Bearing Capacity on the PE Exam.

[HSNIC]

I am studying up on for the April 16, 2010 Exam. I have the Civil Engineering Ref Manual 10th ed. and the 6 Minute Solutions: Geotechnical.

For a problem with the GWT between the surface and the foot base, The CERM advises to subtract 62.4 from the dry unit weight before multiplying by the quantity depth to GWT minus Depth to footing. This is to calculated the surcharge term.

((Pq+Gammadry*Dw+(Gammadry-62.4)(Df-Dw))Nq

Six minute has something similar with the dry unit weight multiplied by the Df plus 62.4 times the quantity depth to GWT minus depth to footing base.

(pq+Gammadry*Df+(62.4)(2.5-4))Nq.

The solutions use this too. I don't see using the dry unit weight to get the buoyant weight. Seems on the conservative side.

Pg 36-8 of the C.E.R.M 10th Ed. Michael Lindeburg, PE
Pg 60 Six-Minute Solutions for Civil PE Exam Geotechnical Problems. Bruce Wolle, PE.

 

[fattdad]

It always helps when there is a question on the original post. Not sure what you'd like to get from the forum. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[HSNIC]

My question: Is using the dry unit weight to subtract 62.4 good practice to calculate for the surcharge term in the Bearing Equation?

 

[fattdad]

I would subtract the unit weight of water from the saturated unit weight - after all, Terzhagi has total stress minus pore pressure in his calculation of effective stress.

Just how I do it.

f-d

¡papá gordo ain’t no madre flaca!

 

[HSNIC]

Yeah me too. Saturated Gamma minus 62.4! That's how I learned it in Undergrad and Grad. We never made correction for overburden, but I understand that part.

I get these study books and my answer for bearing capacity is closer to the wrong answer than the correct one.

I hope the correct answer choice on the NCEES is by way of Saturated minus 62.4, not Dry minus 62.4.