Conventional Foundation in Soft Clays 1

[cbear1]

I am a civil engineer reviewing the geotechnical investigation for a foundation retrofit of a house in an area with firm to soft clays. (Blow counts of 6-12 in upper 10 feet, drastically dropping to 4-5 down to about 41 feet) Groundwater is at 5' below grade.

The only testing done has been moisture content (30-40%) and pocket penetrometer (hovers around 1-1.2 tsf for the first 10 feet, drops to <0.5 below that) and the geotech slaps a value of 1000 psf for soil bearing pressure - based on a conservative guess?? First of all, I'm extremely leary of pocket penetrometers (especially taken on recovered samples, not in-situ) and I would assume that for a sensitive soil such as clays that there would be some consolidation tests run, as well as an undrained shear tests (something beyond a liner sampler..)

I want to make sure that we are getting real values and not just assumptions for what this soil will do, as right now the geotech has recommended simple conventional foundations 2' below grade for the site, while the soft soils and high groundwater would indicate more analysis is necessary...and can you really get compressibility, strength and expansion characteristics from the natural moisture content??

Any thoughts?

Thank you all in advance!!

 

[BigH]

cbear1 - I would look at your site, given the data you gave as a 10 ft desicated crust (N 6 to 12 presumably with the higher values higher up) over more normally consolidated clay below 10 ft. Your N values of 4 to 5 do not really hit me as "soft" like you have described - I would say it is more like low end firm.
Very roughly, there are correlations (and purists say they are bunk as SPTs do not give "good" values of undrained shear strength) - but, in any event, an SPT value of, say 8 would imply an undrained shear strength in the order of 1000 psf. SPT value of, say, 4, would give an undrained shear strength of about 500 psf. As a rule of thumb, the allowable bearing capacity is about 2x the undrained shear strength. So, given the above, your allowable bearing capacity in the upper 10 ft would be in the order of 2000 psf and below about 1000 psf (what your geotech gave you). As you indicated, you would found at a depth of, say, 2 ft. This gives 8 ft of "okay" firm to stiff soil (oops, medium stiff to stiff for you Yanks) below your footing. Using strip footing depth of influence for shear as about 2 max (1 is more likely) for bearing capacity, you would not expect the allowable bearing capacity to be less than about 2000 psf.
Now, the kicker is the allowable bearing pressure that is based on the serviceability limits - i.e., settlement. For strip footings, the depth of influence is about 4x the width of the strip or 2x the width for square or round footings. Again, the "bottom" of the stiffer upper layer is about 4x the width of a normal housing footing (24 inches - sometimes 18 inches) so I would think that the full influence of the footing bearing pressure at 2000 would be in the upper stiffer zone - to be safe, a limiting value of about 1500 to 1750 might be used as the "design" bearing pressure. So, I think your geotech might be a bit on the low side. But, in the end, too, there is usually a minimum footing width used - say 20 inches (8 inch wall and 6 inch projections each side). Given your house is likely only two storey wood frame, the actual bearing values might only be in the order of 1000 to 1250 psf. So, your geotech was being pragmatic in giving you what you needed, not necessarily what you could use, say if your structure was bit more robust. He was happy to ensure that settlements wouldn't be large (leading to cracking, etc) and didn't give you a bearing pressure that you couldn't use anyway due to minimm footing widths, etc. In cases like this, I used to say in reports "For the type of foundations envisaged for your project, actual bearing pressures on minimum width footings would be in the order of 1000 psf and at this bearing pressure, expected settlements would be within tolerable limits." In this fashion, I didn't give a "real" allowable bearing pressure (like if the project was a storage tank or a 3 to 4 storey commercial building). I know that you could "get more" bearing - but you don't need it so why give it.
Now to your question on the soil tests, etc. It is quite likely that the cost of the investigation was in the order of only $1000 or so - maybe even a bit lower. This does not give the geotechnical engineer much money to do "fancy" tests (consolidation - usually at costs of $250 per test or more), undrained quick triaxials (likely about $75/test or so) - he would be doing a fire sale. A few moisture contents (and an Atterberg limit would be nice) would be all that he would likely do for what, to him, is a simple routine job in an area he knows well. He found no surprises. I, too, am not a big fan of the pocket penetrometer but your values "justify" the general SPT correlations.
In the end, even if the geotechnical engineer gave you a design bearing value less than you might have had, it would be likley be less than a cubic metre or so of extra concrete for the footings, in all liklihood. The real reason for the geotech, in this instance, is that (1) he ruled out any big surprises (using (hopefully) his experience in the area) such as swelling clays, peat or other unsuitable materials (poorly placed fills) at the site and (2) gave you a bearing value that is justified for your project although it might be understated for, say, more important types of structures (no knock on residential developments).
Sorry for the long post, but hope this clarifies some aspects of "geotech" engineering for you. For the type of project you have, "text book" or "university" computations just aren't done.

 

[hokie66]

You mentioned that this is a "foundation retrofit". What are you retrofitting? With this type material, I would want to know how the material will behave through droughts and wet periods. Volume change of clay under houses is usually more important than strength. If it is a moisture sensitive material, strip footings may not be the answer.