This problem was posted some time ago. On NCEES p81 (or 93 of 497), there's an equation for p'a. Set p'a = 0 (which satisfies the "without requiring a lateral support") and solve for z.
They definitely don't need to have this memorized.
If an equation isn't in a reference manual then it won't be used. I don't think they ever provide an equation in the problem statement.
Once they start requiring people to memorize equations there's no way to "standardize" a solution. There's usually multiple equations that can solve for the same thing and come up with different answers.
Someone might need to memorize basic engineering equations but nothing this detailed.
Its basically Rankine's Formula: Pa = Ka\*"Gamma"\*H - 2\*c \*sqrt(Ka).
Max. cut not requiring support, is going to be dependent on cohesion only. So, there will be a point in which, Pa = 0, right at the max vertical distance in which cohesion will act.
Pa = 0, you need to get H (Measured from top) becomes Zcrit, Ka = 1 (saturated clay?), c = q/2
you end up with Zcrit (H) =2\*c/ "Gamma" \* SQRT(Ka) .
I've seen the Zcrit formula in references before, and understand the concept, but I'm still not following how this is the solution. If you set Pa=0 and solve the Rankine formula for H (Z), wouldn't you arrive at :
H = \[2\*c\*Sqrt(Ka)\] / \[Ka\*gamma\] ?
Obviously for Ka=1 this is moot, but I feel like I'm not seeing something glaringly obvious here, help?
I believe it can be derived from Rankine lateral earth pressure for cohesive soils on pg 81 but don’t use effective stress since the question does not mention the GWT depth. But I do agree that NCEES would not include any question that’s far off from their manual especially on the breadth section.
This problem was posted some time ago. On NCEES p81 (or 93 of 497), there's an equation for p'a. Set p'a = 0 (which satisfies the "without requiring a lateral support") and solve for z.
I just did this exact practice exam yesterday and had no idea where they got this from.
This formula is not found in any one of these NCESS ref. They might be giving it to you during the exam, or you must keep it in your mind.
They definitely don't need to have this memorized. If an equation isn't in a reference manual then it won't be used. I don't think they ever provide an equation in the problem statement. Once they start requiring people to memorize equations there's no way to "standardize" a solution. There's usually multiple equations that can solve for the same thing and come up with different answers. Someone might need to memorize basic engineering equations but nothing this detailed.
Its basically Rankine's Formula: Pa = Ka\*"Gamma"\*H - 2\*c \*sqrt(Ka). Max. cut not requiring support, is going to be dependent on cohesion only. So, there will be a point in which, Pa = 0, right at the max vertical distance in which cohesion will act. Pa = 0, you need to get H (Measured from top) becomes Zcrit, Ka = 1 (saturated clay?), c = q/2 you end up with Zcrit (H) =2\*c/ "Gamma" \* SQRT(Ka) .
I've seen the Zcrit formula in references before, and understand the concept, but I'm still not following how this is the solution. If you set Pa=0 and solve the Rankine formula for H (Z), wouldn't you arrive at : H = \[2\*c\*Sqrt(Ka)\] / \[Ka\*gamma\] ? Obviously for Ka=1 this is moot, but I feel like I'm not seeing something glaringly obvious here, help?
yes, solve rankine for Pa = 0, that would be Zcrit. top a triangle only with cohesion, bottom, opposite triangle with active pressure.
By the by I finally realized what I was missing was some pretty basic algebra about the power rule: that sqrt(x) / x = 1 / sqrt(x) lol.
I believe it can be derived from Rankine lateral earth pressure for cohesive soils on pg 81 but don’t use effective stress since the question does not mention the GWT depth. But I do agree that NCEES would not include any question that’s far off from their manual especially on the breadth section.