This feat is currently listed and used as a 8-C feat in the only two profiles of this series, Nozomi Kaminashi and Kawai Hanabi, which is a big assumption as we don't see the size of the mentioned boulder in this panel.
So I'm going to calc this feat with a safe assumption for now, I will look the manga again to revise the verse in another occasion.
A boulder should have a size at least of 25.6 cm in diameter, so I'll use that with the sphere volume equation.
V = 4/3 * π * r^3
V = 4/3 * π * (12.8 cm)^3 = 8784.529756 cc
E = 8784.529756 cc * 8 j/cc = 70276.23804 J
70276.23804 J - 70.27623804 KJ - Wall Level
Energy to break a boulder = 70276.23804 J - 70.27623804 KJ, 9-B - Wall Level