geotech six minute solutions prob 38

[jamie]

can anyone explain the solution to this problem? I understand the concept, but don't understand the math they are using. The total head loss in the problem is the difference between the two free water surfaces - which is 12 + 15 = 27 on the left side and 12 on the downstream side, so the total head loss is 15 ft. Each layer of soil contributes to this head loss, h1 for head loss through layer one and h2 for headloss through layer 2. Why do they show these as adding to 12, the head at the bottom of the second layer? Shouldn't the head loss through both layers equal the total head loss, being 15?

doing an energy balance on the water, you should end up with 27 = 12 + h1 + h2

 

[sac_engineer]

I agree that the total head loss is 15 feet, not 12 feet. However, if you solve it through, the closest answer is still "C". You might have to check if there's an errata on ppi2pass.com for the version you have.

Good luck!

 

[crogmobulon]

FYI, I printed the errata for 1st ed. 1st printing of 6-Min.-Geotechnical. There are about 4 pages of it.

The errata for problem #38 is:

"Replace option C with "900 lbf/ft2"."

That's all there was for that problem, but maybe it will make more sense.

I have the complete set of 6-Min. solution books, and printed the publisher's errata for all of them. For some reason, there were more errors in the Geotechnical book than all of the rest combined. I have made it a point to go through each book and write in the corrections before I start working problems. Hopefully that will reduce the frustration level somewhat. I'm also doing the same with CERM & CERM Practice Problems.

I agree that the total head loss is 15 feet, not 12 feet. However, if you solve it through, the closest answer is still "C". You might have to check if there's an errata on ppi2pass.com for the version you have.
Good luck!

 

[jamie]

thanks all, i checked the errata for the edition and printing that i have (2 and 2) and they didn't note anything, but it certainly appears to be an error, unless there's something else i'm not seeing.