Friction angle and N of SPT
Friction angle and N of SPT
(OP)
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
-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





RE: Friction angle and N of SPT
I don't agree with this statement and am not sure of your direct question.
f-d
¡papá gordo ain't no madre flaca!
RE: Friction angle and N of SPT
So, to answer your question, the friction angle does depend on the density.
RE: Friction angle and N of SPT
RE: Friction angle and N of SPT
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)
RE: Friction angle and N of SPT
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
RE: Friction angle and N of SPT
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!
RE: Friction angle and N of SPT
"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
RE: Friction angle and N of SPT
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.
RE: Friction angle and N of SPT
RE: Friction angle and N of SPT
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!
RE: Friction angle and N of SPT
RE: Friction angle and N of SPT
RE: Friction angle and N of SPT