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Clay stratum N SPT > 50 settlement

Clay stratum N SPT > 50 settlement

Clay stratum N SPT > 50 settlement

Hello, I am new to the forum, but I've had my geotechnical lab for several years now. I have a doubt which I hope someone can solve: Can a clay stratum with N SPT > 50 be considered incompressible regarding to consolidation settlement? Thanx beforehand.

RE: Clay stratum N SPT > 50 settlement

It would depend on the load and the area over which the load is applied. No strata is truely incompressible.

Mike Lambert

RE: Clay stratum N SPT > 50 settlement

The behaviour of this clay will be close to an elastic behaviour and not a consolidation one.

RE: Clay stratum N SPT > 50 settlement

If it's saturated and there's volume change there'll be "consolidation." That said, from a practical standpoint, I'd just use elastic theory as a gauge of settlement and divine some value for C-alpha to assess long term void ratio change under constant effective stress.

Unless you are dealing with a large areal load, it's not likely to be to compressible though.


¡papá gordo ain’t no madre flaca!

RE: Clay stratum N SPT > 50 settlement

You don't provide much information? It is probably relatively incompressible for most applications. I can provide many examples of when it's not incompressibe or elastic as Big Harvey stated.

RE: Clay stratum N SPT > 50 settlement

Well, I agree with your comments and it's nice to know we share the same engineering judgment smile. I normally calculate the elastic settlement (with a very high modulus of elasticity, the SPT samples are very hard) and assume there will be no considerable consolidation. I can't find any information on how these type of stratum settle, but I agree with you that it should behave as a relatively incompressible stratum.

Answering to morelat221, a road bridge in northeast Mexico is going to be replaced with a new one (I didn't ask why, most probably is to increase hidraulic area) and the strata with N SPT > 50 are at about 10 meters deep, and after that, they increase. So I am proposing piles embeded in these stratum to support the new abutments. The liquid limits on the support strata are OK, from 20 to 35 and void ratios of about 0.5. The water level is about 6 meter deep. I am also proposing an optional shallow foundation, because the upper strata have lesser N SPT values but are low compressibility clays and sands, too.

I also made my original question because it has happened to me that the N SPT > 50 strata, is made of CH clays. So we have an stratum of a material with a very high cohesion (sometimes with rejection to the SPT head) but also high liquid limits.

Thanks everyone for your comments.

RE: Clay stratum N SPT > 50 settlement

In clayey Strata SPT > 50 Possible? juss question nt answer..

RE: Clay stratum N SPT > 50 settlement

Now to worry about scour. . .


¡papá gordo ain’t no madre flaca!

RE: Clay stratum N SPT > 50 settlement

Look at "Soil Mechanics", 1969, Lamb & Whitman, page 321.

RE: Clay stratum N SPT > 50 settlement

In my case, common practice is SPT-N > 50 could be considered rock. In this case I would call it a "claystone" instead of just a clay.

Due the high SPT values, consolidation settlements are not a concern. If you treat this as elastic settlement, settlement values will be very low as moduli of elasticity (E) for SPT>50 is way too high.

roadrunner13 said the strata above these SPT-N>50 layer "have lesser N SPT values but are low compressibility clays and sands, too". "have lesser N SPT values" is very vague comparison.

You might have SPT-N between 20 and 30 for sands (considered medium dense) and they might be subject to liquefaction potential. So that something you need to clearfy.

Other than that, having low compressibility soils above this "clay layer", I don't see the use of deep foundations. Probably shallow footings will be the best option.

RE: Clay stratum N SPT > 50 settlement

Just watch out for freezing and thawing, wetting and drying. Those can turn exposed claystone to clay pretty quickly.

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