Soil Mechanics

Time to end Scholasticism in Soil Mechanics. #2: Hypothesis A vs. Hypothesis B

Posted in Scholasticism by Paul Joseph on April 7, 2016

This is my second post on the subject of scholasticism in soil mechanics–unfortunately, scholasticism remains alive and well in soil mechanics.  Even 500 years after Galileo, Newton, and Bacon, some researchers/academics in soil mechanics still do not seem to “get” this basic/fundamental concept. It is high time that we put an end to scholasticism in soil mechanics, it is high time we raise the bar in terms of rigor of research in soil mechanics. (My first post on the subject of scholasticism is here.)

This second post on this topic concerns a “debate” that has been going on in soil mechanics these past four decades, one which has wasted a lot of time and energy for some researchers, needlessly confused other researchers, and in general has wasted space in the research literature, not counting numerous beautiful trees that have been felled to publish this needless debate.

The debate I mean is that of “Hypothesis A vs. Hypothesis B,”a debate  that has been going on, as I have said these past four decades, with neither side able to convince the other.  The reason that neither side is able to convince the other is because both sides are scholastic–stuck at the phenomenological level of the problem, a level where the root cause driving behavior cannot be found.  Both sides seemingly lacking the intellectual ability or desire to penetrate deeper to the underlying physical bases of the phenomena to do a root cause analysis.  Or worse, they may be unaware of a basic concept–that the full resolution of a problem requires a root cause analysis and that the root cause is typically to be found at the material/physical basis of the problem.

Because these researchers remain stuck at the superficial phenomenological level, the level of merely manifest phenomena, because they seem unable to go below this level, they have failed in their efforts to convince each other, and the “world” as a whole.  This is because, being stuck at the phenomenological level means that they can, according to their biases, select some manifest behaviors while ignoring others according to their particular likes and dislikes.  This innately predisposes them to avoid going to the nitty-gritty, i.e., the material bases where the root cause solution lies.

In short, my own root cause analysis of why neither side has prevailed is this–both sides are scholastic, because both sides selectively examine the available manifest phenomena selectively,  carefully picking and choosing only that empirical evidence (phenomenological) that supports their own hypothesis, while scrupulously ignoring other (also phenomenological) data.

Rigorous thinking and *really good engineers and scientists* seek root causes for behaviors–they try as soon as possible to go below mere manifest behavior (phenomena).  Once the physical root cause is identified, ambiguities are such as this one are usually permanently and incontrovertibly resolved.  In this debate of Hypothesis A vs. B, both sides studiously avoid a root cause analysis, one which by definition, must be at the physical basis.  Because to do so would force them to examine data that does not support their hypothesis.

In this case, not surprisingly, a root cause analysis reveals that both sides are both right and wrong–that  like the proverbial blind wise men and the elephant, each side has a part of the whole picture, and having only a part, comes up with bizarre understandings and claims, that, naturally fail to convince not just the “other side,” but, neutral observers who don’t give a damn which hypothesis governs, but just want to get their job done.  A root cause analysis of the underlying physical basis drives researchers relentlessly past mere Aristotelian logic, past personal biases, to a complete picture based on a ALL the empirically evidenced data.  To do otherwise is to be but a scholastic.

It is time we end this amateurish, scholastic research, emanating from both groups.

So on the one side we have Hypothesis A and the other Hypothesis B. According to Hypothesis A, creep compression strain due to viscous effects occurs only after the End of Primary consolidation (EOP), and that consequently the strain at EOP is the same in both thick layers in-situ and thin layers in the laboratory test. Hypothesis B, on the other hand, assumes that creep compression strain due to viscous effects occurs during both primary consolidation and secondary consolidation, and that consequently, the compressive strain at EOP for a given stress is also a function of strain rate, i.e., closely tied to the thickness of the consolidating layer.

Today the followers of Hypothesis A are by and large led by Mesri while those of Hypothesis B are led by Watabe and Leroueil. A recent discussion between the two (one which unfortunately I was not aware of till it was too late to join in) is this one: Discussions and Closures Discussion of “Settlement of the Kansai International Airport Islands” by G. Mesri and J. R. Funk.

As I mentioned earlier, both sides scrupulously stick to selected observed behavior, i.e., the manifest phenomena, and strictly avoid any root cause analyses for these observed phenomena.  To do so would be to resolve this debate and in the process reveal that both sides have but a partial glimpse of the truth.  To NOT do so, for both to talk past each other and to remain stuck in a debate that will last till the cows come home as they say.  Such an amateurish approach will continue to waste valuable time and energy, and to follow what is essentially an intellectually lacking approach to problem solving, one I have seen many times in various fields and where, until the root cause physical analysis is done, the problem, like this one, remains unresolved, permanently in play.  

It astonishes me that the bar in soil mechanics is so low among academics and researchers, who come across as intellectually bankrupt. When I was young an older gentleman warned me against doing a PhD.  

“Your mind will become warped” he said.  “You will start counting the number of angels that dance on pin heads.”

 And being young and filled with hubris I disbelieved him.  Now though, I think he was right as far as many PhD’s, at least in soil mechanics, are concerned.  Consider the present case: if Hypothesis A is correct, then what explains the lack of strain-rate dependent friction?  Have the Hypothesis A people found (and counted) angels dancing on interparticle contacts and holding oil cans with which to lubricate the contacts to reduce friction related strain-rate effects? (These must be angels that are “mechanics” on the side it seems.)  Or the Hypothesis B folks–have they found and counted angels also dancing on interparticle contacts but this time, armed with “sand paper” to roughen up the contacts (one assumes that these angels are “carpenters”)?

The strange thing is whether they realize it or not, both sides have indeed pointed to where the physical basis of the root cause lies.  Thus Mesri in his various writings has pointed out that Terzaghi held strongly that the root cause of viscous behavior lay in interparticle friction seated at the viscous layer.  Likewise Watabe and Leroueil have published a remarkable figure (also reproduced in the discussion above) that points to strain-rate behavior being dominated by viscous layer governed interparticle friction.

Now, before we dive in, let us step back and ask, what exactly is one-dimensional consolidation?  A little consideration shows that it is nothing but shear along the Ko line.  In other words, it is but a special case of shearing and the standard one dimensional consolidation lab test is really nothing but a shear test where the sample is always under a Ko condition.  

Strain rate effects in drained and undrained soil shear have been studied extensively and are well known to reside in interparticle friction and the viscous layer.  These strain-rate effects map (to the extent the data exist) reasonably straight-forwardly to one-dimensional consolidation tests–see this paper for example.  The net-net is that strain rate effects exist in both drained and undrained shear, and also, that they are very small.

At a consulting company I once worked at (GEI Consultants), one of its founders, Dr. Dan LaGatta, formerly from the soil mechanics department at Harvard University and a world expert on strain rate, told me of his thumb-rule for strain rate effects.  And though I personally detest thumb-rules, this one I have to say has its place.  He told me that extensive tests he ran at Harvard and at GEI showed that for most soils, an increase in strain rate by about three orders of magnitude of strain-rate typically results in approximately a 10% increase in both peak and steady-state strengths.  In short, strain-rate effects for most engineering problems can by and large be ignored.

Likewise with one-dimensional consolidation, i.e., shearing along the Ko line.  The root cause of strain-rate effects, as in the case of shear in a triaxial compression test or any shear test, is interparticle friction seated in the viscous layer.  Consequently as in other shear tests, here also, strain-rate driven changes here too are small and need not be considered except for very thick layers (10 meters or more?) of soft clays subject to extensive consolidation.

And this is why both sides are right and wrong and why they prefer instead to stick at an amateurish “manifest behavior/phenomena” level, where as a result of their cherry picking the phenomenological data, they remain unable to convince each other for four decades.  Each side has a partial hold on the truth (i.e., each side is partially also wrong), and so each holds that they are right while the other is wrong.

So Hypothesis A groupies are correct in their claim that the EOP line is unique and that the ratio of creep rate to compression index is constant for a given soil (and they are correct in that this is a law no less) and approximately constant for groups of soils (see here for why). But despite their being correct on these important matters, they are fundamentally wrong when say claim that creep compression strains due to viscous effects occur only after EOP.  In short, despite the correctness of their other, claims Hypothesis A is fundamentally wrong.

Likewise, Hypothesis B groupies are partially correct–they are correct in the core issue, i.e., correct to say that viscous effects occur DURING consolidation as well as after.  But they are wrong to say that this matters in the majority of geotechnical problems.  Standard methods (which implicitly assume the erroneous Hypothesis A) of calculating consolidation and creep settlements suffice for the vast majority of geotechnical projects.  This is because of various factors including, chiefly, the small effect of strain-rate during consolidation, sample disturbance in “undisturbed” samples taken from the ground by the average commercial drilling company, and the fact that the thick highly compressible layers that take decades to consolidate and where strain-rate effects might play a role in the design are encountered fairly rarely in real life.  It is only for this small minority of field situations that the isotache method together with high quality sampling has a role in design.

Both sides have a part of the truth; both sides stick to the phenomenal evidence; both sides drive their arguments based on their emotional biases; both sides amateurishly and with a complete lack of rigor seem to not understand the importance of a root cause analysis that fundamentally derives from the physical bases of the behavior observed.  Both sides, instead, amateurishly remain stuck at the superficial level of mere manifest phenomenal data.

In short, BOTH SIDES ARE BUT SCHOLASTIC–hewing to faulty (being partial) logic (Aristotelian), stuck at mere phenomenology, and blind to root cause, physically based empirically evidenced data.

The next time more squabbling on this topic is published, I plan to submit a discussion raising these issues to the authors in question.

Comments welcomed–email me at pjoseph@soilmechanics.us

 

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