huangyhg

soil pressure under a matt foundation
i have a storage tank (leg supported) within a concrete containment area. the floor of the contaiment area will serve as the "footing" for the tank legs. i have seen calculations by others that state the soil pressure under a matt foundation is 10,000 psf at a site that has an allowable soil pressure of 1500 psf for a typical footing design. is the fact that the tank (one of 4 in the same containment) is on a large matt foundation warrant 6 times increase in the soil pressure? is there some reference? thank you in advance.
i have seen calculations by others that state the soil pressure under a matt foundation is 10,000 psf at a site that has an allowable soil pressure of 1500 psf for a typical footing design. is the fact that the tank (one of 4 in the same containment) is on a large matt foundation warrant 6 times increase in the soil pressure?
generally, the answer is "no." but you haven't provided us with any geotechnical details that would allow for a reasoned opinion. and you haven't told us what the project geotechnical engineer has said about the disparity in allowable bearing pressure -
if you will provide some site-specific information, we'll be glad to comment on the approach.
by the way, the proposed allowable bearing pressure of 10 ksf is pretty high for a mat foundation. please provide the mat's dimensions and the locations of the legs on the mat. i see quite a few potential problems with the proposed approach.
perhaps it was 1000 psf for mat rather than 1500?? but, even that seems high for a mat foundation if the normal footing allowable pressure is only 1500 - mats will effect so much more soil with resulting settlements. i'd get a copy of that "viewed" data and really look into it. sometimes, even in the "english" system, zeros are lost or added.
i had the same thought about an extra zero - thought i'd wait to hear what the geotech said first...
thanks to you all. i also considered the "extra" zero error. however, the calculation actually takes the tank pier load and divides it by 10 ksf (after clearly stating 'assume 10,000 psf allowable soil pressure for cohesionless soil under a matt foundation' almost like from a reference). the note was so descriptive i started to think that perhaps because of the stiff nature of a matt foundation that there was a good reason for bumping the allowable up. glad i asked!
my design will consider the 1500 psf allowable, ending up with a huge area required...in fact the "areas" influenced by the 6 piers will overlap.
thanks again for your time!
jtpe,
something still does not seem quite right in the information you have provided, as focht3 has already noted. where did the 1500 psf come from? how did that get traslated into 10,000 psf? was there a geotech. report?
here in the midwest us 1500 psf is generally the code required assumed soil bearing if there is no geotech. soils report. also, in much of our surrounding area we have glacial deposits of sand and gravel and the usual minimum value of 3000 psf is used in those areas, although i have seen geotech. soil reports that would allow up to 8000 psf for strip and spot footings beaing on dense s&g deposits.
in your case the difference between 1500 and 10,000 is so great, i think you should try to track down how the two values were arrived at or get a second soils engineer's opinion. the cost would be more than offset in a more economical mat design.

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