Meyerhof's Bearing Capacity on Layered soil: how much layers are enough

[Grendizer]

Dear all

how many layers and what (and at what depth to stop) in case I am studying Meyerhof's Bearing Capacity.
Also, could you propose a manual calculation example?

I guess, reading Braja example 3.2 (for a single layer) that we stop at where the SPT stops, and we take the average of
different PHI' values?

 

[BigH]

Are you a student? Just wondering.

"where the SPT stops" ? Does this make sense if you have a 2 m wide footing and the SPTs go down 20 m?? No, it doesn't.

There are two issues - one is bearing capacity which is a shear phenomenon; the second is serviceability - a fancy term for settlement (or limiting settlement - including differential).

I would say - what is the depth of influence of the shear planes below the footing. See Das' Principals of Fdn Engineering, 7th Ed (pages 137-138) it is explained how the shear planes are formed. This is the zone in which you would need to determine the characteristic/acting phi for your bearing capacity.

However, given the above, it is seldom a controlling factor in the size of the foundation as the foundation size is almost always controlled by settlement issues - either a standard "acceptable" settlement such as 25 mm (traditionally 1 inch) or 40 mm (as per Indian practice). However, there are other times that more stringent settlement constraints are required. In these cases, it is usual to take the "zone of settlement influence" to be that location where the Bousinesque pressure increase is in the order of 10% of the applied stress. Typically for square or round footings this is 2B where B is the width of the footing. For strip footings, it is typically taken as 4B.

 

[Grendizer]

Dear BigH

No I am not a student, but this is a long story...
In brief, due to my country economical situations, I did not get to work in the same engineering field all the time
(I also spent a quarter of the post graduation time in catering management/tendering business)

Still I like to finish my career (still 10 years) as a strong engineer. Actually I am developing several
Excel calculation sheets, and came across Meyerhof and found it weird that Braja did not provide layered shear capacity
examples.

Anyway, your explanation is very clear. Thanks, and I hope you do not mind further questions in the future.

 

[BigH]

No problem at all - all questions are welcome . . .

 

[oldestguy]

I'd concentrate more on finding the settlement that may occur under foundations, since bearing capacity based on shearing strength seldom is the governing factor in selecting the foundation pressure on the soil. And yes there are methods to find settlement even for granular soil. One being the Hough method. Doing a Google search will bring up many methods.