ok here goes. the circumfrence of earth’s orbit is 940,000,000km when you find the volume of a ring that long, 50,000km wide, and 100 meters deep, you come up with 47e14 cubic meters. versus 1.08e12 volume of the earth.
somewhere one of us made a mistake.
your calculations also assume that 100% of the material on earth would be useful...
I used 1.083×10^21 m^3 for the volume of Earth as this is what I got from Wolfram Alpha.
http://www.wolframalpha.com/input/?i=volume+of+earth+in+cubic+meters