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!


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! :slight_smile: 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.


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.


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


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!