# Practice Exam Form 1 Question #95

Could someone inform me as to what formula this is? I couldn’t seem to find it in the reference material. Or is this just something I should know?

Hey @jlangsdorf, this is one of those formulas you’ll want to know; it’s the formula for hydrostatic pressure (earth + water) against a retaining wall. You can find the formulas for it in the Building Structures book on page 283, but Brightwood also had a useful couple pages on this. I’m working to get some screenshots to share – stand by!

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Thanks @cat.heard9! This is useful information since I hadn’t come across it so far in any of my other studies. Black Spectacles videos included. Those pictures will also be helpful! Thanks again cat.

Hmmm alright, this might take me a day to find, as I need to run to my office to get the book (can’t find it online, unfortunately). @heatherrivera, do you know of any places that help explain the hydrostatic pressure equation pretty well? Building Structures is fairly complicated.

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I’ve yet to find a helpful diagram on this which surprises me because it is so vivid in my memory from structure class. As best I can explain it’s pressure = 1/2bh^2. The diagram is the cross section of the retaining wall; and the forces create a right triangle - with the greatest force at the base. You’re taking the area of this triangle. In this example the 40’ doesn’t matter.

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Yeah that formula is close to the ones I am finding but this is the base one I have found referenced for retaining wall design.image|369x500 Not the same thing but seems more right to my memory. It just seems the problem is ignoring active soil pressure coefficient.

Hey @jlangsdorf; I wasn’t able to find that diagram I was thinking of, but the one you linked is very similar. I think you’re right, the problem is ignoring the active soil pressure coefficient. It sounds like you’ve got a pretty good understanding of the question/formula, so I wouldn’t be concerned. Like Heather said, as long as you know the base formula and realize you don’t need the 40’ from the question, I think you’re good to go

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Great find! The equations look to be the same. You’re not ignoring the active soil pressure coefficient. In the given example, the lateral earth pressure (Pa) is the 30 lbs/ft^3; it already did the math of the coefficient. Finding Ka typically involves trig with finding sins and cosins which is a bit above what we need to know for the exam.

As @cat.heard9 mentioned, it sounds like you’ve got a pretty good understanding on the question and formula. You’ve already dug in deeper than most will; and NCARB will give you all variables you need to know.

Keep up the awesome work!

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