Select Design Undrained Shear strength in clay with varied Su for strip footings

[rainandcm]

I had a bunch of in-situ testing that gave me varied Su for clay soil, with values from 1000 psf to 4000 psf. If I am using strip footing, it seems to be really conservative to simply use the lowest value. I think strip footing should be able to bridge over the weak spots, but I couldn't find any reference to back up my thought. Any suggestions?

 

[BigH]

Have you plotted the Su value to depth? Sounds like you have a desiccated crust. This is common in many places - for instance in the Sarnia Ontario area. By doing this you should be able to see a pattern of high at shallow depth lowering down, then regaining strength with depth. Many now use the "average" line and apply statistics and all sorts of things to the data; we used to draw in a "biased" design line with a few points lower but most above the line. Yes it is conservative but it is easier to deal with rehabing a superstructure than a substructure.

Once you have this, then by placing your footing at various depths you can make an estimate of the allowable bearing capacity; then, carry out analyses for estimated settlement and adjust the allowable bearing pressure from the two values.

 

[rainandcm]

Thanks. I think the biased design line with a few points lower should make sense. Thanks!

 

[Mccoy]

The biased design line which BigH mention ( I read it 'conservative trend') is what the Eurocodes have regulated as a trend of characteristic (conservative) values.
The solution might avoid the trend and involve multiple layers, and this is what is done sometimes when the structural engineer uses directly his geotechnical analysis add-ins which are linked to the structural routines of the software.
At least, this is the SOP now in seismic projects in my place.

The question if a strip footing can bridge over weak spots (weaker random fluctuations of the shear strenght of the soil) is not an easy one. It depends on the strip size, especially its width, and the size of the random fluctuations of the soil properties. If strip footings are connected into a rigid foundation framework, than the foundation is likely to behave as a large unit and it is not sensitive to weak flucuations (that is: the values which govern are not the minimum ones, rather the average ones).

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