How does Compressive Strength relate to Bearing Capacity 1

[NUSOTEX]

Hi All,

I have a question regarding bearing capacity. I work in the oil&gas industry designing what we call "pads" that support the drilling rig. I have calculated the max load appplied to the soil to be 18,000 psf or 125 psi. I ultimately want to know if my soils can handle the loads according to terzaghi's bearing capacity equation. We do not use concrete foundations but instead make a soil/cement mixture of about 8-12% cement content by weight to support the rig. I have taken core samples of these mixtures after curing with compressive strength test results ranging from 300-450 psi. the soil cement layer is usually 12-18" thick. Average CBR values of the in-situ soils beneath the soil-cement layer range from 10-20

My question is: Does the compressive strength of a sample equal the bearing capacity of the soil its taken from? I do not think so, given that the bearing capacity is derived from terzaghi's parameters and is based on a different failure mechanism. Maybe i am wrong and this is easier than I think. If not, then how can I determine the ultimate bearing capacity of the pad? Is it dangerous to assume that because the compressive strength of the core samples exceed the max load then the soil is adequate. Some people have said "psi is psi". I dont beleive that.


I am an EIT in my first job. I can get alot of data but dont always know how to interpret. Any help is appreciated. Thanks
If you need more information ill be happy to provide.

 

[BigH]

This is pretty basic - pick up an introductory soils book - say by Coduto - assuming your pad is concrete?. The "unconfined" compressive strength is equal to 2 times the undrained shear strength. For all intents and purposes the ultimate bearing capacity is 6 times the undrained shear strength (where c in cNc is the undrained shear strength) or 3 times the unconfined compressive strength. Apply an appropriate factor of safety (say 3) to the these - or the allowable bearing capacity = 2 times undrained shear sterngth or is equivalent to the unconfined compressive strength. You may want to check the allowable settlement which may reduce the allowable bearing capacity to a lower allowable bearing pressure. If you have the capability to shimmy the drilling pads then the allowable bearing capacity should be suitable.

This has been discussed many times in the various geotechnical forums - try searching through them. A recent one:
thread274-347923 (Under Geotechnical Engineering Other Topics)

 

[civeng80]

Shouldn't the ultimate bearing capacity be about 5 times the undrained strength ?
Nc = 5.14 (Meyerhof Bearing Capacity fFactors)
or are you using
Nc= 5.7 (Terzaghi Bearing Capacity Factors)
either way wouldn't it be more conservative to adopt 5 times ?
(or am I missing something here?)

 

[BigHarvey]

I confirm the 2 given by BigH, just draw the corresponding Mohr's circle.

 

[BigH]

There are various factors - 5.14, 5.7, 6.14 (i.e., 2pi) - 6/3=2 is a nice number (5.7/2 = 2.85 ~ 3 (safety factor) - so no big deal. Remember that most pressures obtained this way have to be modified anyway due to settlement. You might want to see various articles by Malcolm Bolton.

 

[Ron]

Measure it with a micrometer, mark it with a crayon, then cut it with an axe......BigH is right...in the geotech world, there is no functional difference between 2.85 and 3.

 

[civeng80]

Lets say Cu from a Vane test gives 150kPA (SI units here), a stiff sandy clay.
so qu = 900 kPa
phix qu = 400 kPa (capacity reduction factor)
qallowable = 300kPa

Nothing in report about settlements, but I like to be conservative so Im keeping pressures between 100kPa to about 200kPa (all ultimate values !) when sizing pads and strip footings.

Reasonable, under or over conservative (also taking in account settlements) ???

 

[BigH]

I really do think that you need to address the settlement issue - that is the one that usually governs . . . You can do a chart of footing size vs bearing capacity (at say a SF of 3) and then for each footing size, determine the settlement and, applying a 25 mm or 40 mm permissible settlement, you can get the allowable bearing pressure - if this is lower than the one using shear, then it will govern.

 

[civeng80]

Thanks BigH,
Typically soil reports for these sort of jobs do not contain any information on settlement analysis. All there is in the report is just bearing strength and allowable bearing.
Some pad footings have eccentric loadings (not alot of eccentricity but enough to give extra bearing pressure),
so how would you do a settlement analysis and what parameters are needed ?

 

[civeng80]

Ive gone to Coduto with Schmertmann spreadsheet for settlement and plugged in a few values.
Coduto suggests roughly Es = 300 x Cu i.e. 45000kPa roughly. footing size 1.5m x 1.2m x 0.6m deep, Load 270KN. No detectable rock at least to about 6m, Cu = 150kPa (at least).
Also says that Schmertmann spreadsheet is for sands.
Anyone know if I can apply it for clay with the above values.
It gives a settlement of about 10mm which is very small (but could be right).
I really only need a ball park number here, even 20mm would be OK.

 

[BigH]

civeng80 - I am surprised that the geotechnical firm didn't say something to the effect that "anticipated total settlement would be less than 25 mm with differential settlement in the order of 15 mm".

Settlement isn't only "elastic" in clay - but must take into account consolidation. There have been a few threads on this in the past. You can look to determine the maximum preconsolidation pressure. Su/overburden pressure = 0.23 approximately. With Su about 150 kPa, a very rough estimate of the maximum preconsolidation pressure (either from earlier loading (e.g., glaciers) or by aging) is about 150/0.23 (say 0.25) or 600 kPa. So long as you don't increase the bearing pressure above the existing overburden pressure at this point, by more than 600 kPa (tempered, say, to 400 to 500), you should still be in the recompression range and your settlements would be small. See standard texts on this.

Another approach has been put forth by Osman and Bolton (2005 Geotechnique V55) but I'd have to really spend some time on it again.

 

[civeng80]

BigH
Very useful comments.
Thanks.

 

[civeng80]

Also BigH, if clay is overconsolidated wouldn't the consolidation settlement from the new footing loads be small if not negligable (if pressures are below precompression pressures) since the clay has already consolidated in the past ?
So wouldn't settlement be maily elastic ?

 

[Mccoy]

Also says that Schmertmann spreadsheet is for sands.
Anyone know if I can apply it for clay with the above values.

Not exactly the same method, similar ones though.
A Schmertmann-like spreadsheet has been presented by Mayne & Poulos 1999,

http://ascelibrary.org/doi/abs/10.1061/(ASCE)1090-0241(1999)125:6(453)

you can use it with the edometric moduli to estimate total settlements like BigH suggests. Choice of the moduli without lab tests is a Tricky issue as BigH also said. I can add that choice of the proper edometric moduli is a tricy issue even with lab tests at disposal.
Also, John Focht3 in these fora hinted that he experimented successfully a Schmertmann like proceure in OC clays (didn't provide details though and he's no more writing here'.

 

[civeng80]

Once again thankyou very much Mccoy.
The link gives me an error unfortunately.
After a bit of research overconsolidated clay rebounds an amount with the overburden removed.
So after pressure is applied settlement is elastic with some consolidation (again).

I see your from central Italy. Great place ! we actually come from a small village near Treviso (Veneto Region)

Regards!

 

[Mccoy]

Civeng, good to hear your folks are Italian, ole Italy keeps being a nice place to live in but alas working is becoming exceedingly difficult.

Pls try this preview

http://www.scribd.com/doc/132292944...uence-Factors-for-Elastic-Shallow-Foundations

otherwise just send me a PM

Usually a degree of rebound occurs in deep excavations. If that's your case then some further reasoning is necessary, probably the elastic settlement should be treated with full load whereas the net load (foundation load minus overburden) may be considered in the case of oedometric settlements.

 

[civeng80]

Once again thankyou Mccoy.
I will read the paper out of interest, although I now have the information to design and construct my project.
By the way I live and work in Australia(a great place to be!)

I find geotechnical engineering fascinating and Im getting the book by Coduto (Foundation Design Principles and Practices) for my library.

Regards.

 

[civeng80]

Does anyone know if the Spreadsheet (INFLUENCE) in the above paper actually exists, or do you write your own ?

 

[civeng80]

Does Mayne and Poulos settlement include consolidation settlement as well or just elastic (immediate) settlement?

 

[aeoliantexan]

Going back to the original post, there is a red flag not yet discussed. 18,000psf is a very high bearing pressure for a shallow footing on soil. An unreinforced soil-cement pad may distribute the footing load at something like 45 degrees, but if the footing is large, the bearing pressure is still high for soil. CBR of 10 - 12 is not high, and may represent only a thin compacted subgrade. The pad may support load until the soil-cement shears, then a punching failure occurs catastrophically. BE CAREFUL! You need experienced help supplied with all the details.