Static fluid pressure. What did I do wrong?

[frozenguy]

Homework Statement

Calculate the depth h of mud for which the 3m concrete retaining wall is on the verge of tipping about its forward edge A. The density of mud may be taken to be 1760 kg/m³ and that of concrete to be 2400 kg/m³

The Attempt at a Solution

So I made A actually at 1.5m down from the top of the concrete wall. I figured Rb=R from the mud because of Sum of Fx. Then I just considered the weight of the wall as well.

I assumed for a 1m wide slab.

 

[RTW69]

Does the problem state that the concrete slab is rotating about "A"? It looks to me that if the slab gets pushed over by the force of the mud it will rotate about "B". If you sum the moments about B you can get rid of the force exerted by the ground on the slab at that point. Where did the .8 m slab width come from?

 

[frozenguy]

Does the problem state that the concrete slab is rotating about "A"? It looks to me that if the slab gets pushed over by the force of the mud it will rotate about "B". If you sum the moments about B you can get rid of the force exerted by the ground on the slab at that point. Where did the .8 m slab width come from?

The 0.8 came from the book, I assumed the 1m depth into the paper.

Yeah that makes about it tipping over its forward lower edge, I just thought I could work it out with b in there.

But summing around that point seems like it makes more sense, plus my moment arms will be easier to calculate.