## Bearing Capacity using N Value

## Bearing Capacity using N Value

(OP)

I have been practicing for my PE exam and came across a problem which is throwing me off. The problem is to determine the allowable bearing capacity of a mat foundation 40ft x 80 ft with soil unit wt 120lb/ft^3 located 8ft below ground surface with SPT of the soil being 18. Sounds simple, so I use the N value to get the friction angle (~32°) and then go on to get the Terzaghi's bearing capacity factors for that and compute the bearing capacity turns out be 19.74 Te/ft^2 after applying a FOS of 2. The answer however turned out to be 2.49 Te/ft^2. The author (lindberg) used the formula for a FOS of 2 qa = 0.22*Cn*N, where Cn is the depth correction factor (~0.69). This was very puzzling because of the difference of magnitude. I was hoping somebody with more experience can explain why?

The 2nd part of my question comes after doing more research I realized that most of the bearing capacity equations based on N Value for such as Terzhaghi and Peck (as given in Boweles) qa = 0.720(N-3)*(B+1/2B)^2

or Meyerhof qa = N/4(B+1/B)^2*Kd (where Kd = 1+ 0.33D/B) are very counter-intuitive. It almost appears that the bearing capacity is reducing with width of the foundation. I am assuming this is because of stress concentration near the center leading to greater settlement and thereby a lower bearing capacity, Can somebody please confirm if this is correct?

The 2nd part of my question comes after doing more research I realized that most of the bearing capacity equations based on N Value for such as Terzhaghi and Peck (as given in Boweles) qa = 0.720(N-3)*(B+1/2B)^2

or Meyerhof qa = N/4(B+1/B)^2*Kd (where Kd = 1+ 0.33D/B) are very counter-intuitive. It almost appears that the bearing capacity is reducing with width of the foundation. I am assuming this is because of stress concentration near the center leading to greater settlement and thereby a lower bearing capacity, Can somebody please confirm if this is correct?

## RE: Bearing Capacity using N Value

## RE: Bearing Capacity using N Value

## RE: Bearing Capacity using N Value

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## RE: Bearing Capacity using N Value

Also, for your second question, I think that the answer is related to the above: depth corrections are smaller for deeper locations BUT this is considered for sands where you have the same "soil resistance/penetration value" (i.e. SPT N-values) thru the entire depth of consideration. For example, the soil's shear strength parameters (in case of sands, friction angles) are different for the same N-value at say 5 meters and 10 meters (phi @ 5 meters > phi @ 10 meters for the same N-value). I will post later a chart which this can visualized.

## RE: Bearing Capacity using N Value