predisting existing bearing capacity 1

[Roupen]

I have the existing plans of a house. I have calculated the soil's working stresses under the home's isolated and strip footings.



The working stress under the isolated footings is 2,000 psf.
The working stress under the strip footings is 1,000 psf.


Am I right in saying that based on this information, the soil's allowable bearing capacity is at least 2,000 psf?



And that I have a reserve of 1,000 psf under the strip footings, thus I can safely add another 1000 psf of pressure?

 

[BigH]

You need to know the type of soil you have and determine its capacity to hold up the loads you are imposing both in shear and with respect to settlement. You have worked out the working stresses - now compare these the safe allowable bearing capacity (shear) and the safe allowable bearing pressure (settlement).

 

[Roupen]

Thanks BigH,

There is no soil report and there is no way of determining the type of soil, that's the whole point of my question.

The question stated in a simpler way and in more general terms is as follows:

You have two footings under an existing building, one has a working stress of 2,000 psf the other has a working stress of 1,000 psf.
Is it a valid assumption to say that based on this information, the minimum allowable bearing pressure at the site of the building is 2,000 psf?



It's a question of judgment.

 

[salman85]

the allowable bearing pressure divided with working bearing pressure, must have value more than 2.5..known as SF..
the more undefined soil underneath the structure, usually has bigger its SF..

But, it is important thing to ensure the settlement (cohessive has more settlement than granular) beside considering safe or allowable bearing pressure..like Big H said on above..

 

[salman85]

Working stress = 2000 psf
Allowable Bearing Pressure = 2.5 * 2000 psf = 5000 psf or 239.4 Kpa..is such a little value..

well, do insitu test (eg. SPT, CPT, DCP). I'd rather do DCP test to defined bearing capacity underlaying without losing much time..(Correlation DCP to Bearing Capacity topic already discussed in geotechnical forum)

Then the DCP result (after converted), shall be at least or greater than 5000 psf..

Just suggestion...

 

[aeoliantexan]

Three issues:
1. Have the foundations really been subjected to 2000 psf, or does that calculated stress include live loads that may or may not have occurred? If it is all dead load and known sustained live load, then you can take the individual footings as a test.

2. The ultimate bearing capacity is different for square or rectangular footings than for a continuous footing on the same soil. For clay, a square footing has about 125% of the ultimate bearing capacity of a strip footing. For sand, more like 85%. These numbers can also be affected by footing embedment.

3. A strip footing tends to settle more than a square footing of the same width, because the stress bulbs are deeper.