Soil bearing capacity from Cone Penetration Test and SPT

[JoeH78]

Dear All,

I have an existing soil report where I'd like to determine the soil bearing capacity of a soil. In the existing report there is Cone Penetration Test and standart Penetration Test result performed at the different location of site. Based on that there is detailed soil stratum (silty sand, sand, clay) average Qc , equivalent SPT blows, m value, Su undrained shear value, Phi (internal friction angle) drained, Dr (relative density) printed for CPT. SPT results are simply plotted on borehole log which shows avg blows as per depth.

Normally if I have simply SPT in hands I'd go simply with Terzaghi & Peck (1967) or Meyerhof(1956 and 1974) formulation to determine the soil bearing capacity.
For CPT results case I plan to use (Caquot equation), (Koppejan equation) or (DeBeer equation) for determining soil bearing capacity.

Can you shed some lights whether my reasoning for getting the soil bearing capacity is correct or not ?

 

[oldestguy]

Unless you do this sort of thing for a living, I'd be very wary of this "cook-book" method of answering your question. There is more to it than an easy way. Consulting with a geotechncal engineer is the least you should do.

 

[JoeH78]

Hi oldestguy,

Please feel comfortable at answering my question, it won't be a cook-book that I'll built Taipei on three easy step, I already get in touch with a geotechnical engineer and yes generally I don't do soil mechanics or dynamics for living. I simply use those parameters provided by the geotechnical engineering for structural design.

But since soil mechanics is a part of Civil Enginnering and once we calculated all those settlement, soil bearing, lateral stability etc.. in college its a kinda ugly for me not to know basic steps.

 

[WARose]

I'm with oldestguy, I've looked at SPT data and ran some numbers [Bowles's book has a formula for this; I can look it up if you want] and came out with a bearing capacity far greater than the geotech gave me. At the end of the day, it's a professional judgment call not just based on experience but other factors as well (settlement and so forth). By the way, if you have a geotech report, it doesn't recommend a bearing capacity? It's just raw data?

 

[BigH]

WARose made an important distinction. The safe "bearing capacity" is a value determined by the shear strength of the soils - along with geometric and load application modifications. This, in my view, is seldom, the value one would use in design as in most instances it is the safe "bearing pressure" which is based on the serviceability conditions imposed that controls. In the charts that have been mentioned, the "permissible" settlements are based on 25 mm of permitted settlement - although Bowles does increase his values from those originally posed by T&P and others. Why 25 mm? - and again, these are not the bearing capacity but a safe bearing pressure. Why not 40 mm which, if my memory serves me correctly, is specified by the Indian Standards for settlement? Why not 10 mm which might apply for a foundation with a very settlement sensitive machine or structure?

Most geotechnical engineers will give the safe bearing pressures in their report. Now that you have LRFD coming into being, it will become a bit more confusing I am sure as the safe bearing capacity will be required and then, as explained above, a safe bearing pressure based on serviceability requirements will need to be provided. All to make structural engineers happy (my personal view as I believe much more in the traditional methods for geotechnical engineering).