bearing capacity for clay soils

[ONENGINEER]

Could somebody enlighten me on the followings:

When samples extracted from the boreholes, their strength was determined by Hand Pentameters. The samples were hard with 4.5+ tsf. This suggests a C = 2250 psf and a bearing capacity of approx. 6000 psf.

However, the clays are moist to dry (summer time) and expected to get wet when it rains and when the moisture increases i would expect a lower C value. How much strength will then the clay have when it gets wet. Thanks.

 

[oldestguy]

It depends. That is there probably are some factors that will change and some that won't. For instance the clay mineral can make a big difference and maybe not, so you should know what the usual mineral is and what it does. In my neck of the woods,we have enough experience through the seasons to know about what to expect, so a dry summer will show those conditions that you describe. I'd defer to those who regularly practice in the area.

 

[ONENGINEER]

Thank you Oldestguy for the reply. Specifically the project is in Houston, Texas, if someone has experience there.

 

[fattdad]

pp of 4.5 tsf would equate to an undrained shear strength of 4.5 ksf. For phi=0 conditions and using Nc=5.7, that'd return an ultimate bearing capacity of 25 ksf! Using a safety factor of 3 would allow 8,550 psf.

If you are measuring unsaturated strength, that'd totally affect safe design. Of course, if you really wanted safe design you'd obtain strength using something other than the pocket penetrometer.

f-d

ípapß gordo ainÆt no madre flaca!

 

[ONENGINEER]

Hi Fattdad

You are right that the PPT is not accurate but I can see that the clay samples are excessively hard (measured in the summer). Also after reviewing the local design works, almost all designs used an allowable pressure of 1500 psf. I guess this is because they believe the hard clay materials soften during the rainy seasons. Is this right? But how much softening one could allow by moisture increase?

 

[BigH]

As fattdad states normally accepted allowable bearing capacity (shear) is 2xSu or equal to qu as determined by pocket penetrometer (roughly - I'd use 9000 psf as rule of thumb over 8550 - wouldn't make any difference).

Using 1500? this seems very off even if the soil is permitted to "wet" during the rainy season - and an assessment of the service limit on allowable bearing pressure (settlement)

I'm just wondering if you are in an expansive soil area of TX .. . they have a lot of problems in some parts of the state.

 

[ONENGINEER]

BigH

You are quite right that the question is referred to the expansive soils in Texas. Curious to know how the low bearing capacity that was discussed could be affected by soil expansion. Of course the soil expansiveness would be dealt with swelling and shrinikg parameters as an input to the structural engineer's slab design but those parameters are independent of hearing capacity to the best of my knowledge. Thank you again if you could clarify as I feel I may be missing something important.

 

[fattdad]

if the clay is truly hard and the shear strength is truly 4.5 ksf, there'd be no softening when the formation got wet. Your question about softening by water implies that the original shear strength is for some unsaturated condition.

I deal with strange perceptions all the time. That said, the shear strength of soil is (almost) always determined in saturated/wet samples. Direct shear test, triaxial strength test (CU-bar) and such are done under saturation.

f-d

ípapß gordo ainÆt no madre flaca!

 

[oldestguy]

If you could sample, undisturbed clay from the area whereby those samples are at differing moisture content and run unconfined compression tests on each, my bet would you would find a definite relationship between the moisture contents and the strengths. Using disturbed samples that also is likely but dependent on other factors. Therefore your assumption that shrink and swell is not tied to bearing capacity is questionable, depending on the sitesituation. That said, depending on foundation depth,you may not have structural movement with seasons, but then the expansion-vs- pressure applied relationships will come into play.

 

[oldestguy]

One more from OG. Limiting the "bearing capacity" definition to shear strength, as apparently is the subject 0of this post, may of may not apply to real life. Raising and lowering of foundations due to moisture changes, without a shear failure or consolidating (lower void ratio) due to loads may have limits that dictate what pressure to use for foundation design. In my experience it is very rare that shear strength is a limiting factor in deciding design bearing pressure.

 

[ONENGINEER]

i am wondering while shelby tube samples are very hard, why almost al the local consultants use an allowable bearing pressure of 1500 psf. Too low(?)

 

[Okiryu]

Perhaps other local consultants are not doing actual geotechnical investigations and therefore they are just using tabulated values from available codes. For example, the IBC has a table for presumptive values for allowable bearing pressure which shows 1,500 psf for clays...

 

[ONENGINEER]

Thanks Okiryu. As well, the ground water is usually > 10 ft deep. Being in a hot climate, clay soils are subject to alternative swell and shrinkage processes based on surface water and precipitation conditions. I guess when the soil swells, it means that it was not saturated (?), supporting the Fattdad's comment above. I am certainly enlightened, though a new question arises about a rule of thumb relationship between the unsaturated strength and saturated strength of a clay samples. Let's say if the shear strength of an unsaturated clay soil (e.g. moisture 15%) is 4.5 tsf, how much would be the strength if it swelled and become completely saturated.

 

[Okiryu]

Although this is not a direct answer to your question (relationship of saturated and unsaturated clays shear strengths), you may also use Professor Stark's spreadsheet which relates LL to residual and fully softened shear strength for clays. Note also that the shear strength depends on the effective vertical stress.

This is Professor's Stark website from where you can download his spreadsheet:

http://tstark.net/geotechnical-software

 

[fattdad]

Stark's relationships also require the hydrometer assessment for weight percent of clay-sized fraction. He's big on the torsional ring shear! An alternate to ring shear would be to perform direct shear tests on normally consolidated reconstituted samples. CGPR (Virginia Tech) has a workshop paper on softened strength.

f-d

ípapß gordo ainÆt no madre flaca!

 

[ONENGINEER]

thank you for your detailed answers. Just to add that my question is that of professional practice. Simply in the hot summer time the majority if Pocket Penetrometer tests on Shelby samples of clayey materials show 4.5 tsf or more, i.e. very hard clays. I would like to know in the raining seasons that the soils get wet, would the strength be the same as 4.5 tsf? Thank you again.

 

[fattdad]

in response. . . It depends. . .

If the 4.5 tsf clay sample is from the upper 4 or 5 ft, then there's likely a change in moisture during the year and unsaturated strength may be found during the summer - strength that is temporal. At greater depths, there is much less change in moisture and the temporal variability much less.

f-d

ípapß gordo ainÆt no madre flaca!

 

[Okiryu]

ONENGINEER, appears that there is not a straight answer to your question, but why not push the pocket penetrometer during investigations in rainy season, so you can develop your own correlations for your local clays?

 

[geomane]

From what I have read, saturation has no effect on true cohesion (colloidal size particles). It does however reduce apparent cohesion such as aggregate interlock and capillarity. There is a table in the FHWA Manual for Soils and Foundations Vol I that shows this.

 

[BigInch]

As I am originally from Houston and have worked on many projects there and all along the Gulf Coast, I can tell you that typically "net" soil bearing capacities for Gulf Coast clays are usually within 3000 to 6000 psf, unless your project is located in the middle of a bayou where the clay remains relatively unconsolidated, in which case you should be on piles. 1500 psf for the typically very stiff clays would be low and I have only seen those kinds of numbers used when nobody on the project had had any geotechnical experience at all. Expansion and contraction of the clay with moisture content swings between winter and summer can be a severe problem, but that effect is almost completely independent of the bearing capacity used.