Allowable Bearing Pressure and Settlement

[SuperSandman]

Hi Guys

Structural engineers normally ask for the allowable bearing pressure of the soils. Allowable bearing pressure is then determined by carrying out a bearing capacity analysis and a settlement analysis. The settlement analysis is carried out using the foundation dimensions provided by the structural engineer.

In most cases, the allowable bearing pressure is limited due to the maximum allowable settlement. Once we give the structural engineer the allowable bearing pressure, they tend to start re-sizing the foundation dimensions which affect the settlement predictions, and in some cases this results in further settlement than the maximum allowable settlement.

The trouble I have is, how do I discuss this concept in a geotechnical report? I want to say "The maximum allowable bearing pressure is dependent on the dimensions of the proposed foundations" OR "The maximum allowable bearing pressure of XXX kPa is only applicable to the foundation dimensions provided by the structural engineer at the time of preparation of this report".

Any advice?

SS

 

[oldestguy]

One way is to provide the designers with a chart. Chart plots results of settlement analysis as a series if equal settlement curves relating load on one axis to footing width (either square or long) on the other axis.

You may question how to prepare this chart efficiently. I use an old 1959 ASCE paper SM 4 2135 by B.K.Hough "Compressibility as the Basis of Soil Bearing Value". I believe at lest on State DOT has adopted that method. Can't recall which.

 

[L0k]

In my opinion you should give a restriction in using the allowable bearing capacity, for instance:
- the next footing should be a certain distance to avoid the overlapping load or
- the heavy column load is used pile.

 

[fattdad]

it my opinion you should publish in the report the anticipated wall and column loads. Then there's a hook between the analyses and conclusions. If the loads change (i.e., they later decide to add a mezzanine) you have an opportunity to reevaluation your conclusions.

f-d

ípapß gordo ainÆt no madre flaca!

 

[emmgjld]

TO follow up on oldestguy, see also chapter 9-16 Basic Soils Engineering, B.K. Hough, 2nd Edition.

 

[Ron]

Unless the soil is highly stratified immediately below the foundation, settlement and bearing capacity can be treated separately.

 

[BigH]

A chart of allowable bearing pressure vs footing dimension as suggested by OG is a good way to go. I remember making up quite a few of these back in the 70s (before spreadsheets and computers) for cases of single footing and, as well, interacting footings (i.e., how close footings were to each other). This gives the designer the information he needs.

As to bearing capacity - I never (well, seldom ever) gave a bearing capacity or allowable bearing capacity since, in almost all cases you run into in reality, it is the settlement criteria that will restrict the allowable bearing pressure - to me, capacity is a shear failure issue, pressure is a serviceability issue and the one which almost always rules the roost.

 

[SuperSandman]

Thank you all for the advice...

I have read many of the threads related to bearing capacity on this forum. I am really pleased to see that structural engineers have such positive input into the queries.

However, I am not proud to say, that thus far in my career as a geotech, I have worked with many structural engineers who distance themselves in understanding and absorbing the recommendations given by a geotech report, instead all they want to see are numbers which they can plug into their software programs. And its now my understanding that in my country, it would be a rare occasion for a structural engineer to actually grasp the facts and comments that a geotech gives them.

Especially trying to convince them that allowable bearing pressures are, as BigH mentioned, governed by allowable settlement. I have an entertaining path ahead of me.

Thanks Guys!!!

 

[oldestguy]

Dear Super:

One of the lessons I learned, somewhat the hard way, back then was when a client engineer read my geotech report and then called to say he wondered what bearing capacity he should use for design. There was so much "standard language" in the report that he could not find it. He could care less what different soils were found or how they were found. All he wanted was that one number that was there but buried in the print somewhere. The fancy cover and the beautiful logs didn't impress him. That was even before word processors were available.

Considering the designer is working with maximum loadings that may or may not occur, and the a proximate precision of our reports, going to the extent of the chart I mentioned is rarely needed.

 

[SuperSandman]

Thanks Oldestguy... I haven't seen these charts used often in recent reports by our local geotechs. But I have seen charts extensively in old geotech reports dating some 20 to 40 years back. I honestly enjoy reading reports written in that era

 

[BigH]

Agree with you SuperSandman . . . I started reading our company's older reports (1960s to early 70s - sort of passed on the 50s) when I joined back in 1977. They were written by some pretty fine engineers who, by the way, were the founders of Golders. I kept up the tradition until I went oveseas in 1995.