Approximate relationship between allowable soil bearing pressure and sub grade modulus Ks 1

I do not know where that diagram comes from, but am interested. We always seem to have a hodge-podge of imperfect geo-tech info in our office.

I did come across a Structure magazine article that discusses the subject though. Here is a link to the article:

Link

Halfway down they start to discuss bearing capacity & subgrade mod correlation:

"The most common – and probably the safest – answer to the question of correlation between bearing capacity and the modulus of subgrade reaction is that there is no correlation. But there should be one, as both are the measurements of soil capacities and any of these two parameters can be used to design a regular foundation."

One rule of thumb I've heard is that the allowable soil bearing pressure will result in about 1" of soil deflection. But that is very rough and clearly not applicable for the extremes of the spectrum.

If I don't know otherwise, I assume the allowable soil bearing pressure causes 1/2" of settlement. I think this is conservative.

For example:
2000 PSF = 27.8 PCI
3000 PSF = 41.7 PCI
4000 PSF = 55.6 PCI

DaveAtkins

This diagram seems like from Bowles book. But that's just a guess.

There shouldn't be a relationship because you are talking two different types of loads and behaviors. Allowable bearing is almost always controlled by settlement; which is related to size of the foundation and actual loads. Modulus of subgrade reaction is normally based on a plate 12-inches square and 1-inch of settlement. Completely different conditions.

Best advice is talk with your geotechnical engineer about what kind of performance you need, your loads, and a good geotechnical engineer can give you good values. If you continue to get bad values, then you need to retain better geotechnical engineer for your projects.

Mike Lambert

GeoPaveTraffic said:

Allowable bearing is almost always controlled by settlement

Click to expand...

To me, this means there IS some relationship between allowable bearing pressure and subgrade modulus. Geotechnical engineers typically limit the allowable bearing pressure to the pressure which causes 1/2" settlement. That is how I came up with my numbers.

But--I do agree the relationship is rough. A good geotechnical engineer will be able to come up with more accurate values.

DaveAtkins

The reason they are not really related is the size of the footing versus the size of the plate. Footings are almost always much larger than 12-inches square. As a result they load the soil to a much greater depth. Since natural soil deposits tend to change with depth, and footing load the soil to what can be a significant depth, then the bearing pressure is influenced by soils at depth where the subgrade modulus is just dependent on the near surface soil.

I realize this may sound like splitting hairs, but the difference can be substantial in many soil deposits.

Mike Lambert