drained vs/ undrained strength
drained vs/ undrained strength
(OP)
Dear Friends,
I have the following theoretical doubt and maybe I am doing a stupid question: let you suppose that by means of triaxial CU tests the effective shear strength parameters (c' & phi') of a saturated clay have been derived. Could I derive from the effective envelope the undrained strength (cu) for the examined stress condition? Is this undrained strength coincident with the available effective shear strength?
Please, let you notice that I am not referring to the values of c(CU) and phi(CU) that I could derive from the same CU test, but as I said to the cu (i.e. with phiu=0).
Thank you in advance for your comments..
I have the following theoretical doubt and maybe I am doing a stupid question: let you suppose that by means of triaxial CU tests the effective shear strength parameters (c' & phi') of a saturated clay have been derived. Could I derive from the effective envelope the undrained strength (cu) for the examined stress condition? Is this undrained strength coincident with the available effective shear strength?
Please, let you notice that I am not referring to the values of c(CU) and phi(CU) that I could derive from the same CU test, but as I said to the cu (i.e. with phiu=0).
Thank you in advance for your comments..





RE: drained vs/ undrained strength
Maybe you could use the total stress measurements to back something out of the data.
Jeff
RE: drained vs/ undrained strength
pretty sure I got this right. . . . .
f-d
¡papá gordo ain’t no madre flaca!
RE: drained vs/ undrained strength
Jordan
RE: drained vs/ undrained strength
RE: drained vs/ undrained strength
I just thinking out loud here so forgive me in advance. . . .
f-d
¡papá gordo ain’t no madre flaca!
RE: drained vs/ undrained strength
Is it correct?
RE: drained vs/ undrained strength
I enjoy this discussion and may be off-track somewhere. Maybe I'll be further informed by others.
f-d
¡papá gordo ain’t no madre flaca!
RE: drained vs/ undrained strength
(Quote): ". . . for a given clay with a given stress history, there is a unique qf..p'f..wf relation which applies independent of the type of loading and the degree of drainage during loading. (new para) The foregoing result provides a complete unity for the shearing resistnace of clay under a variety of loading conditios. That is, regardless of how the soil is sheared, the relationsip between strength and effective stress remains the same. However, if two specimens of a given clay are consolidated to the sae stress, p'o, and one then is sheared with full drainage and the other without further drainage, different values of srength qf will result. This difference is explained by the difference in the pore pressures, and hence effective stresses, existing within the two specimens. (new para) By using the pore pressure parameter Af, it is possible to dervie an expression connecting undrained shear strength and intial consolidation stress. . . . the undrained shear strength depends upon the conditions existing before shear, i.e., upon p'o and also upon Af, phi' and c' which are functions of stress history. For normally consolidated Weald clay with Af=0.89, c'=0 and phi'=22, qf = 0.29p'o."
For c'=0, qf/p'o = ( (sin phi')/(1+(2Af-1)sin phi') ) (see equation 28.2. In your case, having the effective stress parameters (and the test results), you can determine Af at failure - then compute qf using these values. Equation 28.1 of Lambe and Whitman can be used if c'o does not equal zero.
I hope this helps you - but it is better to think in terms of stress paths than Mohr's circles per se.
RE: drained vs/ undrained strength
f-d
¡papá gordo ain’t no madre flaca!
RE: drained vs/ undrained strength
Ciao!
RE: drained vs/ undrained strength
RE: drained vs/ undrained strength
f-d
¡papá gordo ain’t no madre flaca!
RE: drained vs/ undrained strength
Thank you very much in advance..
Jordan
RE: drained vs/ undrained strength
RE: drained vs/ undrained strength
here's L&W's equation 28.1:
qf = [c*cosPhi + (po-2Af*qf)sinPhi ] / (1 - sinPhi)
Phi, po and c are barred (effective stress values)
RE: drained vs/ undrained strength
I found also in the Book "Soil Mechanics" (Verruijt, 2001) the following similar equation:
su = c' [cos phi'/(1 ? (1/3)sin phi')] + p0' [sin phi'/(1 ? (1/3)sin phi')]
In this equation there is Af=1/3 and then a more general formulation should be obtained by substituing -1/3 with (2Af-1).
Coming back to my original question, I have derived the following conclusions:
1) in the case when the drained parameters are known, it is possible to derive the undrained strength only if the preconsolidation pressure is known.
2) in the opposite case when the undrained strength is known, it is possible to derive the drained parameters only if the deltaU is known (or eventually the OCR, given that the Af can be empirically derived).
Do you agree?
RE: drained vs/ undrained strength
I disagree that you need to know Pc' in order to determine Su if you know effective stress parameters (I am presuming that you have the back-up data from which the parameters are derived). If the soil is overconsolidated, its Af value will differ from that of normally consolidated clay - and heavily overconsolidated soil will have a very very different Af since the stress path of the HOC soil is to the "right" of the drained condition line (HOC soils are less conservative in the long term and you design not for Su but for c'-phi'. In other words, if you have Af, then you have enough information to determine Su.
RE: drained vs/ undrained strength
You can find Su from the effective-stress envelope and Af, but it's a heck of a lot easier if you have Pc', and probably more reliable, because of sampling disturbance, stress path issues (CIUC vs CAUC vs DSS vs CAUE), etc. One of the main reasons SHANSEP was created was dealing with sampling disturbance. If I could only have one test run on the soil in question, I would ask for oedometers, then look at typical Su/sigma-vc' ratios, such as published in CC Ladd's Terzaghi lecture on SHANSEP in the ASCE JGGE in about 1989. (The lecture was actually in 1986, but he and the editors had a disagreement over number of pages, etc. There is too much meat in the paper for the JGGE's standard number of pages.) I think this would be easier and better than using Af from a CIUC test, and it would allow consideration of CAUC vs DSS vs CAUE.
Are we talking about a real case where somebody has 3x tests but not Pc'? Why would somebody go to the trouble of "undisturbed" sampling, but not do oedometer tests?
RE: drained vs/ undrained strength
RE: drained vs/ undrained strength
Undrained strength ratio in triaxial compression, su/sigmav0', can be calculated as follows:
su/sigmav0'=(M/2)*(R/r)^Lambda
M=6sin(phi)/(3-sin(phi))
R=overconsolidation ratio from isotropic compression
r=spacing ratio, average 2 for most soils
Lambda typically 0.8 (as in SHANSEP)
Going throught the math, this results in similar undrained strength ratios as the SHANSEP procedure, and the authors make the comment that this relationship is what led to the SHANSEP method.