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 firstname.lastname@example.org
500 years after Galileo, Newton, and Bacon, one would think that Aristotelian scholasticism is dead.
Not so apparently in soil mechanics, where some academics still don’t seem to “get” this basic/fundamental concept.
It is time to put an end to this. To this end, see below my review of two recent books on “hyperplasticity.”
Fingers crossed, but the “Leibnizian pigsty style of trade school heuristics” soil mechanics pioneered by MIT seems to be a thing of the past. Now its time to take aim at scholasticism in soil mechanics. Towards ending this scholasticism, I plan to subject any future book I encounter in like vein, to the same, standard, 500 year old test–one that school children (at least in the US) are well aware of–that of empirical evidence.
Comments are welcomed.
For background information on this please read the following:
2) http://www.soilmechanics.us/dssm/soil-mechanics/chapter-9-conclusion/ and
NOTE: I wrote the review you see below, in 2007. At that time I naively did not realize that the concepts in this book would be used to inform text books on geotechnical engineering, and realized this only when I was recently asked to evaluate a proposal for one such text book. This led me to finally appreciate the dangerous nature of the understanding on liquefaction Andrew Schofield proposes. As a result of this new appreciation, today I consider this book to be both dangerous and scholastic, and feel strongly that I have to make this corrective comment. Andrew Schofield claims that soils on the contractive side of the steady-state line do not liquefy. This is wrong! It derives from the scholastic assumption that particles are isotropic; isotropic particles on the contractive side exhibit stress-strain curves that rise to a maximum and stay there. Real soils are not made of isotropic particles and so, on the contractive side, such particles exhibit stress-strain curves that rise to a peak, then decrease to their steady-state value. Such soils can absolutely liquefy! For more details read Section 4 below, on liquefaction, which remains largely as I wrote it in 2007. Further, as I point out below, I understand CAM clay, which is the basis of this book, as to be but an exercise in scholasticism. Soils cannot be treated as metals made up implicitly of isotropic point particles. Any model based on the Drucker and Prager failure criterion makes this underlying assumption that does not apply to real soils. Rather, soils are constituted of finite particles that have anisotropic properties and these govern behavior. Consequently models based theories that implicitly (or explicitly) assume isotropic point particles routinely fail to match readily available data from real soils, i.e., such theories can be routinely falsified. This inability to match real data was known almost as soon as the elasto-plastic critical state based model was first introduced. Paul G. Joseph, Tuesday, August 10, 2015.
Disturbed soil properties and geotechnical design
A.N. Schofield, Thomas Telford, London, 2006, 216 pages
Und wenn du lange in einen Abgrund blickst, blickt der Abgrund auch in dich hinein.
[And when you look long into the abyss, the abyss also looks into you.]
– Friedrich Nietzsche (Jenseits von Gut und Böse [Beyond Good and Evil])
Long story short—buy this book and master it, for this little book is a rarity in its field and worth every cent of its price. It is an important book, a snapshot of the current state of soil shear as viewed by one of the founders of Critical State based soil mechanics, and taken from the perspective of the Cambridge, England, school of soil mechanics . It is essential reading for anyone who thinks they have something to say about soil shear. Should you master it, whether you agree with its concepts or not, you will be very knowledgeable about soil shear. The book is not a textbook, and is much the better for it. It is the only one of its kind in geotechnical engineering that I am aware of; it stands in welcome contrast to the pile of mediocre textbooks available at twice its price.
A good book reads the reader and not only does this book read the reader, it reaches out further and reads both the profession and, its author. Hence the quote from Nietzsche above as it seemed to have the needed gravitas as opposed to the simple “a good book reads the reader.” Once you read this book, you will realize the gaps in your education, the gaps in the profession here in the US, the gaps in the profession in the UK, and the gaps in the book.
My review consists of four parts—this introduction, comments on the book in general, discussion of the specific points made, and a conclusion. I have tried to make it “modular” so that each part (and sub-part) is self-contained–if you are pressed for time you can skip a part and jump to the next. You can even jump straight to the conclusion should you be very pressed for time!
In the first line of the Tao Te Ching Lao Tzu famously said that “The way that can be written is not the way,” and I find him to be exactly right. Consequently, I will revise this review as and when I am presented with new information or insights. I encourage you to stop by at least once a year to read my latest version. I also enjoy and learn from your comments, so please do post a response to my write up–what you like about it, what you disagree with, as well as anything else you would like to add. Should you think your colleagues will find this review of interest, please forward it on to them.
[NOTE 1: if you came here only for my critique of critical state soil mechanics you can go to the conclusion without loss of continuity.]
[NOTE 2: The opinions expressed here are mine and mine alone.]
COMMENTS ON THE BOOK IN GENERAL
One drawback about this book is the figures, some of which are less than clear to me. For example, the frontispiece and figure 55 are key figures, but were confusing to me. References to figures scattered throughout the book force one to scurry from figure to figure and page to page, all to understand a single paragraph.
The other cavil is that Schofield is in general gives short shrift to both Terzaghi and Harvard. In this sense, the book reads Schofield. Thus while it is true that Terzaghi may have been wrong regarding cohesion, it nonetheless does not diminish his achievement of discovering the effective stress principle and formulating a mathematical model of pore pressure dissipation. Harvard also gets short shrift but the book presents only a partial picture of the activities at Harvard with no mention made of important concepts from Harvard such as the steady-state condition and it being the reason for Casagrande’s recommendation regarding liquefaction that is posited as wrong on the back cover of the book.
As someone long used to the KISS (Keep it Simple and Stupid) style of writing often used in the corporate world, I did not find this book an easy read and often wished that it used the KISS approach. I read only about three pages a day, but made sure I knew which parts of the three pages I understood and which I did not. There is still much in this book that I do not understand and which I suspect to understand requires me to fundamentally shift my interests. In this way, the book read me, exposing large gaps, selectivity, and bias, in my knowledge of soil mechanics in general and of shear strength in particular.
These are but minor cavils and more than balanced by the points made and by the interesting quotes from still very important original works by Coloumb, Rankine, Terzaghi etc. Rather than review the book chapter by chapter, I have identified what to me were important themes that thread through the book and have commented on these.
Prof. Schofield was kind enough to answer numerous questions from me by e-mail and this review contains some of this information. As Prof. Schofield recommends, first read the book with a “credulous mind.” He says that his earlier book—Critical State Soil Mechanics (CSSM)—is “widely quoted, but little understood.” I hope I do not make that mistake in this review, but if I do, I do so in the sense of Blake’s “If the fool would persist in his (or her!) folly, he (or she!) would become wise.”
DISCUSSION OF IMPORTANT THEMES
1. Isotropy, shear planes and stress paths vs. tensors and the implications thereof
Some themes run throughout the book; isotropic continuum vs. shear planes; stress paths vs. tensor representations of stresses. Per critical state soil mechanics, the aggregated grains that are shearing and which are of interest are always isotropic, with grains of all shapes and sizes distributed randomly and having no distinct geometrical patterns be they on the wet or dry side of critical. Grains form a three-dimensional skeletal structure with lines of highly loaded grains that buckle as the loads change. The grains move into new three-dimensional structures. Aggregates are elastically loaded and unloaded and deform plastically. As a whole, the aggregate behaves as an elasto-plastic material.
On the wet side of critical, a volume of soil reaches the critical state without formation of a shear plane. On the dry side, though, a shear plane forms. Hence samples initially on the wet side of critical fail in a sort of bulging manner with no definite failure plane, whereas those on the dry side develop failure planes.
In the lab, once a sample fails with a shear plane, per Schofield’s approach, the sample is no longer in a known state in terms of measurements of strains with sliding occurring on a plane between two rigid blocks. In the field, failures on the dry side as in the case of over-compacted soil can cause dangerous instabilities, whereas soils that are on the wet-side are ductile and fail in a stable manner. Analyses in the lab or in the field that use stress-paths as is commonly done in the US are not correct as on the wet side, one is not dealing with a distinct plane of failure, but with a volume of failure. Tensor analysis of volumes of soils is required, but since this approach appears more “complicated” than the stress-path approach, engineers (especially in the US) do not use it.
Implications of the importance given to the failure plane by Schofield are of great significance I briefly explore how it relates to the steady state shear strength. There are three differences between the critical state and the steady state. One is that the steady state has a requirement that the deformation explicitly occur at a constant velocity. Fair enough, but since the critical state implicitly occurs also at effectively a constant velocity—the quasi-static velocity—it meets this requirement of the steady state. So does the critical state correspond to the steady state with deformation at quasi-static velocity?
Not quite, we have the second and third differences. First, that shown in Fig. 8.18 of CSSM where soil strength decays below that shown to be the critical state and second, that at the steady state, there is an explicitly oriented structure as opposed to the implied random structure of the critical state.
According to Schofield, Figure 8.18 distinguishes between two different ideas. The first is the idea of a test path of successive states that lead to the critical state that obtains for an initial single volume of aggregated grains which becomes three distinct bodies; two blocks that are in sliding contact but not yielding because they are being unloaded and a third thin smear of CS paste in which all displacement becomes localized on the slip surface.
This distinguishes against the second idea—that of residual strength on a slip surface where the relative displacement has a slip length longer than the length of many typical grains, with particle alignment in the direction of slip. The basis of the critical state concept is the concept of isotropic aggregates, continuing to remain isotropic during shear even if the thin smear is a layer that is only 15 or 20 grains thick after failure, i.e. there is no particle alignment in the direction of slip. Fig 8.18 shows the resulting contrast. The path shown to the critical state is the behavior of the ideal theoretical model following the ideal test path to the critical state. Real test data may not follow this test path, leading instead to concepts such as the steady state, with its associated rotation and alignment of soil grains, which the critical state model does not represent.
Per Schofield, tests he ran at Manchester on a model of a trench cut into a large sample of stiff fissured clay showed that the little block of clay that had slipped had a slip surface with a thin smear that was a slickenside. However, it was clear that “the clay first deformed as a rubble of blocks and then as these moved their faces had slipped against each other and a continuous surface had formed with many smooth bumps as it passed over the separate blocks in the sliding faces.” Per Schofield, “…the residual strength is something for an engineer to have in mind in site investigation. If there is evidence that at some past time there were large displacements of many feet on an old slip surface and it is found now to be a slick surface then the strength must be expected to be much less than CS.” He tells me that a horizontal, think layer of Kaolinite clay within limestone or chalk will allow the blocks of rock above and below it to have very large relative displacements on a slip plane.
Were Schofield right in that particle alignment plays no role in shear, then he has a point. I feel though that it probably does play a role, that conditions in failure are not as unknown as Schofield makes them out to be, and that consequently, the steady state is a robust “real life” concept as opposed to the more theoretical but more developed critical state concept.
2. Use of plasticity theory in soil shear theory
Another theme that runs through the book is that of “plasticity.” The book has a chapter on the plastic design of structures, and in Schofield’s book concepts of plasticity app directly apply to a structure such as a dam made of compacted soil. A comparison is made of the Empire State building, vs. the Murrah and the World Trade Center (WTC) buildings. At the time Schofield wrote the book, the prevalent theory on how the World Trade Towers had failed emphasised the high temperature of the fire and supposed that the connections between the horizontal, wide, open floors and the outside tubular wall of windows and columns gave way suddenly and the floors “pancaked” on one an other and that both the WTC and the Murrah building were not designed to fail “plastically”.
Schofield notes that fire at the top of a WTC tower did not heat lower floors. People were still escaping down “cool” stairwells at the time of the final “pancake” collapse. He also considers that failure of US educators to teach student engineers the lessons learned from World War II in bomb damaged UK, had the result that the WTC and the Murrah building were not designed with floor-to-column connections allowing sufficiently large lateral displacement. Ductile connections are essential for such structures to fail plastically. Experts now think the WTC failed plastically initially allowing many to escape, but that ultimately it underwent a brittle failure. This though does not nullify, but rather supports Schofield’s larger point that buildings should be designed to fail plastically.
Schofield extends the soft plastic yielding of structures to soil and recommends that soils be brought to a ductile state where they will yield in a stable manner. Using the analogy of the blocks of stone used to build Kings College chapel in Cambridge, he points out that masonry recover any energy used for dilation. He describes work done at Cambridge by numerous researchers using the Cam-clay model in which the work input by the movement of forces on the specimen boundaries plus or minus changes in the stored elastic energy in the aggregate of grains, is dissipated in friction.
In using theories of plasticity and flow rules upon which to describe and quantify soil shear, Schofield puts soil shear firmly on a “scientific” footing, points to the future, and offers a welcome contrast to empirical, thumb-rule driven approaches such as SHANSEP. However, plasticity theory treats soil as a “paste” and so is unable to provide insight into soil structure and changes in soil structure with shear.
3. Terzaghi’s Error
The back of the book states that Harvard soil mechanics teaching contained two errors. The first of these was that Terzaghi was wrong to assert that soil has true cohesion and friction. i.e. that soil strength came from “pure” cohesion and friction. A major thread running through this book is how and why Terzaghi was wrong. For Schofield, Taylor’s discovery with sands holds equally for clays–soil strength is due to interlocking structure plus friction. Terzaghi thought that surface chemistry of fine clay mineral grains and dissolved salts in pore water was the basis of adhesion between grains i.e. of true cohesion.
Schofield’s claim that Terzaghi was wrong rests on two main points–one that Hvorselv’s data that Terzaghi used as a basis for his claim, plots a line that ends where it intersects the critical state line. Hvorslev had no data to the right of this line, but Terzaghi and Hvorslev extended their interpretation of this line without any limit, into this region where he had no data points (normalized confining stress > 0.6 in Fig. 27.) Second, any true cohesion would exist on both the wet side and dry side of the critical state line, or not exist at all, and soil states on the wet side of critical show no sign of cohesion. In short, the apparent cohesion measured on samples on the dry side of critical is the result of interlocking and is not true cohesion. Engineers should not rely on it in their designs.
Underlying Schofield’s premise regarding the lack of true cohesion is a far deeper principle with important implications. This principle is that the movement of soil aggregates is due to grain-to-grain contact and not due to surface chemistry i.e. viscous layers on the surfaces of soil grains sliding past each other. This in turn means that the movement of importance in soil shear is of the type driven by changes in shear and effective confining stresses and not by viscous effects of surface chemistry and that viscous effects of the surface chemistry such as strain rate dependence and creep are of small importance.
Hence, per this view, there is no need to give strain rate undue weight–and the steady state’s requirement of explicitly specifying the deformation rate is of secondary importance as the only rate that is important is the quasi-static rate. Research at Harvard by one of Casagrande’s students indicated that the dependence on strain rate of the steady state strength, while it did exist, was small. In addition, as explained later on, the emphasis of the Soviet Union’s centrifuge studies on creep, took its soil mechanics in a different direction to the soil mechanics based on grain movement due to shear and effective stress changes.
While Schofield has a point that the geotechnical engineer should not rely on cohesion for his designs, one wonders with another reviewer, if given the fact that the bulk of the tests done at Cambridge were on kaolin, a clay with relatively low surface chemistry, whether the results would be different if highly plastic clays such as smectites had been tested. In short, while it may be correct that surface chemistry plays a small role at the stress levels encountered in geotechnical design problems and that one should not use it in design, in reality it probably exists and play a small role as revealed through the slight dependence of the stress-strain curves on strain rates, and by soil creep.
The second error in the teaching of soil mechanics at Harvard according to Schofield is that Casagrande was wrong to assert that contractive soil liquefies. Strangely, the unabridged Oxford English Dictionary does not have a meaning for “liquefaction” in the context of soil mechanics and earthquakes. Perhaps if it did, a lot of the confusion and misunderstanding in this area would just simply go away. It turns out, for reasons explained below, that Cassagrande remains correct–and that Schofield is dangerously wrong–soils on the contractive side of the steady-state line do indeed liquify! (Note: Schofield and others use the term “wet side” and “dry side” to indicate soils states on either side of the steady-state line. Steve Poulos does not like these terms at all, as they don’t describe behavior, i.e., he prefers instead the terms “contractive” and “dilative” to “wet side” and “dry side” respectively, as they describe the actual soil behavior.)
Liquefaction for Schofield consists of two types—Hazen’s and Herrick’s. Per the Harvard school of thought, there are five terms that are related, overlap, and which need to be clearly distinguished from each other. These terms are “liquefaction”, the “z-condition”, “deformation during cyclic loading”, “sand blows”, and “quicksand”.
Off these, “Harvard terms”, the “liquefaction” term is the one that Casagrande had in mind when he said that that if a soil state moved to the contractive (wet) side of the critical or steady state line, then it was at risk of liquefying. Liquefaction per the Harvard definition is a stability failure due to undrained shear of highly contractive, fully saturated masses of soil, where the driving shear stresses are greater than the undrained steady state shear strengths such that failure involves acceleration of the mass and its subsequent coming to rest as the driving stresses are reduced and the inertia of the accelerated mass is overcome by the undrained steady state shear strength. As so defined, it corresponds to Hazen’s liquefaction.
Per the Harvard approach, soils that show a peak with subsequent decrease of their shear strength are candidates for liquefaction, if they were loaded to a meta-stable condition (in-situ static shear stresses were between peak and steady state values.) Strain caused by an earthquake would cause strengths to first increase, but then post peak, they would decrease, a result of soil particles aligning in the direction of the shear. Once they decrease, post-peak, below the in-situ static shear stress, there would be failure (liquefaction) with the soil failing in a process that Casagrande called a “chain-reaction” that starts locally and then spreads.
Per Schofield though, Casagrande’s “chain reaction” with its particle realignment, flow structure, and decay of shear strength to steady state values does not occur. He says that he could not trigger Casagrande’s chain reaction in his testing when he searched for this (on the wet side.) A stress path for disturbed-soil paste can never lead to Casagrande’s “chain reaction” if the soil particles are such that, shear strengths uniformly increase to their steady state values. This is the case for soils made up of “bulky grains” or grains that resemble “point particles” such as the clays that formed the bulk of the soils that Andrew used. Consequently, though there may be deformation due to yielding, there would not be a stability failure as the driving stresses in-situ prior to the event would be lower than the critical state shear strength that the system would reach during and after the earthquake. For soils with platy or elongated grains though, the situation is different and here after peaking, soil strengths decrease to their steady state value. Here, one should indeed see the “chain reaction” and a liquefaction failure, if the soil had been initially subject to a shear stress value that was between the peak and the steady state shear stress values.
As Steve Poulos wrote me: “The contractive sands and clays that lead to flow failures (liquefaction) have a peak and a drop down to the steady state strength. The reason that the drop occurs is that the peak is formed as the undisturbed structure is modified by the shear loading until it breaks down and the strength drops as the strains increase, until the steady state strength is reached.”
So I asked Schofield and he clarified: ” The Critical State point of view gives simple equations of the mechanics of an isotropic aggregate that dissipates work as it deforms in shear but continues to remain isotropic. My ideal aggregate has the behavior that Taylor observed. It contracts or dilates depending on whether it is on the wet or the dry side of the Critical State…The Original Cam Clay (OCC) equations provide a full prediction of the stress strain path resulting from loading or unloading; it applies when grains form aggregates that are isotropic. I do not doubt that other soil grains may align themselves to form an aggregate with transverse isotropy.”
In a nutshell, the Critical State concept applies to an idealized aggregate, not a real soil, and this idealized aggregate contracts or dilates depending if it is on the dry side or wet side of the Critical State. The book though does not emphasized the “idealized” nature of the soil to which the critical state concept applies and as a result, wrongly indicates that one does not need to study the risk of liquefaction if one is meta-stable on the contractive (wet) side. So I asked Schofield: what about “real life” soils? Schofield answered indicating that first one had to understand the behavior of idealized aggregates before one could really understand “real life” soils.
Casagrande’s “chain reaction” can indeed be systematically triggered in a triaxial test. I will never forget an experience I had in the early 90’s, standing with Gonzalo Castro next to a triaxial test under “meta-stable” conditions, along with the lab director Gregory Thomas who was running the test. The sample had been loaded undrained to a shear stress approximately half way between its maximum shear strength and its steady state shear strength (i.e. it was “meta-stable.”) The undrained sample was then “tickled” by subjecting it to very small cyclic strains. As expected, when the cumulative strain reached a “triggering” value corresponding approximately to the post-peak shear strain at which the available shear strength was less than the imposed shear stress, the sample failed catastrophically, and with a loud noise like a bomb going off, as the dead-weight crashed down and slammed into the floor.
I was so startled that I raised my hands and ducked under a nearby heavy laboratory workbench to protect myself against what I thought had surely been an explosion of the lab’s huge air-pressure cylinder. Castro being the kind hearted man he is pretended not to notice either my antics, nor the fact that in my subsequent embarrassment, I had blushed a deep red. The “chain reaction” does exist and can be readily reproduced on samples consolidated to the contractive aka “wet” side–there is no magic about it, and the test method has been published.
When I described this test to Schofield, he said that “failure appears most sudden when the unloading path lets elastic strain energy in a specimen flow into some localized damage mechanism and allows sudden displacement of a loading platen. I have never seen grains being re-oriented as a ‘chain-reaction’ propagated through an aggregate.” I for one do not believe that a non-brittle solid such as the triaxial sample of sandy clay, can store this much elastic energy to the point that it behaves like a crystalline solid that “shatters.” I do not understand why Andrew feels that this is a case of storing “elastic energy.” We continue to discuss this by e-mail, but here is a classic case where one must test to validate ones thinking, and where we can continue to go in circles, if appropriate tests are not run. Also, in case you were wondering, the loading equipment used was made of very heavy and rigid steel, i.e., did not store any “internal elastic energy,” and was used routinely to test similar samples.
The best way to study this (Steve Poulos wrote me) is “…if he prepares a contractive specimen of sand, an undrained triaxial test is okay. Best to make the specimen highly contractive and use strain control. Load control will cause a failure after peak (liquefaction) is reached and nothing will be recordable during the large strain portion of the test.” The sand should be made of particles that are neither “bulky” nor approximately “point particles” else one won’t see a peak in the curve.
Schofield also says that in the disturbed soil from which he made centrifuge models it was impossible to follow stress paths that brought soil paste to a state of perfectly spherical compression without shear stresses. This is very interesting–the fact he could not find it is really a blow against elasto-plastic theories because such these theories imply such a state exists, and separate out the effects of shear and confining stress with one occurring independent of the other. This is fundamentally a wrong assumption–soil structure can be usefully analogized to a “house of cards” such as the ones we used to build as children; the slightest compression on such structures results in immediate shear deformation, as any child can attest.
The book describes the failure of an early concrete drilling platform in the Norwegian Frigg offshore oil field. This platform stood on a slab that did not have the skirt of sheet piles that later became a standard feature. In the earlier design, for one platform, the designer in an attempt at preventing possible liquefaction moved the soil state to the dry side of critical. This ultimately caused the platform to fail. This example needs to clarify two important concepts. First, that moving the soil state to the dry side of critical did not absolve the designer from also investigating other modes of failure such as the “pumping action” that subsequently caused the failure in this case. Second, that had he been on the wet side of critical, Casagrande would have told him that if in-situ driving stresses were less than the corresponding steady state undrained shear strength, then, the worst damage that an earthquake could cause would be significant deformation but not liquefaction.
As is, this claim by Schofield that soils on the wet (contractive) side of the steady-state line cannot liquefy is very dangerous and a huge mistake in his book.
5. Model testing and the centrifuge
The book has a section on modeling using the centrifuge that ties together major themes in the book—the use of plastic design in geotechnical engineering, model analysis and the role of surface chemistry. It presents interesting information about centrifuge research in the former Soviet Union along with the general concept of the use of the centrifuge by engineers to experience failure of their own designs in an orderly way, to the betterment of their own design skills.
As he recently put it to me: “In the Soviet view TOTAL stresses on soil cause VISCOUS creep time effects. Their Academician Vyalov modeled such changes using a power law with an exponent that was found by experiment. His analysis was applied to both permafrost and to the centrifuge data of tests with primary consolidation. I have read about “models of models”, not about centrifuge testing of permafrost models. I believe that when training of Soviet civil engineers taught about TOTAL stress that intended to prepare them to work on new military construction and new East/West transport routes in frozen regions. In contrast, our training of civil engineers in Harvard prepared them to work in the un-frozen zones with soils that are left after an Ice Age. It made sense for Harvard soil mechanics to concentrate on effective stress and neglect creep while Moscow soil mechanics taught total stress analysis.”
Schofield presents interesting case studies of work done by his centrifuge group on the modeling and failure analysis of levees on the Mississippi river, the Teton dam, and a North Sea oil platform. Information in the book on geotechnical related centrifuge development at Cambridge University and companies that Schofield set up to further centrifuge research show the impressive amount of effort he personally put into developing the use of the centrifuge to study geotechnical issues.
[Note: to fully understand what follows you should be familiar with the views of the philosopher of science Paul Feyerabend]
1. Parochialism in soil mechanics
In the mid-eighties at MIT, the basic and advanced graduate courses on soil mechanics, taught by a “Terzaghi lecturer,” centered on the empirical, thumb-rule riven SHANSEP concept, with not a single mention of either Critical State or Steady State soil mechanics. The course on soil dynamics, taught by another “Terzaghi lecturer” said nothing about the different schools of thought on liquefaction. This though both Poulos and Castro who together represent the Harvard University approach to soil mechanics post-Casagrande, consulted a scant eight miles away.
A meme in soil mechanics is parochialism in the worst, most blinkered, sense of the word, a practice seemingly initiated by the “Founding Father” himself—Karl Terzaghi (then at Harvard)—who (with the assistance of Peck, his junior colleague) tried to prevent and then delay the publication of the theory of interlocking and friction in sands proposed by Taylor at MIT. The impact of this action is still felt today, as we shall see later in this article, in MIT’ and Cambridge University, with both ignoring the important Steady State concept from Harvard.
Schofield’s book at least mentions the Harvard school of thought. though it does not do it full justice. It studiously avoids mention of that key concept, central to the Harvard approach–the steady state condition. In this way, Schofield is also parochial. I wrote to Schofield asking him about “this fly in the ointment.” He replied that he was indeed parochial in the sense of the word given in most dictionaries–“taking care of one’s parish.”
This is true, and perhaps the word I am looking for is “partisanship.” However, the end result is the same as one of the meanings of the word in the unabridged Oxford English Dictionary, where it is defined as “/esp./ confinement of one’s interests to the local sphere; lack of global perspective; narrowness of view; petty provincialism.”
In the end, it is disappointing that each school appears to puff themselves with pride and preaches their “own true religion.”
2. Empiricism in soil mechanics
To his credit, Schofield’s approach, based on the concept of work done on the soil and the energy it dissipates, is rational and based on general mechanics. It leverages and extends standard concepts of plasticity that would be familiar to structural engineers, places soil strength in a rational framework, and shuns the thumb-rule driven empiricism to which soil mechanics, at MIT for example, has atrophied.
Ironically, it was at MIT that Taylor pioneered a rational approach to soil shear. Roscoe, Schofield, and others at Cambridge then applied these concepts to clays and sands whereas Taylor’s successors at MIT, unfortunately, were only able to follow an empirical, thumb-rule based approach, thus allowing Cambridge University to lead in fundamental research on soil shear.
Bob Whitman wrote me saying “I’m all for fundamental understandings (and steady-state soil mechanics is one such), but a good blend of fundamentals and empiricism can be very useful for practical work.”
And this would be a perfect approach at a trade or vocational school.
Richard Goodman wrote me that he had once queried Harry Seed about pursuing a more “fundamental” research program. Seed replied to him that “the implicit message of Hooke’s law is empiricism.” The point is taken, but one could just as easily substitute “gravity” in place of Hooke’s law.
So is empiricism the implicit message of gravity? The answer of course is that our current inability to explain gravity currently drives a vast amount of fundamental research and that at root, empiricism speaks of an inability to reach to the fundamental cause of the observed behavior. Science has rightly held crude “trade-school”/”thumb-rule” based empirical approaches a distant second to more rigorous empirical or theoretical approaches. (One cannot but recall Leibniz, attempting to distinguish human intelligence from animal knowledge: “And men, in so far as they are empiricists, that is to say in three-fourths of their actions, only act like brutes.”)
Whitman did not address my earlier comments on parochialism at MIT. Nor did he reply to my follow up e-mail asking why, if he thought steady-state soil mechanics was a “fundamental understanding”, it was not taught at MIT. (Bob Whitman was no parochialist–quite the contrary, I personally heard him during a graduate school luncheon request that the steady state concept be taught, only to be turned down; the person in question trotted out an old “war story” of how someone at Harvard had said of someone at MIT that “the man was an idiot” and that this was why he felt no obligation to teach Harvard’s steady state concept. I think this was but a handy, if dramatic, excuse: the real reason, much more mundane, down to earth, practical, and universal, I discuss later.)
I think that MIT’s crude parochialism and it’s intellectually lazy trade-school empiricism speaks of an approach that is, in the final analysis, not worthy of emulation. Its legacy may be the stagnation in research on soil shear in the US during this period.
But enough of blinkered parochialism! Enough of timorous, trade school empiricism! Enough of pretentious posturing! In short, enough of “my little distraction.” Let us instead put aside this sad shambles and move on, to discuss real soil mechanics.
Schofield, though consciously parochial, does not appear to be a true empiricist. He bases his approach on theory to the point that at times he appears (is?) scholastic, seeming to shy away from the useful empiricism of Francis Bacon’s Novum Organum i.e. he seems to place the experimental validation of the critical state based model a distant second to theory; that as he once wrote me, “…you can torture a sample in a triaxial cell to get whatever result you want.”
Neuroscience tells us that our brain, is not as perfect as it is made out to be–that it’s construction was dictated by evolutionary needs with a “deliberative system” overlaid on an ancestral, reflexive/emotional system. To paraphrase Hume, our brains are not thinking machines that feel, but rather, are the reverse–feeling machines that think. Consequently, our thinking is biased, colored by our emotions, and we very easily construct theories that we wish were true i.e. Bacon’s point in Novum Organum has a physiological basis and so he (Bacon) will remain relevant so long as our brains are physically the way they are. We can discuss soil mechanics theories till the cows come home (or as Schofield put it to me: “till Herrick brings home the Bacon”), but the fact of the matter remains that the proof of the pudding is in the testing (to paraphrase Bacon).
Remember the learned “Divines” before Galileo who would look at you gravely in the eye and tell you very sagely that it was obvious that heavier objects fell faster than lighter ones. Or recall the physicists of the nineteenth century who would also tell you that if you moved in the direction of light, that your velocity relative to light would increase. It took the experiments of Galileo and Michelson-Morley to prove that intuition is often wrong. Bacon: “I contrive that the office of sense shall only be to judge of the experiment, and that the experiment itself shall judge of the thing.”
I repeatedly pushed Schofield over the last few years as to why he did not believe in the concept of the steady state, but was not able to get a clear answer. In the end it appeared to me that as in other things, our beliefs are at root psychological, which is why (again) Bacon’s contribution in Organum is so important–believe what we may, the proof of our beliefs is in the test data.
It continues to amaze me how even today, five hundred years after Bacon, my interactions with senior professors in soil mechanics from all over the world reveals an appalling ignorance of Bacon’s fundamental, foundational concept of science–a simple concept that every school child should be taught if we wish to educate them well. And reviewers too, who should know far better. Sometimes a reviewer of one of my papers will reject it, saying he/she does not believe in the steady-state concept! They ignore results in the same paper that shows that the steady-state driven hypothesis was validated by the experimental data. Probably these reviewers also believe that heavier objects fall faster than lighter ones, and that the earth is flat! At any rate, they should be reminded of the basics of science–again, something a school child should know–that one’s beliefs come a distant second (if at all), to “hypothesis/validation of hypothesis by testing,” the foundational principle of science.
The other thing that also is astonishing is that in soil mechanics when a researcher presents his or her model, they compare it against their own test data!! This is a very low bar indeed and I am surprised these papers even see the light of day–they should be rejected on the spot! Such comparisons are definitely not “acid tests” and I consider these comparisons third rate. After 50 years or more of shear strength testing of soils, stress-strain data are far from rare, even data from true-triaxial testing are freely available. Models should be compared against data from tests conducted by those not responsible for creating the model and who are NOT co-authors of the paper in which the model is being tested, unless of course the data in question is of a very specialized kind involving new kinds of tests. In my papers I take great pains to test my model against data obtained by others and then too, I look specifically for data examined by rival theories. This allows for a direct comparison of the two theories.
This is the reason why I chose to model the Plant data–it was the data set used in Roscoe, Schofield, and Wroth’s paper on the critical state. I also used Gilbert’s data which Roscoe, Schofield, and Wroth also analyzed, but found the data to not be sufficiently rigorous for quantitative modeling–Gilbert only recorded an average of 12 points per curve, which was insufficiently rigorous to define large portions of the curves.
(Note: contrary to what some reviewers think, you cannot join two measured points by a “smooth curve” and use data points from this interpolated curve as input to your model. Such an approach speaks of a basic lack of knowledge of the underlying mathematics.)
Hence when Andrew wrote me that one could get any data one wanted from a triaxial test (the exact reason why a modeller should not use her own data), I was able to tell Andrew that I used data from his own paper.
And one more thing…when testing a model against data one should validate with a lot of tests…lots both in number and kind. A minimum of a 100 tests, drained, undrained, normally and over-consolidated, triaxial, true-triaxial etc. and on clays, silts, and sands, are called for in my opinion. I have seen some papers (one by a so called “Rankine prize winner”) where the model is considered to be validated with comparisons done against as few as three tests on a single clay! Absurd!
3. The critical state and elasto-plastic models
The thrust of Schofield’s book is largely pedagogical. Consequently, his coverage of modeling does not go beyond the pedagogically useful CAM clay model, and he does not address controversial issues that surround the critical state concept. These include questions such as why, despite their unwieldy complexity, the latest models based on the critical state fail to provide good matches with stress-strain-void ratio curves obtained from tests on soils such as sands and moderately to highly over consolidated clays. Can a theory founded on an idealized isotropic aggregate, apply to real soils? Recall that most real soils show a peak when contractive samples on the wet side are tested, and that this is not possible with an idealized isotropic aggregate.
No real soil is isotropic to the point that it continues to deform isotropically during shear! No real soil is isotropic to the point that under pure hydrostatic compression it does not show shear deformation!
Andrew curbed my naivety with this reply: “The word “real” has the sense of the realm of control in which a sovereign exerts power or the land and property that people sell. That corresponds to the land for a few hundred miles around Harvard. However there are granular aggregations that exist on the sea-bed, or are hypothesized to exist in space before star-dust forms a new star, to which it may not make “real” sense to apply un-diluted Harvard soil mechanics.”
Karl Popper, a philosopher of science, came up with the criterion that to qualify as scientific, a theory should be falsifiable. Is the critical state concept, to the extent it depends on an idealized aggregate, falsifiable? Does it fail Popper’s test? Any deviation from theoretically expected behavior can be disqualified by the ad-hoc reasoning that the test was on a real soil–not the idealized material that the Critical State theory is built upon i.e. isn’t the critical state concept not falsifiable and so not qualify as scientific?
I put the question to Andrew, and here is his reply:
“…There are logical mathematical arguments that tell us that we can only say that a homogeneous isotropic material has a property such as stiffness or strength if we can measure it and ascribe a value to it that is independent of our choice of reference axes. So invariance under transformation of axes can provide us with a test of truth or falsity of any claim about a theory…”
In this limited sense, I concede the point. It is a point about homogeneous isotropic material. No real soil behaves as such.
One should make claims about hypothetical materials with the care of a person walking a tightrope stretched high across an abyss else one may fall into statements that cannot be falsified i.e. fall out of science, and into the absurdity of scholasticism.
Half a century of research later, the question of the endogenous/micro-structural basis of critical state based models remains. Can a theory originating from a metals theory of elasto-plasticity be used to model soil behavior? Does soil really resemble metal? Is soil really elastic? If so, why do I get permanent shear strain when I apply the slightest ISOTROPIC compression to a sample of loose or medium dense sand? Can a structure that is much like a “house of cards” be elastic? Elasticity is only a good APPROXIMATION, one that only works with dense sands and clays and then too at very low strains.
Recall also Schofield’s inability, despite a “determined attempt,” to follow stress paths that brought soil paste to a state of perfectly spherical compression without shear stresses, a state that should readily exist per elasto-plastic theory. (The yield surface of the original Cam-Clay model was a logarithmic arc. This was changed to a ellipse in the Modified Cam Clay model. The reason for this change is that the plastic strain increment vector, which is normal to the yield surface, now becomes horizontal for the largest mean effective stress i.e. no incremental deviatoric plastic strain occurs for a change in mean effective confining stress. This makes it “numerically stable.” However the problem is that real soils do show plastic shear strains when subject to pure hydrostatic stresses, as evidenced from Andrew’s search above, and also from common intuitions as to how complex structures would behave.)
Likewise recall that plasticity theory has no option but to treat soil as a “paste” and thereby is not able to provide insight into soil structure and changes in soil structure with shear.
When in the 8th grade, I remember distinctly being taught about the religions of the world. The teacher then came to the original form of Buddhism as preached by Gautama Siddhartha Buddha and today called, “Theravada” Buddhism. The Buddha did not mention nor need the concept of God, and my teacher called the religion “a fortress without a well.” Being young and impressionable this phrase struck me with force and I remember it to this day. So also with elasto-plastic theories based on the concept of the critical-state. The approach that they use does not have a clear, simple, transparent underlying organizing principle that can describe the shear process in a few easy to understand words and so sometimes seems to me to be “a fortress without a well.”
Roscoe was a mechanical engineer by training and his introduction to soils occurred when bravely tunneling out of a German Nazi prisoner of war camp. Andrew Schofield was mentored by Prof. Baker, one of the great structural engineers whose courses and whose approach to steel structures were based heavily on the theory of plasticity. Given the dearth of any alternate theory, it makes sense then that they applied elasto-plastic theory then available for metals, to soils.
However, this smacks of the well known story where a policeman saw a man searching for something under a lamppost. “What have you lost?” the policeman asked. ” My keys”, said the man. The policeman then helped the man look. After searching for a while he asked the man: “Where exactly did you drop them?” “Over there”, responded the man, pointing towards a dark street a good distance away. The policeman asked exasperatedly “Why are you looking here if you lost your keys over there?” The man replied “Because the light is so much brighter here.”
This is not to disparage Roscoe et al. Rather, this is how such things often happen in science and technology: there was not alternate theory ; one had to be pragmatic and use what was available.
Today elasto-plastic models are “a dime a dozen,” with each model claiming superiority in a niche area, but each uniformly doing a poor job of accurately predicting stress-strain curves over a wide range of soil types and soil conditions. Here too Popper’s falsifiability criteria cannot be applied because the elasto-plastic theoreticians explain away their inability to fit the stress-strain curve post-shear failure in the case of brittle materials that develop distinct failure planes by saying that once localized deformation develops, models that determine their parameters based on the assumption of homogeneity of deformation for the entire stress-strain curve are invalid.
In short it means that we do not know if overall, their approach is right or wrong as their theory is falsifiable only up to shear plane development i.e. once a shear plane develops it does not qualify as a testable theory i.e. is not scientific. (Note though that the dynamical systems based approach provides close fits post failure, indicating that the generation of a failure plane does not significantly disrupt the underlying failure mechanism and that this hypothesis used by critical state theory to explain away poor fits post failure is but an excuse.)
I am reminded of Wolfgang Pauli (who seemed to be channeling Popper) who on reading a paper submitted by a physicist colleague, said: “This isn’t right. It isn’t even wrong.”
The best that one can say about the critical state and elasto-plastic theory based on the critical state is that it is a “heuristic,” useful in engineering. But outside of the world of soil-mechanics, in the more rigorous world of science, it would qualify as a “just so story.”
Perhaps this is the reason why soil-mechanicians so easily forget their “Bacon” as mentioned earlier–we are educated and work largely on the basis of heuristic models which in turn makes it so easy and natural for us to “explain away” differences between test data and theoretical model/hypothesis. The critical-state/elasto-plastic approach is largely unconstrained theory driven by more theory and not theory driven and constrained by real test data. They should recall the late Richard Feynman (who was really only channeling Bacon) when he said: “It doesn’t matter how beautiful your theory is, it doesn’t matter how smart you are. If it doesn’t agree with experiment, it’s wrong.”
Over the years I have slowly but steadily come to believe that the critical-state and the elasto-plastic approach will eventually be seen to have been red-herrings–dead ends in soil shear modeling, or as Bacon would put it, “idols of the (soil-mechanics research) marketplace.” It will take new paradigms and a new generation of researchers for this recognition to occur. Recall Max Planck’s words on new paradims! A new generation is least vested in old models–they have yet to build their careers based on any paradigm and so are more easily able to choose. For the previous generation (myself included) to move to a new paradigm requires “switching boats in mid-stream” which is almost always much harder than simply remaining in one’s old boat!
Already one sees signs of this with the numerous “niche” critical-state models, all tremendously complex and providing poor matches other than their particular niche application. By contrast, the concept of the steady-state places soil shear squarely within a powerful mathematical framework–dynamical systems theory–and with a clear, transparent, physical basis, and which provides close matches with but 2 equations. If you are a young professor seeking to build a research reputation, take note and do not paint yourself into a corner.
The critical-state based approach shows many signs of becoming what Imre Lakatos called a “degenerating research program.” According to Lakatos, a degenerating research program is a scientific enterprise that started out with great promise, showing impressive results in a limited domain. Researchers then apply the program more generally, and if they succeed, the program gains more followers and expands. If on the other hand, researchers encounter important anomalies, that consistently resist explanation with the new concepts, then the program will stagnate. It will be characterized by a lack of growth, or growth of a protective belt of auxiliary hypotheses (such as failure planes or non-uniform particles or anisotropy or lack of shear under pure hydrostatic stress being the reasons for poor fits to real data) behind which researchers will attempt to shield the theoretical core from falsification.
Let me end this section with a quote (with apologies to Nietzsche): “After the Critical State was dead, its shadow was still shown for years in a cave–a tremendous, gruesome shadow. Elasto-plastic soil shear theory is dead; but given the way of men, there may still be caves for decades of years in which its shadow will be shown.”
4. Schizophrenia, “exceptionalism”, and complexity in soil shear research
Current research on soil shear has a “schizophrenic” feel to it. On the one hand are crude, empirical, thumb-rule riven approaches, such as MIT’s , and on the other are approaches where Ph.D’s count angels on the point of a pin and create models (usually some variation of the elasto-plastic model) with over sixty equations, each equation dependent on the previous and each subject to simplifying assumptions. (The assumptions are often subtle and hidden, seemingly even to the model’s creator who proceeds, blithely oblivious.)
In most of these papers authors apply their model to data from their own tests. As mentioned above, Andrew once wrote me, a specimen in a triaxial cell, can be tortured to produce any result you want. After over fifty years of research into soil shear, with most tests run on standard, run-of-the-mill triaxial or true-triaxial tests surely the bar has to be raised. In my opinion, researchers who fit models to their own data should not be allowed to publish the results of their fits. Rather, they should demonstrate the validity of their hypothesis by applying their model to data collected independently by others, and preferably, from tests run before their hypothesized model was even conceived of.
These papers also usually omit to provide “chi” or “goodness of fit” values when comparing predicted vs. observed curves. The authors of the classic text “Numerical Recipes in C” caustically refer to this approach as “chi by eye” and say that “…its practitioners usually get the treatment they deserve.”
Except that is in soil mechanics where the bar is set much lower and researchers are allowed to get away with statements like: “the fit was good.”
Two other (somewhat minor) examples of this kind of egregious “soil mechanics exceptionalism” are the use of the universal mathematical constant “e” as the symbol for the void ratio and the use of the prime symbol ‘, the universal symbol for the first derivative, as the symbol for effective stress.
Dick Goodman wrote me: “…in Erdbaumechanik, (1925) Terzaghi used the Greek letter epsillon(ε) for the pore number (Porenziffern), which was identified as “the quotient of the volume of voids to the volume of grains”. I looked into Terzaghi’s 1929 book , Ingenieurgeologie (engineering geology) with Redlich and Kampe, (1929) and found he was now using “e” for void ratio. Since in clays he saw this as measure of the distance between particles, I thought he might have selected “e” to represent a special kind of distance – – the word “entfernung” comes to mind. It means “distance”. But I have no information to suggest that this is the origin of the symbol. Of course, he might just have tired of using a Greek symbol and replaced it with the Latin “e”.”
As any scientist will tell you, the number e is one of the most important numbers in mathematics, and is sometimes called Euler’s number. It is an irrational number whose first few digits are: 2.7182818284590452353602874713527… (and forever more).
But the key “exceptionalism” is this: “critical states” are defined in science as states at which a phase boundary ceases to exist. There are multiple types of critical points such as vapor-liquid critical points and liquid-liquid critical points but the only field that I am aware of which (mis)uses this term for a non phase boundary transition is soil-mechanics.
The critical state condition in soils is a stark case of exceptionalism–there is nothing critical about the critical state in soils! One does not find such a condition anywhere else in nature–a stark contrast with the ubiquitous steady state condition. I have repeatedly asked Andrew about this, but he has given me no answer. (Note: an author of a noted text book on critical state soil mechanics wrote me that the sense in which he now uses the term “critical state” is the same as Poulos’s definition of the “steady state”. Such use of nomenclature is to me a grave error–the term “steady state” is rigorously defined not just in soil-mechanics but in numerous other fields and to misuse an old term and use it in a new sense in soil mechanics would be another grievous example of the aforementioned “exceptionalism.”)
The “steady state” is indeed ubiquitous in nature. They can be found everywhere and at all distance scales: in the beating of the heart; in one’s eye; in the formation of an infant in the womb; in the shape of shells; at the heart of tornadoes and hurricanes; the Great Red Spot on Jupiter; in galaxies–where ever you turn you find steady states. (A good book describing examples of the steady state condition is the New York Times best seller “Chaos” by James Gleick.)
Indeed it would be very odd and anomalous were soils under shear not to exhibit a steady state–it would qualify as what that tragic philosopher of science Edward Constant would call a “presumptive anomaly” because a soil under shear is nothing but an “open” (energy coming from outside the system) “dissipative” (energy dissipated through processes such as friction) thermodynamic system. Open, dissipative systems fall into a general class called “non-conservative” systems that have been extensively studied by physicists. All non-conservative systems must involve a “contraction of phase space” with “attractors (steady states) and consequently, that is exactly what one must find in a soil being sheared if one is not to contravene the laws of physics. Unless of course we want to exceptionalize soils even out of the realm of the laws of physics…to where? into the supernatural? Michael Jackson’s Thriller?…or perhaps we could, like MIT, wallow in Leibniz’s pigpen of trade school empiricism?
Back to the bloated 60+ equation elasto-plastic, “state-of-the-art” models of soil shear. Complex models impress only novices. Seasoned innovators know that Nature (of which soil mechanics is a part!) is not schizophrenic and that she acts in radically simple, economical, and elegant ways. As Newton believed and said numerous times in one form or the other: “Nature is exceedingly simple and conformable to herself.”
Consider our own throat, which nature uses for the essential tasks of eating, breathing and vocalizing. This kind of radical simplicity and radical economy is beyond the ability of all but a few to conceive of, and then too perhaps only after much introspection. But that is no reason why nature should not be radically simple and economical. As Albert Einstein said, “Nature did not deem it her business to make the discover of her laws easy for us.”
Complex models usually are indicative of an approach that is basically flawed and schizophrenia indicates that things as a whole are not wired together right. Complex models, paradoxically, are not that hard to create. It is radically simple and economical models that explain observed behavior that are hard to create, hard for our minds to conceive of. Very hard. Again, Mr. Einstein: “Nature rarely surrenders one of her magnificent secrets!”
I have observed that if there is little or nothing to be said, or if the point is not “robust”, then the person often couches what they have to say in complex, difficult to understand wording or mathematics. Nietzsche got it exactly right: “Being deep and appearing deep–Whoever knows he is deep strives for clarity; whoever would like to appear deep to the crowd strives for obscurity. For the crowd considers anything deep if only it cannot see to the bottom: the crowd is so timid and afraid of going into the water.”
Beneath all the various stress-strain-void ratio curves lies a simple explanation. It is indicative of how puny the “great human mind” really is that we have taken so long to find it.
5. Secret Knowledge
Senior geotechnical engineers on hearing about the steady state concept react with surprise. “I have been practicing for 25 years but haven’t heard of this.” they say. Why is this knowledge “secret.” In truth, it is not “secret” so much as “hidden.”
Secret/hidden knowledge like this exists in literally hundreds of fields as diverse as real estate, finance, human spirituality and so on, protected by vested interests who would rather not publicize certain knowledge. A technical word used by people who study this fascinating phenomenon is “information asymmetry” where, in the absence of a free flow of information, institutions or organizations which control access to information, can “hide” information that they felt didn’t contribute to their own best interests. Thus a powerful theory from Harvard was not appetizing to certain universities seeking to “take the lead.”
The impact of this has been a blight in soil shear knowledge and research. Schools that consider themselves among the best seem to be completely ignorant of the steady state condition. Ph.D dissertations done at these schools and which focus on shear strength do not even mention the steady state in their literature review.
An additional result of this parochialism, is a great trans-Atlantic divide in the teaching and research on soil shear. Schools in the US tend to follow the mathematically impoverished, thumb-rule driven, trade school MIT approach with no good theoretical soil model being taught and with the emphasis on soil behavior. Schools in the UK and Europe on the other hand have a far more theoretical and mathematical approach driven largely by critical state based models, and with no real constraint from empirical data.
But with the advent of the internet, information asymmetry is steadily decreasing as the ability to distribute information becomes democratized and any old Joe (Paul Joe??) can quite effectively communicate to the targets of the information asymmetry using mechanisms such as this very blog…, and that too, for free. Sure, this facility is often misused and “mavericks and unsound thinkers” abound–you will have to decide on your own the trustworthiness of the information presented here.
So how do you know I am not an “unsound thinker?” As Gautama Siddhartha Buddha put it 2,500 years ago: “Believe nothing, O bhikku (learned monk), just because you have been told it, or it is commonly believed, or because it is traditional or because you yourselves have imagined it. Do not believe what your teacher tells you merely out of respect for the teacher. But whatsoever, after due examination and analysis, you find to be conducive to the good, the benefit, the welfare of all beings – that doctrine believe and cling to and take as your guide.” Today he would probably have added a few choice words on Bacon.
6. Which model will “win?”
Jim Graham, a researcher from Canada of elasto-plastic models has provided input to my research on a dynamical systems based model. While he agreed with me that the Cam Clay was an idealized model for isotropic clay with low surface energies, and that consequently he has taught Cam Clay only as an idealized component of elasto-plastic soil mechanics, he disagreed in that real clays were different. He said that my comments on elasto-plastic approaches to soil shear constitute “blue snow” (only negative hypotheses can be proven) and that newer elasto-plasticity frameworks that account for anisotropic elasticity and other physical processes etc. are the only way to make sense of what is going on when dealing with unsaturation, temperature effects, chemistry, and viscosity/strain rate effects.
Perhaps. But to me it seems that these issues are best addressed at the root–at the level of the micro-structure. It is here that temperature, pore fluid viscosity and saturation interact with the material friction of the particles and with soil-structure changes . Again, the proof of the pudding is in the testing and not in trading opinions over each other.
The obvious question is which model is going to “win”–the critical state or the steady state? The answer of course is that science has tools (some for centuries) with which to decide the better of two rival theories. These include tests to determine which theory is: a) simpler (“Occam’s razor”) b) a better fit (measured quantitatively) to experimental data (Bacon) c) more falsifiable (Popper), d) more complete (has an endogenous explanation), e) has greater explanatory power (explains for example why the e-log p’ curve in 1-D consolidation is linear) and f) fits in with fundamental concepts from the basic sciences.
Einstein when asked which theories are most likely to survive the test of time answered: “A theory is the more impressive the greater the simplicity of its premises, the more different kinds of things it relates, and the more extended its area of applicability.” (He went on to say that on this basis, he would bet on the first and second laws of thermodynamics.)
Usually though it takes a new generation before a paradigm shift in theory becomes widely accepted. As Max Planck said: “a new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.” What happens also is that the current generation is very vested in the existing paradigm and simply cannot afford to simply dump a life time of research and beliefs, without putting up a good fight. The next generation though has little vested in either of two competing paradigms and so is likely to consider rival theories, free from any “emotional baggage” and to choose what they see is the most promising approach.
There is nothing special in soil mechanics having two competing soil shear theories–what is special is how one theory has been kept tightly under wraps. Ultimately, with the Internet fostering the free flow of information, “information asymmetry” of all kinds will reduce and slowly but steadily wither away; the standard tools of science will sort things out without any “Sturm und Drang.” One should remember that if one does not understand the theory, but if it none the less makes predictions that conform closely with experiment, then the problem may not lie with the theory, but rather with the particular person’s inability to understand it.
One major hindrance to progress in research in soil shear is that over 60 years after World War II, a time when soil mechanics developed momentum, we are still missing a free and universally available data set on soil shear, that researchers all over the world can use to test their constitutive models against. In 1992, the NSF, under a visionary director (Dr. C. J. Astill) funded an ambitious program called VELACS, with duplicate tests run in independent soils laboratories on the East and West Coast of the US and following previously agreed upon test standards. You can find more details here. The VELACS data set is freely available and has played a fundamental role in my own papers.
If we could do it in 1992, we surely can do it two decades later when test equipment, procedure, and communications (email/internet) have all become so much more sophisticated and ubiquitous. We cannot claim that the “old timers” knew how to test better than we do today, and that so, we can no longer do this. Some of the best test data I have seen in over three decades was obtained by a 22 year old graduate student who built and used a true-triaxial machine for the testing. When I ran a soils laboratory, I worked closely with an “old timer,” and I can assure you, “old timers” come with their own set of issues and with rigid opinions as to how things should be done. I firmly believe that it is very difficult to outdo a modern automated testing system that requires minimal “art” on the part of the tester, in order to produce good data. The days of manually recording a dozen or so data points should be well behind us.
In short, it beggars common sense that standardized, publicly available datasets developed along the lines of VELACS are not available, and that we proceed on an amateurish, ad-hoc basis. I wrote to Richard Fragaszy, Program Director of Geomechanics and Geomaterials at the NSF to ask if they would fund the creation of such datasets.
Richard said most researchers believe that there were an abundance of high quality laboratory data sets available to validate numerical models, that many would say that additional tests would only be of minimal benefit, and that the difference with VELACS was that VELACS added centrifuge modeling of full-scale problems, not additional laboratory element tests. He made a valuable suggestion: instead of individual tests on soil elements a better comparison with VELACS would be centrifuge model tests of foundations systems, excavations, embankments, etc.
Based again on his extensive experience with the geotechnical research community, he still did not have a good feeling as to how such a test program might review. He anticipated the argument that there were enough high quality centrifuge tests already to do what I proposed without needing to do new tests, and that the proposal would have to make a strong argument for new tests. He suggested that I give some thought to using existing data from appropriate tests. He added that he was not saying this was better, but rather that I should be open to the idea and be able to defend either use of old data or justification for new data.
I am open to using old (existing) data—in fact I agree completely with Richard’s thinking that simply running additional tests that simply repeat what was done before would be a complete waste of tax payer money that could be put to better use elsewhere. However, where are these publicly available data sets? I asked Richard. He replied that I could do a search on the NSF web site for funded awards that have centrifuge tests included. Since our taxpayer money funded the creation of these data, they would be available to the public for the incremental cost of providing it. Richard also pointed me to the NEES Project Warehouse.
I searched both the NSF Funded Awards and the NEES Project Warehouse, using the key word “centrifuge” and in the “Engineering – Civil” field of application. The search returned 86 distinct centrifuge test programs that had been funded by the NSF over the last 40 years. Of these 86, I found none that modeled a non-dynamic loading of an excavation or embankment. If my finding bears up to a more detailed and thorough investigation, it sounds to me that a good case can be made to ask the NSF to fund a program for this purpose.
So in the end I did not find the centrifuge tests I was looking for. I know that these data exist (in the UK for example) but they seem closely held, gathering dust on some academic’s shelf somewhere. Also, I do know that there is real data from an instrumented embankment that was part of a research program done in the US in the 1970’s. Should you know of any such data, do email me with the details and we can strive to release it to the public.
I proposed to Richard that were such data available, we could put it up on a website using a “wiki” format. Researchers who had a constitutive model that they had published in an international peer reviewed journal would be eligible to use the data, provided they described in their section of the wiki transparently, exactly, and in gory detail, how they used their model so that any one who chose to, could independently confirm that the values reported were correct.
With an approach like this, the community can directly compare different models, and quickly “separate the wheat from the chaff.” The approach is voluntary in that if your model does not appear to provide good fits, you simply do not contribute to the wiki. Once I suggested this to Richard, it dawned on me that one reason why this seemingly commonsense approach has not happened in 60 years is perhaps this is actually the LAST thing that some may want to happen–empirical evidence that their model is wrong!. I for one would rather know that my model was wrong, then to live with the uncertainty of not knowing how accurate it really was.
In the end, for me, this wonderful book by Schofield read the author, the current state of the soil mechanics research profession both in the US and in the UK, and the national agencies that fund highly empirical research that does not fit into a general framework of science. Everything reads everything else and in the end, I suppose, this review also reads me (“Wer mit Ungeheuern kaempft, mag zusehn, dass er nicht dabei zum Ungeheuer wird“).
Ultimately, Schofield’s book raises two fundamental issues that need resolution. First, how important is surface chemistry in soil shear—the book says it is not of primary importance, and I would agree (while guiltily doffing my cap to Bacon!). Second, do soil grains move into a steady state structure or are they randomly oriented? If the former, you have steady state soil mechanics, residual strengths and the approach to liquefaction as prescribed by Harvard; if the latter, you have critical state soil mechanics and the approach discussed in this book.
I came away amazed that 40 years after the Harvard soil mechanics department shut down, its concepts are still powerfully relevant–a reflection that, all said and done, thanks to overt parochialism, research in soil shear has essentially stagnated.
Another prime cause of this stagnation is the way the highest honors appear to be awarded in soil-mechanics–for example the Terzaghi award. Unlike the Nobel prize which can be awarded to (say) James Watson for a specific discovery he made when he was 23, the Terzaghi award is given every two years for an opaque “outstanding contributions to knowledge in the fields of soil mechanics, subsurface and earthwork engineering, and subsurface and earthwork construction.” I believe that this allows for the smooth and seamless operation of an old boy network. The committee appointed to select the awardee usually lacks the technical depth to identify, rank, and reward deserving innovators, but instead appear to select the awardee from a list submitted by members in a process open to hidden influence. Further, the selection of awardees by and large tends to be nationalist and in this additional sense, blinkered, and parochial.
Specific discoveries/innovations are more to be prized than opaque “contributions” (which is the reason why the Nobel is awarded this way) and you will, in the case of the Terzaghi award, be hard pressed to find what specific discovery or innovation awardees have made to our understanding of soils. Deserving innovators are ignored–can someone tell me why Steve J. Poulos is yet to be honored for his seminal 1981 paper “The Steady State of Deformation” or Gonzalo Castro for important work on liquefaction? Or G. Mesri for seminal work on creep and consolidation? In my opinion, the opacity of the reasons behind the award has allowed the award to be made in the past to a petty parochialist, a timorous trade-school empiricist, a pretentious junior author of a text book, and combinations thereof, whose specific innovations are unknown at worst and murky at best.
Along these lines, when someone tells me they have written N papers where N is often over 100, I ask them, what their most important finding has been from among these N papers. Surprisingly often (initially my question was asked in all innocence), the result is a lot of sputtering and stammering followed by a haughty “you will have to read them to find out!” Yes, indeed. Any when I do read them I find that they usually consist of “filigree on top of filigree” in short–no major finding or advance. Sometimes though they tell me frankly and clearly what it is and I am able to place them accordingly. Either way, I quickly know their true “soil mechanics merit.”
So the next time someone tries to impress you with their awards or with their “chairs” or with the number of papers they have published, ask them a simple question–tell me in one sentence of less than twentyfive words, what has been your most important finding? Judge their “soil mechanics merit” accordingly, keeping this quote from Albert Einstein (my last quote from him, I promise!) in mind: “I have little patience with scientists who take a board of wood, look for its thinnest part, and drill a great number of holes where drilling is easy.” Misguided though I believe the critical state and elasto-plastic concepts to be, Andrew Schofield deserves kudos and respect for drilling most useful holes where the wood is thickest.
In the early 20th century Karl Terzaghi created a narrow bridge connecting science to soil-mechanics. Since then this narrow bridge has been submerged by mindless trade school empiricism, crude parochialism, and Aristotelian scholasticism. As a result, it seems to me that today soil-mechanics has become an isolated island separated from a continent of general, applicable, scientific knowledge. It is high time we uncover a connection to the main land by a wholesale (not retail) import of elementary science and scientific thinking into the subject. Schofield’s concluding call for more research in the fundamentals of soil shear is apt; it is up to current researchers to take up the challenge.
If you think that you know about soil shear then you should be knowledgeable about the critical state condition, the steady state condition, and the concepts presented in this book. Should this describe you then do write to me with your thoughts on soil shear and this review–I would like very much to hear from you. But should this not describe you, then be warned! You walk on thin ice! You rush where angels fear to tread!