Bearing Capacity Calculation

[nagatalluri]

Dear All:

I am wondering how to consider the effect of soft soils (Soils with low 'N' values)in the boring while calculating the bearing capacity using General Bearing Capacity equation. I mean the soft/weak soils are not at footing depth but are present at a depth between 25-40 ft along the profile and the N values spiked later. How do account or reduce bearing capacity for this type of situation.


Thanks
NT

 

[aeoliantexan]

The Navy Manual, DM-7.2, addresses bearing capacity of two-layer foundations on Pages 137 and 145. You can download the manual for free.

Generally, I would rough-size the footing based on the strength of the upper layer, then compare the thickness of the upper layer to the width of the footing. If the depth to the soft layer is more than twice the footing width, it may have no effect, but check it using Page 145.

You will need to address settlement separately. Remember that a point in the deep layer may be stressed by more than one footing.

 

[nagatalluri]

Aeoliantexan:

Thanks for the reference.

NT

 

[fattdad]

if you are using N-values to correlate strength, take the weighted N-value to a depth of 2B (4B for strip footings) and use that average value to derive bearing pressure. It'll likely be o.k. On the other hand, you need to look at elastic solutions to determine the stress increase in the layer to determine how much settlement will result from the loads acting on this layer. You may (will likely) need to adjust the foundation bearing stress to limit settlement from this layer. It'll not likely be designed based on ultimate bearing pressure, but on the basis of foundation performance (i.e., settlement).

f-d

¡papá gordo ain’t no madre flaca!