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.

 

[geomane]

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.

 

[ONENGINEER]

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.

 

[BigH]

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.

 

[BigInch]

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+.