Limiting Foundation Width 3

[dawn836]

Hi all
I am a structural engineer with I think satisfactory understanding of soil mechanics , I got soil report for geotechnical engineer for design of a foundation , there was a note in the report stating that Isolated footings or strip footings shall be used with maximum width 3 Ms.
Just I wonder why the geotechnical engineer limited the foundation type and its width?!!

The foundation type is shallow with Allowable bearing capacity 110kN/m2 and recommended foundation level is (-1.50Ms) at this level and under Soil was giving results around 25 and increasing for SPT. Borehole depth is 10Ms.

 

[TDAA]

With an increase in the width, you have an increase in the depth of influence. This will affect the settlement calculations. The limit in this case (with improving blow counts with depth) may be to limit the depth of influence to the depth investigated, although I could not say for sure. With the numbers you have shown, your depth the foundation would influence would be about 7.5m, leaving a bit of a buffer.

He may not be limiting the foundation type. This may just be a poor choice of words. He may be alluding to "If" footings are used, limit the width to 3m.

Ask the geotech for some clarification. He probably has read that line so many times it makes sense to him, and he doesn't realize it could be misinterpreted.

 

[UcfSE]

Maybe those limits are the extent of the parameters the geotech used in determining capacities for the report. Remember that as you increase footing size you decrease bearing pressure but increase settlement. Did you ask the geotech about it?

 

[dawn836]

Thanks all for quick reply but considering the point of deflection, I made analysis using finite element analysis modeling footing as a shell and the soil foundation interaction as springs with modulus of subgrade reaction equals bearing capacity/settlement used for B.C. calculation, actually sometimes I was forced to increase footing thickness to achieve uniform stress under footing and sure after that settlement was less than maximum.
B.c.=110kN/m2
Max. Allowable settlement =25mm

 

[oldestguy]

Dawn836

Not sure where this sort of a statement comes from. It would be assumed that the load does not change as you vary footing width.

Per UcFSE
'as you increase footing size you decrease bearing pressure but increase settlement." THIS IS NOT TRUE. For the same loading and with a wider footing, Bearing pressure is less and the setlement is less. That's the controlling factor (settlement).

If the statement were true, one would use very small footings. We widen footings to decrease settlement potential. For closely spaced load points not much is gained when overlaps of stress bulbs occur however.


The best advice is to get the report writer to explain the statement.

 

[Mccoy]

Unless at one point (not too deep) in the sequence there are very compressible layers, and the increased depth of influence, although with a decreased pressure, shifts the balance toward greater settlements.

 

[eric1037]

Is it possible that footings greater than 3 m in diameter are not economical in your area? If loads are high enough, deep foundations may be more economical.

Also, I agree with McCoy. If there are lower lying loose or soft layers, then the depth of influence of wider foundations may cause additional settlement.

It is also possible that they want to limit the influence of the foundation to the depth of the boring.

Did they recommend deeper borings for larger loads?

 

[dawn836]

well we received today 2 soil reports for other sites from same laboratory he gave also same recommendations for other different sites in different areas B.C. was different in each report but same recommendation for foundation width!!
Depth of borehole was 10 Ms in the three reports, I wonder if he is limiting foundation width to borehole depth!!
Is not 10 Ms enough as a borehole for shallow foundation?!

About loads I have is that max. Load is 2100kN where 4.5x4.5 square footing is enough. The building is only 2 stories. other problem is if I have a concrete core for lift which is not heavy loaded at all but the foundation must be sized for each dimension so that it is 5.00Msx5.00Ms this was considered unaccepted by other firm which is checking our work because they said the statement I mentioned about footing width, there refusal was not because of technical reasons but just soil say this (just people dealing with papers)

 

[UcfSE]

oldestguy, you are correct. I was thinking one thing and typed another. Thanks!

 

[oldestguy]

Hi all:

Let's not get all mixed up with zones of influence until you compute the ratio of added pressure to existing pressure as far down as you think is important, not just to bore hole depth. That is what is important and causes the settlement A high strength layer over a soft one may tend to distribute the stresses better below, but I doubt that that was the reason. See next para here.

I strongly suspect the report writer uses a boiler plate report that he or she just edits for each new job report. When in a hurry a lot of standard language comes along. I've made the same mistake a few times.

 

[BigH]

Just another point in the oldestguy vs original UcfSE posts. Oldest guy is correct given uniform soil stratum of unlimited depth that for the same "load", increasing the footing width will decrease the applied bearing pressure and settlements will be less. I think that the orinal post by UcfSE was intended to say that for increased footing widths, to "maintain" a given settlement, the applied bearing pressure will decrease. This latter is the well-known bearing pressure vs footing width charts for settlements of 25mm at different N values and charts that can be developed for clays (but needing to take into account immediate and reconsolidation/consolidation settlements). Layered soils, overlapping pressure bulbs of adjacent footings - changes the ball game requiring a relief pitcher, I would think.
Oldestguy is absolutely correct in that this must be a boiler-plate recommendation. We all have them and use them; but it shows the absolute necessity (to avoid embarassment) of thoroughly confirming that the boiler-plate recommedations make sense for a specific project.

 

[casimmons]

Here's an idea- why don't you ask your geotechnical engineer about his logic with that statement?

 

[dawn836]

well i have been discusing it with the geotechnical engineer who made the report he told me the only reason is to limit to 25 mm because depth of borehole was only 10 Ms

here is another point what if i used isolated footing following his regulations and i have maximum width of it to be 3 Ms but the distance between them 20 cms?!

 

[BigH]

dawn836: If this was the reasoning of the geotechnical engineer - it is not appropriate and does you a dis-service. His investigation should be to determine the most suitable types of foundations, bearing pressures under the various conditions inherent in the project structure - not to say - well, since I only took my boring to 30 ft (10m), then I won't give any recommendations that will exceed "MY" zone of influence. I'm sorry, but I would get another geotechnical engineer on this one. What he is saying to you is not to use a 4m footing since he didn't investigate for it. I would say - why not? If he needed to go back for additional depth, he should have let you, as the client, know this and the reasoning for it.