Bearing Capacity in Relation to Shear Capacity 1

[JohnnyB_]

Hello -

I am reviewing a geotechnical report. My questions stems from the recommended bearing capacity of the soil being stated as 3000 psf at 12 inches below grade. Later in the report it is noted that the soil stresses beneath the foundation should not exceed the shear capacity of the soil.

My question is, shouldn't the bearing capacity of the soil reflect a capacity that keeps the shear stresses within allowable limits? I have always assumed that is why the bearing capacity of the soil increases with depth.

Thanks in advance for your help in this matter.

 

[Mccoy]

My first impression is the same as your impression, that is the report appears to state the obvious since if the soil stress exceeds the shear capacity, a rupture or shear plane is formed which will compromise the stability of the foundation.

 

[JohnnyB_]

Thanks McCoy. The more I think about it, and assuming a consistent soil strata through the effected depth, the shear stress failure dictates the bearing strength of the soil as the only mode of failure. I could be wrong, but this is what I have convinced my self of, currently.

 

[EireChch]

The bearing capacity of the soil is one thing, the bearing pressure applied by the foundation is another thing. This is based on the load the foundation sees.

I think the statement is a bit silly and likely doenst need to be stated but isnt it just saying to not apply a load to the foundation that results in a bearing pressure of more than 3000psf.

 

[JohnnyB_]

Yes sir, I would have to agree. I was just curious if there was a hidden something I was missing or just fodder for the sake of it.

 

[dik]

If Mohr's circle has anything to do with it, it should be 0.5 times bearing with whatever safety factor/load factor you use...

So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[JohnnyB_]

Good point dik, thanks for that reminder. I'll dig into that for a deeper understanding.

 

[BigH]

For me, the report should if it suspects that shear is the issue, provide an allowable bearing capacity to use for the foundation - the bearing capacity would basically be dependent on the geometry of the footing and does not take into account the actual load on the footing - it provides the safe allowable bearing pressure that can be applied for shear.

Similarly, if the geotechnical engineer suspects it is settlement (and it usually is) is the issue, then an allowable bearing pressure should be given. The latter will govern your footing dimensions as it will take into account the footing geometry and the loading. Now one can develop a relationship between footing width and allowable bearing pressure to produce a given maximum settlement. In effect this is what the graphs of N value vs footing width does - usually taken for 25 mm of settlement.

I seldom ever had a need to provide shear based bearing capacity.

 

[JohnnyB_]

Thanks for the clarification BigH.