spt = soil stiffness or soil strength
spt = soil stiffness or soil strength
(OP)
can some one tell me do we determine soil strength or soil stiffness by using spt value??
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spt = soil stiffness or soil strength
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RE: spt = soil stiffness or soil strength
For practical use, there are correlations between SPT and both soil density and allowable bearing capacity for cohesionless soils, and for strength of cohesive soils.
RE: spt = soil stiffness or soil strength
SPT is a vast subject and I am not able to give you the complete picture. You can find enough literature in many good manuals on the subject and perhaps look a little deeper in the theory behind it (Bowles books are a good start, I have read the subject in his Foundation engineering).
From a practical point there are correlations between N values and density, unconfined compression strength AND modulus of deformation. In each case the proper correlation depends on the nature of the soil examined. So you need to have properly identified the soil before using a correlation formula. There arealso cases when you must not use any correlation, for example if you have extra high N values and the soil is identified as containing big gravels then these values are not representative of the global behavior of the formation.
In my opinion the type of the test reflects some characteristics that are both deformation and failure. In hitting a soil by the sampler there is failure as the grains break their connections and further compaction shows how the soil becomes stiffer when stress continues to apply.
There are more on the subject that i don't know and a lot on the use of these type of empirical indices that are not easily understood, at least to people of my level of knowledge, although we use these indices frequently.
Good luck in your enquiry and it is always nice to see people wondering on subjects that we have been used to use without many questions :).
Dimitrios
RE: spt = soil stiffness or soil strength
The problem is to obtain a proper definition of what is being obtained by the test and what is required by the project to be designed.
I oftentimes deal with unsaturated soils, oftentimes dessicated. Very dry alluvial clay my have high SPT, be very stiff and exhibit a lot of strength, IN THAT DESSICATED CONDITION. After development of the site, usually with landscape irrigation, drainage water accumulations and decreased soil moisture evaporation beneath structures and pavements, the soils soften. A new SPT may be radically lower lower, strength is less and the soil is less stiff.
If I am measuring a sand SPT, the change in the strength and 'stiffness' ( actually 'firmness' for granular soils) may not be near as 'radical' as for the clays.
The problem then becoms, is the SPT actually measuring what I need? If I need a saturated clay measurement, the SPT in a dry clay may be only a crude approximation. If I am dealing with soils cemented with a soluable mineral (gypsum), the dry SPT may be very high but, the soils may actually be collapsible (metastable) when saturated. The high SPT is not only useless but, may be dangerous!!
So I repeat ... The problem is to obtain a proper definition of what is being obtained by the test and what is required by the project to be designed.
RE: spt = soil stiffness or soil strength
RE: spt = soil stiffness or soil strength
It must be remembered that soil is compacted or densified to change its properties. We must think soil modulus or structural stiffness (depends on application) instead of density to more accurately create what is really needed.