The problem with fortifying just the top 80 feet is stone shape. Bore a 1' wide hole (which is enough for Line of effect) down (let's say 75 feet), then set off the spell, since you now have a nice LOE. A wand of stone shape at CL 5 drills down 15' per casting, and only costs 5625gp and 225 xp. With 50 charges, you can dig down a total of 750 feet, although for each casting you'll only go down far enough to ensure maximum tunnel destruction.
If you use 5 charges, you get down 75 feet. That ensures you use all almost the spell's area of effect, instead of wasting half of it. Since earthquake has a range of long, the spellcaster could be flying above, and not worry about getting any damage himself for being underground.
I'm pretty sure that dwarven tunnel networks cost more than 6k gp to build.