Civil PE Exam - Geotech - How to Solve for Immediate Settlement using Schmertmann's Method (Updated)
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- เผยแพร่เมื่อ 12 ต.ค. 2024
- 👀Today, Cody Sims jumps on to solve a nice civil PE problem dealing with finding immediate settlement using Schmertmann's method. This is a perfect geotech problem that you may be hit with. Hit pause and see how you do it, then jump in and let's solve it together.
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The units for H1, H2, Hi should be in (ft^3)/lb. That way, when you solve for Si, you end up with feet (psf * (ft^3/lb)) = ft. The values you used were in the correct units but you did not write down the correct units
Correct.
You NEED to use NET PRESSURE. NOT THE APPLIED PRESSURE. THIS IS INCORRECT.
Why do you use the full 4’ of layer 1 even though only 1’ of that layer is below bottom of footing?
This is a great point. Looking back, you're exactly right.
Six feet (measured from the existing ground surface to the bottom of the foundation) should not be included in the 10 year settlement calculations because the foundation has no effect on that soil. Expect this type of critical thinking and understanding for the exams. That could be a conceptual problem in itself.
This problem should have only included one foot as the depth for the first layer. I believe this will get you an answer of 5" (D), is that what you got? Thanks for pointing this out!
Three questions.
Why do we solve for all layers of Hi if the depth of our foundation is only in layer 1?
Why does DeltaP only account for the applied force and not the stress from the soil weight? It says in my manual it is the applied force at foundation depth so I had assumed that would include soil stress.
Finally if strain relief is not given and we have to solve for Po I had assumed in this scenario Po = DeltaP as the foundation is above the water table. (Which results in C1=0.5) is this correct?
1) We are solving for the 10 year settlement of the soil beneath the foundation, any soil above the bottom of the foundation will not be effected. As you can see, the deeper you calculate, the less effect the foundation will have on the soil below it. Look at the 2:1 Method and Boussinesq's Theory to see a graphical picture of this.
2) In V1.1 of the PE manual, it uses the wording of "Net applied stress at foundation depth" This means "Applied Force/Width of Continuous Foundation + Specific Weight of Foundation (Concrete=150 lb/cf)*Depth of Foundation - Specific Weight of Soil Above Foundation * Depth of Foundation" This will give you a new delta P of 370 psf. Please see above response to Henry as well for an update on how the layers should be appropriately calculated.
3) Not quite. Po is the effective in-situ overburden stress, aka stress caused by the soil above the depth of the foundation. Po in this problem would be (130 pcf of soil * 6' of soil depth to bottom of foundation)
Does this help at all? My apologies for the confusion, thank you for pointing this out!
@@codysims7465 thanks Cody this does help. Definitely been awhile since looking at foundations, but your explanation helped clear things up!
WHAT IS the "PE Manual?"
The PE manual is the PE Reference Handbook that is a free download located on your NCEES account.
The c2 equation use a + not -, c2= 1.4
I believe a plus sign was used in this problem for C2, can I help with any confusion you may be having about this?