Posted in Soil Mechanics

Van der Merwe’s Method, Adapted for SI Units

Van der Merwe’s method, which was first introduced in the 1960’s, is a simple method for estimating the vertical movement of expansive soils.  The method at its most basic is described in publications such as Foundations in Expansive Soils, and basically looks like this:

Van der Merwe Method

The problem with this presentation is that it is entirely in “Imperial” units, which were the standard in the South Africa of van der Merwe’s day.  We need to convert this to make it usable in SI units as well.  First we present the potential expansion chart, from Soil Mechanics:

Volume Change Potential Classification for Clay Soils

Then we reconstruct van der Merwe’s equation to make it applicable for SI units as well as US (or Imperial) units:

Van der Merwe's Method SI and US Units

Note that the swell potential is now in the denominator.  That’s a convenience to make the PE values of a reasonable order of magnitude.  But now it’s dimensionless. Also note that the depth reduction factor has a different exponent for SI units than it does for US units.

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Posted in Academic Issues

The Problem with "Going Dark" in the Technical Literature

When starting out on a major research project in science or engineering, the first thing to do is to go through “the literature” (which usually means the peer-reviewed body of articles and published books, although internet stuff is becoming increasingly important) and try to figure out the current “state of the science” (we used to say “state of the art” but people are less inclined to use that expression than they used to be).  From here we proceed to do new things which will hopefully advance the state of whatever field of endeavour we are operating in.  As I stated in my master’s thesis:

In any investigation such as this the ideal goal is to come up with something truly novel, and many of such works emphasize their novelty to the denigration of those who have gone on before. While in some fields of endeavour this might be appropriate, in this case such sweeping novelty cannot be claimed. This work fits the mould as outlined by Pascal above: it takes the work that has been done before, advances it a step while realizing that there are many more steps before “perfection” is achieved.

But stepping back to those who have “gone before”, the scientific and engineering literature isn’t as transparent as one would like.  In recent years fraud and misrepresentation of results has required any researcher to be careful as to what he or she believes.  There are also situations where stuff that looks really good at one point in time get abandoned later for various reasons; we have to make sure our research takes a long sweep in time as well as topic.  We also have the problem of “non-novel” papers, which are really rehashes of stuff figured out a long time ago but put back into the literature to give glory to someone else.  These don’t do much for the originality reputation of their writers but, sometimes, can be useful, putting back into currency things which have “gotten lost in the shuffle” over the years.

But one serious problem that deserves some attention–and one that doesn’t get a lot of press–is the matter of “going dark” in the literature.  An overview of the pattern of scientific and engineering advance is in order.

Generally speaking, in any given field there are “seminal papers” (usually more than one), which is where the field was kicked off.  From there we have what comes after, which usually refers back to the seminal paper.  In my field of pile dynamics, we have one paper that gets cited in just about everything written on the subject.  From here the science and technology are developed and things advance.  And then, without much fanfare, the literature “goes dark”.

That doesn’t mean that people stop publishing anything on a given topic.  Far from it; however, it’s like a line from Hogan’s Heroes, when Gestapo Col. Hochtstetter tells Klink that he can make Hogan talk.  Klink’s reply was, “You can make him talk.  He just doesn’t say anything”.  A lot of the literature is little better than fluff or promotion of a new idea without substantive detail on how these “new” improvements really work.  The obvious question is why.

One reason is that the material is classified for military or national security purposes.  Generally speaking, however, that literature doesn’t go dark as much as it’s dark to start with and it’s only later when things come to light.

Another reason is that the field has become inactive, usually temporarily.  There are a number of reasons this happens.  In my field, wave propagation in driven piles was discovered in the early 1930’s in Australia, and the English carried out some research later in the decade.  (The Americans got into a food fight on the subject).  But things went dark for a very big reason: World War II, which focused the participants on other matters, such as rational soil classifications and nuclear weapons.  After that conflagration, things resumed and progressed to the current state.

In my experience, however, the biggest reason the technical literature goes dark is because of commercialisation.  In the early stages, the research is the “property” of academic institutions, individuals and the government (especially since World War II) which funds it.  In these conditions there is a relatively free exchange of ideas and expression of these ideas in articles and books.  However, when technologies are commercialised (especially when it’s done by a relatively small number of organisations) things start getting proprietary, and then things start getting secret.  Although it’s possible to have patents and copyrights to protect oneself in some cases, it’s not possible to copyright an idea; it’s easier to simply use trade secrets, even if those trade secrets are derivative from research from more “open” sources.

The fact that a technology can be commercialised is a good thing in that it shows that it works and is useful.  Over time, however, it happens that organisations use institutional inertia and human habit–to say nothing of our tort system, which stifles innovation by punishing experimentation which can go wrong–to make their proprietary method a “standard” and keep its true nature under wraps to discourage its replacement or even improvement.  In time this slows the advance of science and technology in ways that are not obvious to most people.

Researchers who set out to try to advance methods in areas which have “gone dark”–assuming they can get funding for their work in the first place–face a number of obstacles.  First they must realise that beyond the dark literature are doubtless some improvements the nature of which are obscure.  They may find themselves “reinventing the wheel” in an unavoidable way.  If they get past that, they find that they lack the benefit of the learning curve which those who actually use the existing method.  The road to advancement can be a perilous one under these circumstances.

But advancement is what science and engineering is supposed to be about, isn’t it?

Posted in Academic Issues

Mirroring Our Creator

Not too long ago, while grading homework for a course I was teaching, I saw a “better than usual” performance from one of my students.  I noted that, if she would consistently concentrate on what she was doing, she was capable of very good work.   The response I got to this was as follows:

I just stumbled across the feedback you gave me…Thank you for that. It’s nice to hear those things once in a while, and especially from a professor of your calibre.

My response to this was as follows:

At the beginning of his poem Paradiso, Dante wrote the following:

The glory of Him who moves all things rays forth
through all the universe, and is reflected
from each thing in proportion to its worth.

Our first task in life is to point the mirror in the right direction.

I’m sure that it’s the rare professor in the College of Engineering and Computer Science that would quote Dante in a communication with a student, but doing so brings up some things that need to be said.

Today the concept of “equality” is endlessly paraded before us.  In practice, however, equality is a tricky concept.  It’s one thing to pass some legislation and give each other the high-five that we’ve moved towards a more just society. It’s another to achieve real equality.  To do that would require either that we accept that everyone have the same outcome (which was a goal of Communism) or abolish any kind of reward for performance, and frankly we’re not near either one.

No where is that more evident than in education.  In spite of the levelling efforts of the last fifty years, we still don’t have real equality, not only among the students and faculty but among differing institutions.  There are many reasons for this but the most important one is that people are not the same; thus, inequality is built into the system from the start.

A teacher is presented with a varied lot each time class assembles.  In addition to differing levels of intelligence, there are other things that vary.  Students learn differently one from another.  Some take too many courses in one semester.  Some work full-time jobs and/or have a family.  Some do both, which can be a real disaster.  Some experience personal tragedy, either going into their studies or during them.

It’s tempting for an academic to focus on their “best” students.  Having worked in industry first, I am aware that there is more to life than academic performance, and I’ve seen in class that the “smart” students aren’t always the ones who come up with the best solutions, especially on projects.  That tells me that, as one of my own professors observed, testing may not be the best was to gauge performance, but it’s the best we’ve got.  We need to understand its limitations, along with those of the whole academic system.

Getting back to Dante, he lived in a world where inequality was accepted as a fact of life.  But he also lived in a Christian world where each and every human being had worth to his or her Creator.  Each of those creatures should reflect whatever glory their creator put in them; if they did so, they fulfilled their purpose, and found their value in doing so.

Today our obsession with “equality” leads us to try to do all and be all.  But our God doesn’t expect that, and neither do I.  As a professor, what I want to see from my students is their best, to bring out that which their God and their creator has endowed them with.  If I get that, I’ve succeeded and they’ve succeeded.

That is what I meant by my comment: our first task is to direct ourselves in such a way as to reflect the glory of our Creator best, and that first is towards Him.  But that leads to another point of the Paradiso: we get to the point where we realise we cannot achieve our true goal without God’s help and presence in our lives.  To fully reflect the glory of our Creator and to fulfil his purpose for us requires that step, and for that the provision is his, not ours.

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Posted in Academic Issues

Is the Principles and Practice of Engineering Exam a Barrier Against Women?

The intrepid Toni Airaksinen at Campus Reform has written an article highlighting the research of Drs. Julia Keen & Anna Salvatorelli on this subject.  The statistics are interesting and so are their recommendations for further research:

This study focused on pass rate, and the resultant disparity is only the first step. Additional research should be conducted to identify why women are not passing the PE exam at an equal percentage rate as men. This research should include:

  • Identifying biases in the exam itself

  • Examining the timing of administration of the exam in an engineer’s career progression

  • Exploring the likelihood of women to retake the exam compared to men after failing since the number of attempts was not recorded within the data collected

  • Identify factors that may contribute to higher pass rate for women in some states compared to others.

As someone who has taught civil engineering for more than a decade at the undergraduate level, this has more than a passing interest.  For me, it was also an interesting moment, because I saw this just after I had returned from the dedication of the new headquarters for Division 2 of the Tennessee Department of Transportation, where most of my students who work there are female.

Let me first set forth their “bottom line” cumulative statistics (I strongly urge those of you who can get access to their paper to do so):

  1. About 20% of the people who take the “Principles and Practices” exam are women.  That tracks pretty well with the number of women in my classes.
  2. 51.5% of the women pass the test on the first try, while 63.1% of the men do.

With that out of the way, I’d like to make some observations.

  1. My female students tend to be a very diligent and competent group.  In many ways an engineering curriculum is more of an endurance match than anything else; the women “tough it out” at least as well as the men.
  2. I’ve never noticed women having more difficulty with tests than men in my classes.  That’s saying a lot because my tests tend to be bizarre, as my students will attest.
  3. Women in civil engineering have some built-in advantages because of the diffuse structure of the system by which structures get built and their socialization skills, as I explain this 2014 post.  Because of the nature of our society, engineers tend to get stuck in the caboose on the train of respectability; I think that women are a significant part of the key to change that situation.

Especially considering #2, I find it hard to believe that the test is intrinsically biased against women.  So why is this disparity so?  Our researchers give us four options, and my gut tells me that the second one is the most likely.

My reasoning is simple.  Generally speaking, most engineering students take their first exam (the FE exam) while they’re in undergraduate school.  After they they acquire four years of experience, they can apply for the privilege of taking the P&P exam.  If they pass it and meet other requirements, they obtain their Professional Engineers license.  For most people, that means that the critical moment takes place in their mid- to late twenties.  Millennials aren’t as “progressive” on sorting out tasks between spouses or partners as some might have you believe.  That time in life is also the same time when many marry, have children, etc., and the work associated with those events falls harder on women.  Thus the first opportunity to take the exam takes place at a point in life which is less opportune for women than it is for men.

So what is to be done?  Do we need a special accommodation?  The answer is “no.”  Since venting pet peeves seems to be the thing on this site these days, let me vent one of mine: there is no cogent reason why we should force people to wait several years out from their academic studies to take the P&P exam.  This exam is supposed to reflect experience, but a reality check is in order: it’s just another academic exercise like just most any other test.  Fortunately change is in the wind, as this statement from the National Society of Professional Engineers indicates:

Until relatively recently, candidates for licensure as a professional engineer have needed to gain four years of approved work experience before taking the Principles and Practice of Engineering (PE) Exam. In recent years, however, attitudes within the profession toward the early taking of the PE exam have begun to shift. In 2013, the National Council of Examiners for Engineering and Surveying (NCEES) removed from its Model Law the requirement that candidates earn four years of experience before taking the exam. Separating the experience requirement from eligibility for taking the PE exam is sometimes called decoupling. For the National Society of Professional Engineers, as stated in Position Statement No. 1778,

“Licensing boards and governing jurisdictions are encouraged to provide the option of taking the Principles and Practice of Engineering exam as soon as an applicant for licensure believes they are prepared to take the exam. The applicant would not be eligible for licensure until meeting all requirements for licensure— 4-year Accreditation Board for Engineering and Technology/Engineering Accreditation Commission accredited degree, passing the Fundamentals of Engineering exam and the Principles and Practice of Engineering exam, and 4 years of progressive engineering experience.”

The NSPE would have us think that this concept is a novelty, but that’s not really the case.  When I was an undergraduate at Texas A&M University in the 1970’s, Texas allowed people to take both exams before graduation; our own NSPE student chapter strongly encouraged that, and I did it myself.  Taking the P&P exam not only gets the exam away from major life events in early adulthood, it also eliminates a good deal of remedial work trying to remember things one learned in school but had forgotten in the years before the exam.

I think that, if we do not obscure our thinking with trendy concepts and look at things realistically, we can solve this disparity by making a change that will benefit both men and women and improve our profession.  If this disparity provides motivation to move the process of “decoupling” forward, then so be it.  It’s a change that’s overdue.

Posted in Soil Mechanics

Jean-Louis Briaud’s “Pet Peeve” on the Analysis of Consolidation Settlement Results

In his recent, excellent article on the settlement (and subsidence) of the San Jacinto Monument east of Houston, Briaud (2018) takes an opportunity to vent a “pet peeve” of his relative to the way consolidation tests are reduced and consolidation properties reported:

A Chance to Share a Pet Peeve

The consolidation e versus log p’ curve is a stress-strain curve.  Typically, stress-strain curves are plotted as stress on the vertical axis and strain on the horizontal axis.  Both axes are on normal scales, not log scales.  It’s my view that consolidation curves should be plotted in a similar fashion: effective vertical stresses on the vertical axis in arithmetic scale, and normal strain on the horizontal axis in arithmetic scale.  When doing so, the steel ring confining the test specimen influences the the measurements and skews the stiffness data.  Indeed the stress-strain curve, which usually has a downward curvature, has an upward curvature in such a plot. (p. 54)

Is this correct?  And is he the only one who thinks this way?  The two questions are neither the same nor linked.  Although this problem will certainly not be solved in one blog post, it deserves some investigation.

Statement of the Problem

Let’s start with a text we use often here: Verruijt, A., and van Bars, S. (2007). Soil Mechanics. VSSD, Delft, the Netherlands. Early in the presentation on the subject, he presents the following plot:

As Jean-Louis would have us do, the strain (or negative strain, since we’re dealing with compression) is on the abscissa, and the dimensionless stress is on the ordinate.  The difference between the two is that the stress is plotted logarithmically.  But it’s a step.  We’ll come back to that later.

Verruijt defines the relationship between the strain and stress ratio as follows:

\epsilon = -\frac{1}{C}\ln\frac{\sigma}{\sigma_0}

This relationship goes back to Terzaghi’s original tests and formulation of settlement and consolidation theory almost a century ago.

From a “conventional” standpoint there are two things wrong with this formulation.  The first is that it is based on strain, not void ratio.  The second is that it uses the natural logarithm rather than the common one.  The last problem can be fixed by rewriting it as follows:

\epsilon = -\frac{1}{C_{10}}\log\frac{\sigma}{\sigma_0}

This formulation is essentially the same as is used in Hough’s Method for cohesionless soils, once the strains are converted to displacements by considering the thickness of the layer.  So it is not as strange as it looks.

The first problem can be “fixed” by noting the following:

\epsilon = \frac{e-e_0}{1+e_0}

We can substitute this into the equation before it and, with judicious changes of the constants and other subsitutions, come up with the familiar, non-preconsolidated formula for consolidation settlement, or

\Delta H = \frac{C_c H_0}{1+e_0}\log\frac{\sigma}{\sigma_0}

When we reverse the axes, we then get the “classic” plot as follows:

But is there a problem with using strain?  Verruijt explains the two conventions as follows:

In many countries, such as the Scandinavian countries and the USA, the results of a confined compression test are often described in a slightly different form, using the void ratio e to express the deformation, rather than the strain ε…It is of course unfortunate that different coefficients are being used to describe the same phenomenon. This can only be explained by the historical developments in different parts of the world. It is especially inconvenient that in both formulas the constant is denoted by the character C, but in one form it appears in the numerator, and in the other one in the denominator. A large value for C_{10} corresponds to a small value for C_c . It can be expected that the compression index C_c will prevail in the future, as this has been standardized by ISO, the International Organization.

As is often the case, the simplest way to help sort out this issue is with an example.  Briaud (2018) actually has one, but we will use another.

Example of Settlement Plotting

An example we have used frequently in our teaching of Soil Mechanics is this one, from the Bearing Capacity and Settlement publication.  It is a little more complex than the theory shown above because it involves a preconsolidated soil.  The plot (with the simplifications for determination of C_c and C_r is shown below.

With this information in hand, we process the data as follows:

  1. We convert the void ratio data to strains using the formula above.
  2. We convert the stresses to dimensionless stresses by dividing them by the initial stress.
  3. We “split” the data up into compression and decompression portions to allow us to develop separate trend lines for both.

First, the strain-dimensionless stress plot, using natural scales for both.

preconsolidation example strain natural stress

The result is similar to that in Briaud (2018).  The compression portion best fits a second-order polynomial fit.  (Not that we have thrown out the zero point to allow more fit options.)  The decompression portion fits an exponential trend line best.

Below is the same plot with the stress scale now being logarithmic.

preconsolidation example strain logarithmic stress

This is basically the original graph with the axes reversed.  There is no effect using strain; we will discuss the advantages of doing so below.

Now let us look at the data from another angle: the tangent “modulus of elasticity,” defined of course by

E = \frac{\Delta\sigma}{\Delta\epsilon}

We consider natural scales for both modulus and strain.  To obtain the slope, we used a “central difference” technique except at the ends.

preconsolidation example strain natural E

It’s interesting to note that, except for the “kink” caused by preconsolidation, in compression the tangent modulus of elasticity increases somewhat linearly with strain, as it does with the decompression.

Discussion of the Results

There’s a great deal to consider here, and we’ll try to break it down as best as possible.

Use of Strain vs. Void Ratio

The graphs above show that there is no penalty in using strain instead of void ratio to plot the results.  The advantage to doing so is both conceptual and pedagogical.

In the compression and settlement of soils, we traditionally conceive of it as a three-stage process: elastic settlement, primary consolidation settlement, and secondary consolidation settlement.  Consolidation settlement is nothing more than the rearrangement of particles under load; the time it takes to do so is based in part on the permeability of the soil and its ability to expel pore water trapped in shrinking voids.  Elastic settlement is due to the elastic modulus of the material, the strain induced in the material and the geometry of the system.  This distinction, however, obscures the fact that we are dealing with one soil system and one settlement.  Using strain for all types of settlement would both help unify the problem conceptually and ease the transition to numerical methods such as finite element analysis, where strain is used to estimate deflection.  In the past we were able to use a disparate approach without difficulty, but that option is not as viable now as before.

The Natural Scale, Consolidation Settlement Stiffness, and the Ring

Both here and in Briaud (2018) the natural stress-strain curve experiences an upward curvature, which is obviously different from what we normally experience in theory of elasticity/plasticity.  This comes into better focus if we consider the variation of the tangent modulus of elasticity, which (except for the aforementioned preconsolidation effect) linearly increases with stress.  There are two possible explanations for this.

The first is to observe that, as soils compress in consolidation settlement, their particles come closer together, and thus more resistant to further packing.

The second, as suggested by Briaud (2018), is that the presence of the confining ring in the consolidation test augments the resistance of the particles to further compression.  The issue of confinement is an interesting one because in other tests (unconfined compression tests, triaxial tests) confinement is either very flexible or non-existent.  It should be observed that consolidation theory, as originally presented, is one-dimensional consolidation theory.  For true one-dimensional consolidation, we assume a semi-infinite case where the infinite boundary “confines” the physical phenomena.  The use of a confining ring assumes that the ring can replicate this type of confinement in the laboratory.  Conditions in the field, with finite loads and variations in the surrounding soils, may not reflect this.  While it would be difficult to replicate variations in confinement in the laboratory, these variations should be kept in mind by anyone using laboratory-generated consolidation data.

The “Modulus of Elasticity” for Consolidation Settlement

This may strike many geotechnical engineers (especially those in areas where void ratio is used to estimate consolidation settlement) as an odd concept, but if we consider the material strain vs. its deflection, it is a natural one.  Varying moduli of elasticity are nothing new in geotechnical engineering; they have been discussed on this site in detail.  The situation here is somewhat different for a wide variety of reasons, not the least of which is that here we are dealing with a tangent modulus while previously we looked at a secant one.  Also, differing physical phenomena are at work; theory of elasticity implicitly assumes that particle rearrangement is at a minimum, while consolidation settlement (both primary and secondary) is all about particle rearrangement.

A more unified approach to settlement would probably reveal a process where the change in stress vs. the change in strain varies at differing points in the process along a stress path with multiple irreversibilities.  Such an approach would require some significant conceptual changes in the way we look at settlement, but would hopefully result in more accurate results.

Conclusion

Consolidation settlement is a topic that has occupied geotechnical engineering for most of its modern history.  While the theory is considered well established, changes in computational methodology will eventually force changes in the way the theory is applied.  A good start of this process is to use strain (rather than void ratio) as the measure of the relative deflection of structures, and the example from Briaud (2018), along with the demonstration relative to natural scales, is an excellent start.

References

Briaud, J.-L. (2018) “The San Jacinto Monument.”  Geostrata, July/August.  Issue 4, Vol. 22, pp. 50-55.