Relationship between soil parameter phi 2

[GraemeL]

I am confused as to whether there is any difference between the undrained angle of shear resistance (phi) and the drained shear strength parameter (phi').

Would they be the same value (in degrees) for a sand? If so, the failure envelope for the total stress situation would have the same slope as that for effective stress case.

If not, how does one get phi' from a known value of phi?

Is a clay different from a sand in this regard?

 

[ishvaaag]

Well, there's an amount of information regarding this matter and is not so easily manageable by those not highy proficient in geotechnics. Having different name, won't be the same thing, nor for damp or saturated and even submerged sands where they are close. For clays it suffices to see some shear test chart figures to see that the same are not.

There's also the question, what is the approach to security? For most ordinary designs all is a matter of choosing safe values as satisfactory practice ordains, other thing is to become a proficient geotechnical expert. This is true for sands and maybe even more for clays where friction can be altogether disregarded.

Yet apart of our ordinary ways, it is clear that we may also consider the history in the time of the solicitation, where breaking the initial friction plus adhesion (and some adhesion even by capilarity is accounted in the angle of inner friction, p. 101 Bowles 5th edition) costs more than when the bond is broken and is then higher than the final strength, ad the maximum strenght may be used as the referent for strength. Part of this view is no doubt accounted even in the angles of inner friction proposed for the practitioner. This way, in no agitation, one may think a submerged sand has the same friction angle than dry, close to at least...yet in more detailed scrutiny it shouldn't show to be such due to the equalization of water tension around the grains.

Further than that the differential on the stable initial status and one where liquefaction in one or other way, dilution included, -this also happening in soils of sandy nature- appears makes that assuming in general the equality I wouldn't think to be proper.

Other way said, these scientific theories have developed on the need to describe the nature and different phenomena happening in the soils, and then have been arrayed as tools for engineering use. The procedures are many as are the cases, and the suitable tools need be used to properly ascertain the significant aspects of the prediction of the behaviour for a project. As I stated above, 2 different names represent things that are someway never entirely the same, there are always nuances that can make to the aware preferably the use of one than that of the other.

 

[grafter]

Suggest you read a good basic soil mechanics textbook...DAS, BOWLES should suffice.

 

[GraemeL]

I suppose I should have made it clear that my confusion still reigned even after I had gone through various texts on soil mechanics. I have found that this subject is not treated very clearly by those I have read. Hence my resorting to this forum!!!

Terzaghi & Peck, R. F. Craig "Soil Mechanics" and even the good old US Army Corps of Engineers "Bearing Capacity of Soils" etc etc etc.

It would have been of most benefit to me if a reply had said that:

1. There is a difference.... and given an explanation as why or: There isn't a difference.

2. No, they wouldn't be the same for a typical sand ...

3. A relationship between the two angles for a sand = ...... see such and such book or: No relationship been documented.

4. Clays are different in the following areas ..........

Or am I asking too much of a resource such as this? I don't want to sound ungrateful, it is that I am just none the wiser from the above replies other than to go off and do more head scratching and research.

 

[ishvaaag]

You don't sound ungrateful, don't worry. The thing is that quoting the exact phrases of the books that support what I have put still would be limited in ability to satisfy -very reasonably- your need for conviction, and then the study would still be required...and in it you will continue finding the same opposed views that cause your confusion, because lots of times the referred soils are different and so are the focuses of those who write, and even then, deepening in knowledge of what said often makes you so aware of dissent that is difficult to assess to what theory of knowledge one must attach to.

For my practice as an architect only much rarely this will become a critical matter, and the same will happen I think to most structural engineers. Geotechnical people is other thing, since they have deeper training and face more and more critical cases. Wish someone can give you more satisfactory answer, and they will do better than I. No that I am not willing, but giving a list of my geotechnical books where I would deepen for your matter is not a big deal for a serious professional like you sound you are. Good luck.

 

[DRC1]

This really isn't as complicated as it is often made out to be. Concentrating on a sand for the moment,the phi angle is determined by laboratory testing, either direct shear or a triaxle test. The direct shear is a test where sand at a computed density is packed into a mold that is split in two. A weight is applied in the vertical direction. The horizontal load required to shear the soil is measured. As the vertical weight is increased, the horizontal load incrases. A plot of the result yields the phi angle. The direct shear test is a drained test because the water is allowed to drain, so there is no increase in water pessure. A triaxle test is where the sand is placed in a membrane in a chamber The top and bottom of the membrane are conneted a water line. The bottom of the sample rests on a fixed pedistal, at the top of the sample is a ram that can apply a vertical load. The sample is placed inside a chamber an flooded with water. The waterline saturates the sand. The chamber pressure is set to a selected value. Load is applied to the ram until the soil fails. The drainage of the soil can be prevented by closing the drain lines durring the test. The water pressure inside the soil can be measured (this is the pore pressure - the water pressure inside the pores). Repeating the test at several chamber pressures yields different ram loads. A plot of the two yields the phi angle. However this is dependent on the pore presssures. Since we measured the pore pressures, we can subtract their effect and plot the phi' line.
When designing in sands, draiage is ussually fairly rapid, so the phi' is usually used for design. However, it is refered to occasionally as phi. rarely in design do you use the undrained phi angle. If a table lists phi angles for varrious soils, they almost always mean phi'. In a clay, drainage is much more slow. Drained analysis of clays is similar to sand except that the phi' is much flatter. However the undrained strength which occurs the moment the load is applied is strictly a function of the coehsion of the clay. and phi' values do not enter into undrained calculations.
In summary, Phi' is the important value for soil strength for a given type of soil and often phi' is refered to as phi.
I hope this made this cleare, instead of more confusing. Good Luck!

 

[grafter]

My response wasn't meant to sound terse - apologies if it did. I find the textbooks that I referred to quite useful for foundation engineering (particularly Bowles Foundation Engineering - it's excellent) and hoped that you would find the answer that you were looking for in either. I just felt that the subject may be too large to try and cover on-line - that's all.

From my SI and geotechnical design experience in the UK, I would generally reiterate DRC1's commentry but would add the following (apologies if this seems basic and obvious to you and others):

1. SAND soils tend to be free draining and hence pore pressures generated from foundation loading and settlement generally occurs during construction. Therefore the drained soil parameter phi' is used in the calculations as you are considering effective stress analysis. Phi' can be measured indirectly by SPT/CPT correlations or in the laboratory by Shear Box or Triaxial Testing. Indirect methods are generally used, particularly SPT correaltions from well documented charts published by Terzaghi, Peck, Thorburn et al. Laboratory testing can be problematic (i) Shear Box tests fail the soil along a pre-determined failure plane and hence may not be representative of the soil in-situ and (ii) Triaxial Tests require the granular soil to be held in a vacuum in order to fit into the test membrane, hence they are expensive and sensitive to operator error.

2. I have never heard of foundation design in free draining soils using total stress analysis and hence a value of phi for bearing capacity determination (This appears contrary to theory as I know it - so please correct me if I am wrong in order that I improve my soil mechanics knowledge). What is often confusing is the fact that phi is referred to in the literature when dealing with sands and gravels rather than phi'. In any event it is my opinion that effective stress analysis is the design intent for such soils. So perhaps the question that you are asking is not relevant to soil mechanics principles and this is what is causing the confusion? Let me know what you think.

3. CLAY soils exhibit different stress/strain characteristics. For total stress analysis i.e. undrained conditions which occur in the short term, phi is taken as zero and undrained shear strength (reported as either Cu or Su) is attributed to cohesion, c, which is not an inherent soil property. Thus phi = 0 for total stress analysis in Clay soils is used in the calculations. Undrained shear strength is widely measured from Unconsolidated Undrained Triaxial Testing, which is quick and cheap. Many correlations with Cu/Su from SPT/CPT tests exist in the literature, Stroud & Butler suggest Cu=5xSPT N value. Undrained conditions generally govern allowable bearing capacity for foundation design but drained conditions should be checked.

In the longer term (an order of years) excess pore pressures dissipate to give rise to effective stress conditions and hence consolidation settlement. In this instance phi' is used in the calculations and is best obtained from Unconsolidated Drained Triaxial Testing, mainly beceause this method is quicker and cheaper to perform than Consolidated Drained Tests. Phi' of clays will be lower than that obtained for sands and gravels.

4. I have little knowledge of partly saturated clay soils and their stress/strain behaviour, but I would recommend that you read some literature to increase your knowledge.

Hope that this provides you with a better understanding to your design situation. If not please post another.

Regards.

 

Two different names, two different behaviors. You have to sample and measure. Get Shelby tubes if you can and run R-bar D-4767 triaxial tests and get effective stress parameters (note that this test does not give total stress parameters despite what most misinformed consultants will tell you). Second, if you want total stress, you must run Q-type triaxial at desired confining stresses with ASTM D-2850. Third, you can get effective angle of phi with blow counts corrected for overburdened if they are not too low (not below 5 or so, if that loose they are pretty meaningless and you have settlement, not bearing capacity governing). Don't get bogged down on semantics, and get cracking on some field investigations and laboratory testing.

Your fellow American and Celebrant of July the 4th and the Declaration of Independence.