Branch Mining Program (Gravel)

I've been writing a branch mine tunneling program, and it's been working fine until it hits gravel. I've been trying to gravel-proof it and for some reason every loop I try is ruining the program, with the turtle getting stuck in some forward digging loop. For the tunnels I've been trying things like this:
http://pastebin.com/z4YD29ne

Split into new topic.

for i=1,6 do
	for i=1,20 do-- Why have two loops inside eachother?
		turtle.dig()
		turtle.forward() 
		if turtle.forward() == false then -- THIS confuses me...
			repeat
				turtle.dig()
				sleep(0.25)
			until turtle.forward() == true
		end
	end
end

This program ks vry confusing to me.. I suppose you want to mine 120 blocks.. Fixed code here

for i=1,120 do
    turtle.dig()
    while turtle.forward() == false do
        turtle.dig()
        sleep(0.25)
    end
end