Calculate Allowable Soil Bearing Pressure

[hockeyfan960]

I have an existing condition of 4" of unreienforced concrete on top of a sandy soil that has been in place for 10-15 years. I am looking to have the 4" of concrete support a point load and need to find the allowable Soil bearing pressure in order to give the structural engineer the starting point for the load calculations. I have a recent AAShto T-180 Method A soil analysis, ASTM C136 Sieve Analysis and a AASHTO T193 CBR Test....how do I calculate the allowable soil bearing pressure for the subgrade...???

 

[Lion06]

hire a geotech!

 

[DarthSoilsGuy]

At least 50 psf based on consolidation from the slab dead weight being complete by now.


ok, couldn't resist, sorry.. you're going to need other info than those listed. if you have any geotech/CMT companies that are looking to market themselves to you or your client, you should find their pricing reasonable, especially if you're talking about placing racks on an industrial slab (random guess). you should have an idea on the magnitude of the loads and acceptable level of settlement when discussing it.
The lower the bearing pressure the structural egr needs while still keeping the design efficient, the easier it will be for Geot Egr to limit frequency (cost) of testing.

 

[DaveAtkins]

I think bending or punching shear in the slab will control--not soil bearing pressure.

DaveAtkins

 

[hockeyfan960]

The Structural Engineer on the project wants to have the allowable bearing pressure of the sub grade before doing the point load calculation on the concrete. I also have a concrete core sample result for the 4" concrete slab.

 

[jike]

Dave Atkins is right on! The bending in the unreinforced slab most likely will control. A beam on elastic foundation analysis is required or analysis by chart.

 

[fattdad]

In the absence of any real field informaiton, you can't assess the allowable bearing pressure. If you want to keep the structural engineering moving, have the foundation sized for 2,000 AND 3,000 psf - I mean how difficult will it be? Then (this is the important part), hire a geotechnical engineer. If this is not that critical of a structure and if the foundation size is relativelly small maybe the geotechnical engineer can just use a hand auger boring to give you the bearing pressure. Lab testing of surface soils just won't get it done!

f-d

¡papá gordo ain’t no madre flaca!

 

[COEngineeer]

Well dave is right but if you have a small point load sometimes you can get away with bearing it on slab. Make sure punching shear is ok though.

Never, but never question engineer's judgement

 

[dirtsqueezer]

Do you have any historical information on the site? Original plans, reports, ect. Anything?

It seems like (a geotech engineer) may be able to put together a recommendation based on information using a borehole drilled adjacent to the slab- given a large caviot on what this recommendation is based on, that the geo engineer is not responsible for losses, ect. If nothing else maybe a square sawcut and hand auger by someone equally qualified. You can always fill in using grout or more concrete. I've got to agree with you, though. A 4" unreinforced slab is not reasurring, given it's the engineer that's asking the question.

 

[sdz]

You could drill a small hole, e.g 4" dia, in the slab and do a penetration or vane shear test close to the bearing point. Discuss with a geotech.

 

[BigH]

First off, with such a thin slab, why don't you simple cut out the size you need and replace it with a proper footing. I am assuming that you do not have a "heavy" load - you should know from the structural drawings what the allowable bearing pressures (and hopefully the founding stratum) were for the support of the building columns and walls.
If not, then with the slab broken out, you could do some hand samples with the split spoon and resulting SPTs (I did the back breaking job - using standard 140 lb hammer dropping 30 inches by hand - with driller's help on many occasions). You could drive, say 4 continuous spoons (if not hitting refusal first) which would give you the stratigraphy for about 8 ft depth (assuming you drive a 24 inch spoon for 4-6" blow counts. This seems to be a reasonable way to look at the problem.