vast city.

But I digress. If we assume a worse case scenerio of quartzite, then the best a two man/dwarf team could hope to move is about 1/2 a yard to a full yard in a work-day (six to eight hours). This also takes into account that the dwarves were using standard crib and shore methods to keep the ceiling from falling on them (which would bring in additional dwarves to cut and shape the shores, but right now I am only assuming the actual work done in the mines).

Finally, and the big kicker, is that Khazad-dum was founded some time in the First Age, or perhaps earlier (I don't recall right off the top of my head). But the Balrog wasn't discovered until Third Age 1980. Even if all mining ceased (since Nain made a stand against the Balrog, and the city wasn't officially abandoned until 1981) then the dwarves had about 6,003 years to work on their city.

Then, if the dwarves have 365 days per year (and for the life of me I can't remember the number of days in a ME calender years) then that is 2,191,095 days in which to work (I know, I didn't calculate additional time for leap years, nor time off for various birthday parties, general and government holidays, vacation days, or various battle times, but this is an assumptive scenerio) that would be roughly 3,286,642,500 to 6,573,285,000 pounds of material moved.

Not too shaby for the little guys!

-RobRoy