20' x 26' x 0.25' = 130 cubic feet = 975 gallons = 6.5 tons of pea gravel.
Add more weight for the soil which will get vacuumed along with the
gravel. That's a lot of vacuuming, a lot of weight, and a lot of emptying of
a shop vac.

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George.com wrote
a good bagless heavy duty vacuum cleaner and simply vacuum it up.