local high soil pressure under tank base
local high soil pressure under tank base
(OP)
I posted a similar thread in Structural Other Topics (thread507-202072: a column at center of tank bottom plate) about a tank foundation review. The posting here the geotechnical question related.
A 85ft dia tank to contain sour water about 45ft deep. There is a central column supporting roof, the 20" dia column in turn is supported on a 90" dia base plate on the 0.375" thick bottom plate. With computer modelling, I calculated a 0.35" deflection at the column and a 0.18" at the edge of the base plate. The modulus of subgrade reaction is 150pci (10ft granular engineering fill on sand and till fill).
The question is whether the local 0.35" deflection and the 7ksf local high pressure is acceptable. The stress in base plate and differential settlement are within the allowable.
Thanks for advice.
A 85ft dia tank to contain sour water about 45ft deep. There is a central column supporting roof, the 20" dia column in turn is supported on a 90" dia base plate on the 0.375" thick bottom plate. With computer modelling, I calculated a 0.35" deflection at the column and a 0.18" at the edge of the base plate. The modulus of subgrade reaction is 150pci (10ft granular engineering fill on sand and till fill).
The question is whether the local 0.35" deflection and the 7ksf local high pressure is acceptable. The stress in base plate and differential settlement are within the allowable.
Thanks for advice.





RE: local high soil pressure under tank base
Please recognize that the center of large tanks often settle inches - not fractions of inches!
f-d
¡papá gordo ain’t no madre flaca!
RE: local high soil pressure under tank base
The 150pci modulus is given by the geotechnical consultants. And I'll double check with them for the calculated deflections. Just want to get some geotechnical idea if the high stress/deflection is very local, what happens.
RE: local high soil pressure under tank base
for instance, i'll talk in very round figures relating to something i'm pretty familiar with (area fills). here where i'm at, let's say there's 40' of 20-30bpf material and there's going to be 20-25' of new fill over a large area. without running any tests, i would expect fill induced settlement to be 2% of the soil profile thickness. that's 40x.02/.12=6.5" settlement (some of which will work itself out near the end of fill placement but most of which will occur in the weeks following the end of fill placement). now imagine it's a big tank instead 45' deep (center) x say 60pcf = 2700psf contact pressure. well, 25'x115pcf=2875psf. i say you've got yourself a comparison there...and that's on a pretty good soil profile. my scenario is not exactly how it works but it's close enough for a rough idea. a geotech would need to use the appropriate chart, tables, data, etc to perform a real settlement analysis for your case.
consult a geotech. and remember, moderate loads over big areas equals bigtime settlement even in the best of situations. the other folks on this board can definitely provide a more thorough school lesson than i can.
RE: local high soil pressure under tank base
Refer to this thread for more information and a reference to a paper on this topic by Prof. Duncan (Virginia Tech).
http://www
f-d
¡papá gordo ain’t no madre flaca!
RE: local high soil pressure under tank base
I can see the total settlement can reach several inches. What I am most curious here is whether the high soil pressure (about 7000psf, it is larger than the allowable) adjecent to the column is acceptable because the higher stress is very local. The subbase is 10ft deep engineered granular fill, the soill underneath is sand and clay till.
The vendor of the tank has a strong opinion not to use a footing under the central column. From what I can see, using a footing will make the soil pressure less than the allowable even under the column, but when the stress spread several feet down there shouldn't be a big difference. Also the total settlement of the base may not be noticably affected by a footing. Can you comment on this? Thanks again.
RE: local high soil pressure under tank base
RE: local high soil pressure under tank base
Here's where you are kind of lucky: If this site is underlain by glacially consolidated soils, there may be negligable settlement under the load of the tank contents. Not for me to decide, but something for you to evaluate (or your consultant).
¡papá gordo ain’t no madre flaca!
RE: local high soil pressure under tank base
RE: local high soil pressure under tank base
It is not a rigid raft foundation. The tank is to be supported on a concrete ring beam, subbase and a central column for part of the roof. There is a socket at the column base, to make the column length adjustable at least during the construction. The geotechnical consultant originally proposed a 12'x12' footing under the column, but the vendor is strongly against it, saying because the soil at the immediate outside of the footing will settle more than the footing, this will create a high stress in the bottom plate there. Without a footing, the vendor uses a 90" dia, 2.25" thick plate plate at the column to reinforce.
I'm sending a request for additional evaluation of the tank without a footing. From my view the only concern is the very local high bearing pressure. I personally tend to consider the footing is not neccessary.
The allowabe differential settlement for tank is about 2".
RE: local high soil pressure under tank base
That's crazy talk. Two inches for a radius of 42 1/2 ft is nothing. You will likely see much more than that when all is said and done.
¡papá gordo ain’t no madre flaca!
RE: local high soil pressure under tank base
I've sent the evaluation request to the geotechnical consultants. I should be able to tell you what they figure out the total and differenct settlements for the tank described above later.