6-Minute Soln. Problem #13 Terzaghi-Meyerhof

[crogmobulon]

Problem # 13 in 6-Minute Solutions Geotechnical describes a square footing on clayey-sand, and asks to solve for qult using the Terzaghi-Meyerhof equation.

In the CERM (11th) it states that this equation (36.1) is appropriate for a foundation in a continuous wall footing, and that shape factor corrections need to be applied to the Terzaghi bearing capacity factors Nc & Ngamma. The respective shape factor multipliers are provided in Tables 36.4 & 36.5.

Seems pretty straight forward, but when I did problem #13 in 6-Min., I of course came up with the wrong answer. When I checked their solution, they used the same bearing capacity factors that I pulled from Table 36.2 of CERM without applying the shape factor multipliers for the square footing. No reason was given in the solution for why these multipliers were not used.

Which approach is correct?

It's this kind of inconsistency that is really shaking my confidence.

 

[jamie]

i don't have the book with me, i'll look at it later, but i did this problem and don't recall it being ambiguous. are you sure it was a square footing? as i recall there was one they showed as a square footing, but said something about it being a section of a longer footing.

 

[sac_engineer]

The question asks for q-ult for a STRIP footing. Therefore, you need to use the following equation:

q-ult = (c x Nc) + (gamma x depth of footing (in feet) x Nq) + (0.5 x gamma x width of footing (in feet) x N-gamma)

There's no need for shape correction factors. Be sure that you properly identify the different equations for q-ult based on the shape of the footing (strip vs circular vs square).

Also, be sure to verify the Terzaghi bearing capacity factors for a given soil friction angle. I've seen different values of N-gamma for the same angle from different tables. Google these factors to be sure that you will use the most common values compared to those in the CERM.

Good luck!

 

[crogmobulon]

Here is the exact wording of the problem from my copy of 6-Minute Solutions for Geotechnical:

"Problem 13

Using the Terzaghi-Meyerhof bearing capacity equation and Terzaghi bearing capacity factors, what is most nearly the ultimate bearing capacity for the square footing shown?"

The illustration actually has the words "square footing" written across the representation of the footing with B=10'.

If your's asks for qult for a STRIP footing, maybe you have a more recent edition or printing of the 6-minute solution book. Mine is 1st edition 1st printing. I just went to the publisher's site and downloaded the errata, and alas it was there:

"pp. 3 & 4, Problem 13: Change "square footing" to "continuous footing". In the illustration, change the description to "continuous footing".......

So, I stressed out over nothing after all. I printed out the errata for the whole set. It should come in handy.

Thanks for looking into this.

Jeff

The question asks for q-ult for a STRIP footing. Therefore, you need to use the following equation:
q-ult = (c x Nc) + (gamma x depth of footing (in feet) x Nq) + (0.5 x gamma x width of footing (in feet) x N-gamma)

There's no need for shape correction factors. Be sure that you properly identify the different equations for q-ult based on the shape of the footing (strip vs circular vs square).

Also, be sure to verify the Terzaghi bearing capacity factors for a given soil friction angle. I've seen different values of N-gamma for the same angle from different tables. Google these factors to be sure that you will use the most common values compared to those in the CERM.

Good luck!