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bearing capacity for clay soils

bearing capacity for clay soils

(OP)
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.





RE: bearing capacity for clay soils

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.

RE: bearing capacity for clay soils

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

RE: bearing capacity for clay soils

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!

RE: bearing capacity for clay soils

(OP)
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?

RE: bearing capacity for clay soils

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.

RE: bearing capacity for clay soils

(OP)
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.

RE: bearing capacity for clay soils

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!

RE: bearing capacity for clay soils

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.

RE: bearing capacity for clay soils

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.

RE: bearing capacity for clay soils

(OP)
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(?)

RE: bearing capacity for clay soils

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...

RE: bearing capacity for clay soils

(OP)
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.

RE: bearing capacity for clay soils

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

RE: bearing capacity for clay soils

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!

RE: bearing capacity for clay soils

(OP)
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.

RE: bearing capacity for clay soils

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!

RE: bearing capacity for clay soils

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?

RE: bearing capacity for clay soils

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.

RE: bearing capacity for clay soils

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.

RE: bearing capacity for clay soils

Where along the gulf coast were you working? I work in northwest Florida, south Alabama, and south Mississippi and have only recommended an allowable bearing pressure of 3,000 psf a hand full of times (everything else usually 2,500 or maybe 2,000 psf). That is because settlement usually governs.

RE: bearing capacity for clay soils

(OP)
Thank you for all the enlightening comments but still I cannot answer this simple basic soil mechanics question to myself. The projects I referred to were from North and Central Texas. I was previously involved in some projects in Houston, where they also recommend an allowable bearing pressure of 1000 psf in the majority of the documents I reviewed. This could be regarded as an ultimate bearing capacity of approx. 3,000 psf. Still 2-3 folds smaller than what is given by jmcc3265. I arrived in this area in early summer and have not stayed long to experience the shelby tube samples obtained in the wet seasons. I hope local practitioners could provide this information, if I won't have the chance to stay until the next wet season. Based on my observation of the shelby tube soil samples in the relatively dry summer season, the use of an allowable bearing pressure of 1,000 psf seems to be very conservative and one could question the benefit of drilling and testing - unless the project is founded on marshlands. As Fattdad mentioned above, with most of the samples having a PP of > 4.5 tsf, the allowable bearing pressure is expected to be at least 8550 psf. That is why I thought the designers may have assumed that the samples would not be as hard in the wet season and this was the reason behind my original question. I would like to know how BigInch and jmcc3265 reached to the pressures they have reported. Are values given by BigInch and jmcc3265 obtained through bearing capacity formulations or would they quote it based on a good local practice estimate. Thank you again for your comments and I hope I could state my question clearly.

RE: bearing capacity for clay soils

Why can't you ask your associates who are more experienced in the area why they "do what they do"??? Seems reasonable since you are new to the area. To use 1000 psf, the undrained shear strength would have to be in the order of 50 kPa - taking settlement into account - probably more like 40 kPa. The soil has to be quite soft for that kind of bearing pressures.

Could they be using such low values to ensure a minimum footing width - an intimation of such?? We would, in Toronto, use a minimum 600 mm strip footing width and a 1000 mm square footing width - I realize that for "residences" you could go to 450 mm but 600 is a nice number.

RE: bearing capacity for clay soils

You should not use higher end values without a site geotech study that classifies all soil layers that will be affected by the foundations as high strength clays. Settlement, especially with the high degree of variability of soil moisture that the Gulf coast can experience can be a significant problem, especially for certain types of structures. I believe that potential long term settlement issues should be considered separately from defining a general site allowable net soil bearing capacity. I don't think that defining a net soil bearing capacity to include settlement limitations in a general sense is very good engineering practice, although that approch might work for a bunch of light barnyard buildings. All foundations and structures do not have the same settlement limitations or tolerance. Well designed foundations can proportion structure loads to equalize soil loadings resulting in nearly equal settlement across the foundation. Differential settlement is more often the problem, rather than the total absolute value. Foundations for sensitive rotating machinery might have 0 tolerance for differential settlement, but allow some absolute uniform settlement. Soils could be preconsolidated to minimize long term settlements by preloading for a year or so, prior to actual construction. Moisture content variabity under or near the foundation might be controlled by chemical soil treatments, proper drainage, or permanently draining the swamp. If you don't do a geotech analysis and you're placing a foundation in shallow soil near a geologically recent bayou, sure, use 1500 psf and don't build anything that will be sensitive to short or long term settlements, or put posts down to the layer that soil moisture no longer varies. If you have good drainage and are on a good strong clay, or can dig down to layers where moisture content if constant, you can easily get 4000+.

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