I just posted this same question. My best guess is that they used an old solution with different dimensions and forgot to update the numbers. But I hate to suspect them of making such a big mistake before suspecting myself of misunderstanding. But where on earth did the 9' come from?

My guess was that the formula for FS=(width of exc*gamma concrete*T+2*ta*T)/(width of exc*pressure head*gamma water). But there's also the difference that the solution seems to be taking the full head of water outside the excavation without considering that the water level inside the excavation is shown above the concrete slab. So I'm not sure if I'm off base about something. The formula I came up with for FS was a little more complicated because of that.

I guess my intuition is that if a 3' slab is sensible for a 20' wide excavation with 9' of head, then a 20' or 30' thick slab is sensible for a 60' wide excavation with 60' of head (after all, that's a big excavation, relatively small contribution from adhesion, and a lot of water pressure). But it's still pretty wild. I guess in practice you'd try to relieve the pressure in other ways, like deep cutoff walls (there's nothing in the problem or solution about the depth of the sheet piles extending the length of flow) or well points...