Bearing Capacity Determinations

[smcgee]

I am a recent college graduate. It is my understanding that you need the foundation type, width, depth, etc. in order to calculate bearing capacity. I have seen foundation recommendation reports that provide bearing capacity values of the soil even though the foundation specifications are not known. How is this possible?

 

[tincan]

The lab reports are based upon tests performed on soil samples taken from the site in question. Pick up a basic soils engineering text for self study.

 

[MRM]

smcgee,
I've got a few comments. You're absolutely right that you need to have those things before you can make a bearing capacity determination. Look all through the report and see if they don't make some good assumptions (or even knowns) regarding the type of structure, loads, usage, footing type, etc. They should know those things before they develop recommendations. It is very dangerous to say a site has a "bearing capacity" of 2,000psf without knowing what is planned, because that value is highly dependant on what is being built there. There also may be more than a single recommended bearing capacity for a structure too. Maybe for different structural elements or something.

Also keep in mind that the "bearing capacity" value you see (if that's what they call it) in the report is actually a misnomer since the value you're reading is actually an "allowable bearing pressure" figure. See what I mean? The allowable pressure given may be limited by settlement, not an actual shear failure of the soil. For example, the bearing capacity of a given footing on a given site with given loadings might be 8,000psf, whereas your soil contact pressure to limit settlement to an acceptable value may be 2,000psf. Clearly, you can only apply 2,000psf since settlement controls. It is also possible to have instances where bearing capacity (soil failure) may control the recommendations rather than settlement too.

 

[BigH]

Do a search of the various other threads from the recent past. This has been discussed several times elsewhere. MRM is correct - many, if not most, times it is not the bearing capacity that governs but the allowable bearing pressure - and this is based on permissible settlements (total or differential).

Also, in "routine" soils investigations, the writer, with many years experience, "knows" the typical foundation types that are used in, say home or light commercial construction. Therefore, he is likely giving you the "standard" values to use; not the actual site specific value that could be used.

 

[npthao121]

In our country (Vietnam), there is a common practicing in which the report writer caculate the bearing capacity for a conventional foundation with 1x1m (width & length) at 1 m deep from the ground level. they say the bearing capacity of the conventional foundation. besides that, they also mentioned the foundation solution for the project is the full responsibility of the designer. Geotecnical men will not be in charge of the given bearing capacity.
However, some experinces people usually give their recommendation about the foundation solution (type, which soil layer should the foundation be placed...)
But, finally, if you play the role of the designer all parameters must be considered & evaluated by yourself to get the reasonable solution.

 

[jdmm]

This is a problem that I think that every young engineer encounters. In most instances the designer recommends an 'allowable bearing pressure' that considers a wide range of possiblities. We know that bearing capacity (rupture) depends upon the depth of foundation and width of foundation so it depends upon the wall or column loads (as well as the soil conditions). If we make some assumptions for the particular building about the range of wall and columnm loads then we have some idea about the size of the foundations.

We know the soil conditions (hopefully) so we can design the foundations on the basis of the range of probable loads anticipated. Bearing capcity (rupture) or settlement may govern but the allowable bearing pressure recommended considers all of these variabilities.

From a practical perspective, we don't normally recommend allowable bearing pressures that result in unusually small footings. For example, if our bearing pressure resulted in footings that were only 150 mm (6 inch) wide this would not be very practical. Designers are looking for practical recommendations, not complications.

 

[BigH]

jdmm has good point. Usually, along with the allowable bearing pressure, I give a minimum footing width - usually 600mm for strip footings and 1m for square/rectangular footings. One of the governing factors in determining footing width is the thickness of the wall it supports (if strip). Usually you do wall thickness plus 150 each side - hence for cinder block walls, about 600mm.

 

[sc]

Point to clarify:

1. "Allowable bearing pressure (or capacity)" is the estimated (or tested) maximum pressure that can be placed on the soil to cause failure.

2. "Bearing pressure" is the design pressure caused by a structure and its footing system.

In design 1 must always be greater than 2 by some factor of safety.


2 is a pressure not a capacity

2 is often reduced by altering the footing width or depth to take advantage of spreading the load or letting soil cohesion provide some assistance.

1 cannot be altered only proven to be different by testing. Sorry, it may actually be improved by compaction or modification.


regards
sc

 

[smcgee]

Thanks for everyone's response. Makes things a lot clearer.

 

[BigH]

sc - we had a good thread a few months back on this - actually it was about PLT but it "degenerated" into a much broader and useful thread.

I don't agree with your point 1 as far as terminology is concerned: "allowable bearing pressure (capacity)" - you have just mixed apples and oranges , in my view.

Allowable bearing capacity is the capacity governed by shear - e.g., for clay: qult/FS =
(2)(pi)(Su).

Allowable bearing pressure is taken as that bearing pressure permitted for a given permissible settlement. At least this is the definitions that I use as I opine most geotechnical engineers use.

 

[Focht3]

[blue]BigH[/blue] is correct; he beat me to the punch!

But it's important to note that [blue]sc[/blue]'s confusion is really the norm, not the exception, among civil engineers. It's a rather common misunderstanding that we geotechs frequently deal with -



Please see FAQ731-376 by [blue]VPL[/blue] for tips on how to make the best use of Eng-Tips Fora.

 

[sc]

Thanks for the clarification I hadn't been given a simple explaination of the 2 variations on "allowable" before. Most geotechs I know have been happy to let the engineer continue along their merry way.

My point was more about the "allowable bearing pressure (or capacity)" and "bearing pressure" being 2 different beasts, one derived in the field by testing and the other derived by the structural design process.

So based on this anything provided by a geotech report would generally be an "allowable bearing pressure (or capacity)" rather than a "bearing pressure". The geotech would not normally have access to the design building at the early site investigation stage and as such could not make a judgement on the building bearing pressure. He may have access to an estimate building mass at the footings and hence can provide an "allowable bearing capacity" based on field tested soil properties and conservative assumptions of the buildings footing system.

I hope I haven't made my point any more confusing with this.

regards

sc