Six-min solutions question 79

I got the answear 0.68 (A), I would like to know if anybody else got the same answear. The book solution said the answear is 0.80 (

Please respond.

I agree with the book's answer of 0.80. I won't show all of my work because I'm too lazy, but here are the weights and moment arms (m.a.) that I calculated:

Weight of wall: 8700# m.a. = 3 ft.
Weight of soil: 1033# (top layer) + 1904# (bot. layer) = 2937# m.a. = 5.25 ft.
Total resisting moment = 41,520 #ft

Using Rankine's simplified formula: k = .45
Lateral pressure from top layer: 0.45*(110.4)*(1.04)*(6)*(6/2) = 930# m.a. = 14ft
Surcharge pressure from top layer: 0.45*(110.4)*(1.04)*(6)*(12) = 3720# m.a. = 6ft
Lateral pressure from bottom layer: 0.45*(110.4)*(1.15)*(12)*(12/2) = 4113# m.a. = 4ft
Total overturning moment = 51,800 #ft

F = 41,520/51800 = 0.80

Thanks Phalanx, I'll check my results!