# Practice Exam Form 1 Question #95 **URL:** \ **Category:** Project Planning & Design\ **Created:** [March 19, 2020, 7:42pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943 "2020-03-19T19:42:24Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![jlangsdorf](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/jlangsdorf/32/420_2.png) [@jlangsdorf](https://community.blackspectacles.com/u/jlangsdorf)\ **Post date:** [March 19, 2020, 7:42pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/1 "2020-03-19T19:42:24Z")
![image](https://canada1.discourse-cdn.com/flex029/uploads/blackspectacles/original/1X/3bdf98d7f7dabd37b4d1964129b0e032798eccd9.png) 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? ---
**Author:** ![cat.heard9](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/cat.heard9/32/401_2.png) [@cat.heard9](https://community.blackspectacles.com/u/cat.heard9)\ **Post date:** [March 19, 2020, 8:06pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/2 "2020-03-19T20:06:07Z")
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! ---
**Author:** ![jlangsdorf](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/jlangsdorf/32/420_2.png) [@jlangsdorf](https://community.blackspectacles.com/u/jlangsdorf)\ **Post date:** [March 19, 2020, 8:11pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/3 "2020-03-19T20:11:29Z")
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. ---
**Author:** ![cat.heard9](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/cat.heard9/32/401_2.png) [@cat.heard9](https://community.blackspectacles.com/u/cat.heard9)\ **Post date:** [March 19, 2020, 8:34pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/4 "2020-03-19T20:34:30Z")
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. ---
**Author:** ![heatherrivera](https://blackspectacles.com/uploads/images/23.png) [@heatherrivera](https://community.blackspectacles.com/u/heatherrivera)\ **Post date:** [March 19, 2020, 9:28pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/5 "2020-03-19T21:28:08Z")
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. ---
**Author:** ![jlangsdorf](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/jlangsdorf/32/420_2.png) [@jlangsdorf](https://community.blackspectacles.com/u/jlangsdorf)\ **Post date:** [March 20, 2020, 1:53pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/6 "2020-03-20T13:53:00Z")
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](https://community.blackspectacles.com/uploads/short-url/4dWdeom8liIrcibYlr1e0JEz4w0.png) Not the same thing but seems more right to my memory. It just seems the problem is ignoring active soil pressure coefficient. ---
**Author:** ![cat.heard9](https://yyz1.discourse-cdn.com/flex029/user_avatar/community.blackspectacles.com/cat.heard9/32/401_2.png) [@cat.heard9](https://community.blackspectacles.com/u/cat.heard9)\ **Post date:** [March 20, 2020, 5:47pm UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/7 "2020-03-20T17:47:48Z")
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 ---
**Author:** ![heatherrivera](https://blackspectacles.com/uploads/images/23.png) [@heatherrivera](https://community.blackspectacles.com/u/heatherrivera)\ **Post date:** [March 21, 2020, 11:03am UTC](https://community.blackspectacles.com/t/practice-exam-form-1-question-95/943/8 "2020-03-21T11:03:02Z")
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!