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# 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??

### RE: spt = soil stiffness or soil strength

Okay, I'll jump in here.  I would say the SPT (standard penetration test) gives an indication of soil density and soil strength.  It doesn't really indicate stiffness, except that there may be some correlation between stiffness and the other soil properties.  Stiffness is more stress/strain relationship at small deformations, while the SPT actually fails the soil.
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

Hello pink,

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

Soil strength and soil stiffness are both represented in the SPT results. Remember that changes in moisture content may radically change both strength and stiffness AT THE TIME OF THE TEST. The SPT is a valuable test but, must be used with a good deal more thought than I believe is commonly used.

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

There are numerous published correlations between soil stiffness and shear strength and SPT N-values.  Due to the inherent variability in SPT N (delivered energies can vary from 40% to over 90%), these correlations should be used with extreme caution. In addition, the SPT was invented for granular soils, and is less applicable to cohesive soils, further limiting its usefullness. The best thing to do would be to develop your own site or region-specific correlations between N values and more precise data, such as can be derived from in-situ testing or laboratory consolidation/triaxial tests.  If accurate estimation of soil shear strength and compressibility parameters is important on your project, I recommend you consider use of the Marchetti dilatometer.

### RE: spt = soil stiffness or soil strength

I think there is confusion regarding the term 'stiffness'. Stiffness, in general, is resistance to deflection or bending and is a property of a soil structure, i.e., subgrade. For this question the more useful term is modulus, (Young's modulus or modulus of elasticity) which is a rate of change of strain as a function of stress. It is a basic property of the material itself.

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.

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