The volume that needs to be filled is 11 x 45 x 0.5 = 247.5 cubic feet, say 250.
There is a lot of info if you Google pea gravel weight. The most frequent weight I saw in those postings was about 100 lbs per cubic foot. So the weight of 250 cuft of pea gravel is about
25,000 lbs or 12.5 tons.