Friction angle and N of SPT

[geotechniqa]

Engineers always find the drained friction angle of the soil from the N reported in field of SPT tests from Meyrhof oo Peck relationship.;e.g bearing capacity calculations

-My first question should N taken from field be corrected before these formulas are used. Please suggest

-For sand that operates tested under drained condition the Mohr Coulomp failure friction angle is unique (we have one friction angle) regardless of the density of the sand: Loose, Dense, Medium. Since N is changing with density, then how the correlation between N and the Mohr Coulomp failure friction angle could be logical . This makes me really puzzled

Please

 

[fattdad]

-For sand that operates tested under drained condition the Mohr Coulomp failure friction angle is unique (we have one friction angle) regardless of the density of the sand: Loose, Dense, Medium.

I don't agree with this statement and am not sure of your direct question.

f-d

¡papá gordo ain’t no madre flaca!

 

[moe333]

The drained friction angle is a function of density, grading, angularity, mineralogy, etc. These also affect the N Value, and accordingly, the correlation to drained friction angle.

So, to answer your question, the friction angle does depend on the density.

 

[geotechniqa]

Well when you conduct a direct shear test... you perform the test for different confining stress. When the confining pressure changes, the void ratio will also change and with higher confining pressure we get denser sand. Bur at the end of the test and for several samples tested at differet confining pressure;i.e different void ratio (different density), we obtain only one drained friction angle.. do not we..?

 

[geotechniqa]

moe333
The grading, angularity, mineralogy are all inherent parameters and I know that N will differ based on then.
I am talking about the density only (a state parameter and inherent parameter)

 

[DRC1]

When you perform a direct shear test you run the same sample at three diffretent overburden stresses. But the density does not change much. It does change a little and if you use a wide range of confining sttresses, the failure envelope will have a slight curve to it, due in part to densification of the sample. Generally this is ignored and a straight line is assumed. For cohesionless material the angle of friction is highly dependent on density.
I generally use the relationships from Terzaghi and Peck. Prior to corralating N to phi, The n value is corrected to corespond to the effective stress at 20 feet with out groundwater. T&P devloped the corralation for the construction of basements for large buildings which at the time were 20 feet deep. I would assume the Meyerhoff would need to be normalized in a simmilar manner.
A soil with a blow count of 10 10 feet from the surface is much stronger than a material that is 40 feet below the surface with a blow count of 10

 

[fattdad]

When the confining pressure changes, the void ratio will also change

The implication of this statement is not correct. There are MANY instances where you have dense sand overlying loose sand (i.e., where a looser sand is under much greater confining stress). The density (and correspondingly the strength) of sand is determined by depositional environment.

When a sample of sand it sheared, you either generate positive or negative pore pressure depending on whether you are below or above the critical void ratio for that stress condition. That said, the miniscule change in void ratio from varying the confining stress in a direct shear test is not that relavent.

f-d

¡papá gordo ain’t no madre flaca!

 

[geotechniqa]

hello DRC1 You said
"The density does not change much". I do not agree with you.
If you have loose (virgin/normally conslodated) sand fill and you subject it to a high confining stress ( compact it), it well experience large change in density and it will become dense. For, example, you may get at the begining (before compaction N=15 while after applying the high confining pressure (densifying it) you will get N= 35-40. When performing direct shear on that sand at the two different states (using the two confining pressures applied), and using MOHR_COLOUMB ENVELOPE criterion,, classically we are assuming that we have the same effective drained friction angle from the test

 

[moe333]

Similar to what the others have said, you can think of it this way:

Run a direct shear test with a loose sand at a range of confining pressures and you get a particular friction angle. The sand would be lightly compacted.

Run another direct shear test with the same sand, but compact the sand so that it is relatively dense. Do the tests at the same range of confining pressures. You will see that you get a higher friction angle than you obtained with the loose sand. Confining pressure is not the same as relative density.

 

[geotechniqa]

Imagine the Virgin sand/silty sand like a normally consolidated clay: for each confining pressure you have a corresponding void ratio (compression line). The sand in the second case is overconsolidated and it will mostly experience only elatic change in the volume/void ratio/relative density.

 

[fattdad]

Compacting and changing confining stress are two different things.

Good luck, but I'm not sure how many different ways folks can explain that the failure envelop for a loose sand and the failure envelop for a dense sand are different, irrespetive of confining stress at the time of shear.

f-d

¡papá gordo ain’t no madre flaca!

 

[DRC1]

I will take a stab at it. In order to have densifcation of sand, you need to reduce the volume. that is typically done by reducing the ht of the soil through some lateral spred. the lateral spread is less than the drop in ht. The rings in the direct shear bpevent lateral spread and thus volume change. as the rings shear there is a small amount of room created for densifcation, but in general it does not effect the results in a meaningful way. In practice the confinement of the overburden may be sufficent for materials to prevent densification. After vibration or liqufaction where the fefects of overburden are interupteed soil will settle to a denser stronger state.

 

[geotechniqa]

Suppose that the sand has some silt or clay and you are managed to get samples for doing a CD triaxial test. Should my argument above be right.

 

[moe333]

No, it wouldn't. The friction angle would largely depend if the response to shear is contractive (loose or soft soil), or dilative (dense, or stiff). So it is still dependant on the relative density of the soil. And, the N value correlations are really only valid for cohesionless soils. Once you have any significant clay content, you don't want to be estimating friction using an N value.