Size – A Matter of Perspective

Is that stick really holding it up??

That’s one big BOULDER!  To find more pictures like this one, try a quick Internet search for images matching “large glacial erratic”.  You’ll pull up more amazing pictures of huge rocks resting in strange places.  Here are a couple more examples:

Madison Boulder in New Hampshire: 23 ft high and measuring 83 ft long and 37 ft wide, estimated to weigh 5,000 tons.

Or the massive quartzite 16,500 ton Okotoks boulders in Alberta, purportedly the large erratic in the world.

Glacial erratics are pieces of rock (usually boulders – larger than 12″ in diameter per USCS) that have been transported and deposited by glacial action and are dissimilar to the soil and rock around them.  According to Wikipedia, the term erratic comes from the Latin for “to wander, roam, or ramble.”  These rocks have wandered and roamed via glaciers to their new homes.

The size of these monsters illustrates one of the amazing things about glacial soils: the HUGE range in particle sizes that ice can transported.  Erratics can be found resting on or in a matrix of gravel, sand, and even clay particles, which are many, many times smaller. Madison Boulder, for example, has an equivalent diameter of about 51 ft.  Compare that with a common clay particle dimension of 0.002 mm.  The boulder is almost 10 million times bigger!!!  Yet both are the result of glaciation.

But are the glacial erratics the biggest in every way?  Consider the specific surface area of the boulders.  Assuming Madison Boulder is a rectangular prism, it’s SSA is a puny 2.4×10-7 m2/g.  For comparison, the clay particles in our class example problem have a SSA of 37 m2/g – almost 200 billion times larger than Madison Boulder!!  Putting it another way, only 34 g (just over an ounce) of our clay are required to have the same surface area as the boulder.

So the way we look at particle size is in many ways a matter of perspective.  Whether something is big or small depends on what property we’re measuring.  Whatever the case, the astounding range of particle sizes encountered in soils is one of the reasons their properties, such as strength, compressibility, and permeability, vary so much.

Geotechnical Engineering – Assignment 1

Why is my course important?  We’re asked to explain this “to” our students or “for” our students in our syllabus.  Not that I can’t do this, but should I?  Is explaining the “importance” of our class just another example of exerting power over our students, right from the beginning?

Maybe this is another question that we can help and encourage our students to explore for themselves.  I’m envisioning a syllabus and first assignment in an introduction to geotechnical engineering course (a junior-level class) that goes something like this.

Syllabus Intro:  Throw out half a dozen intriguing facts about soil mechanics that will pique the interests of these young civil engineers.  Try to touch on all the major branches of the discipline; cast the net large.  Brief entry-level papers could even be provided as background for the facts for students who want to explore further.  For example:

  • Did you know that expansive soils cause an estimated $13 billion damage to buildings in the US annually, more than the effects of hurricanes, floods, tornadoes, and earthquakes combined (Rendon-Herrero, 2011)?

Assignment #1:  Brainstorm and record the reasons why a knowledge of geotechnical engineering and soil mechanics will be important to you (aside from passing the FE exam).  Use the facts/questions in the beginning of the syllabus as a jumping off point, if necessary.  Also consider the following:

  • If you plan to concentrate in geotechnical engineering, explain why.  What attracts you to the field?  What makes you interested in it?
  • If not, how do you think that your concentration (structural, transportation, etc.) interacts with geotechnical engineers in practice?  How will the content of this course help you with those interactions?
  • Think of this assignment like a journal entry, not a paper.  It will be graded based on the thoughtfulness of the response, rather than the elegance of the prose or the conclusiveness of your argument.

I think an assignment along these lines could really help students think through the purpose of the class and semester before them.  It would be helpful to compile, distribute, and discuss the results with the class and share the student’s various insights with everyone.


Student choices in assignments

Been musing about student choices within assignments, specifically in my field of geotechnical engineering.  How to do this?

Consider an assignment calculating seepage below a dam or levee resting on layered soil, for example.  The designer must have values of permeability for each soil layer.  Rather than the more traditional approach in which students are given these parameters, why not give the students an abundance of test results for the soils and let them choose the parameters from the results as they would in practice.

The assignment would then be evaluated as follows:

  • Say 80% of grade is assigned based on the design analysis (the primary learning objective of the assignment).
  • The remaining 20% would be assessed based on HOW the parameters for the analysis were chosen (secondary objective).

Students would be allowed to use all, some, or none of the provided test results to determine the parameters for their analysis.  If few or none of the tests are used, the students must justify this and explain the judgment calls that they have made.  If more or all of the tests are used, the students will see that getting more technical data is “expensive,” and does not necessarily result in more accurate analyses.  This approach would “cost” the students more study hours just like it would cost their clients more in the future.


The power of a crumpled piece of paper

My office mate, who is teaching for the first time this semester asks, “How can I explain the concept of specific surface area (SSA) to my students and the difference between sands and clays?”

For those uninitiated to soil mechanics, the SSA of a soil is the ratio of particle surface area to mass.  Sand particles are roughly spherical and have a small amount of surface area relative to their mass.  Clay particles on the other hand are extremely thin plates with many times more surface are than those in a sand.  For example, a 100 g (4 oz) of some clay may have enough surface area to cover a football field.

We discussed the matter for a few minutes and devised the following demonstration, which my office mate could perform off-the-cuff during class:

  1. Take two pieces of plain paper.  Same paper = same mass.
  2. Crumple one up into the smallest ball possible.  Ask the class to imagine there is no air left inside the ball.
  3. Compare the surface area of the balled up paper (think – sand particle) and the flat sheet of paper (think – clay particle).  The clay obviously has much greater surface area because of its platy shape.  The class could quickly calculate the surface area of each, if desired.
  4. Optional: Throw balled up paper into the class.  Try not to hit anyone in the head.

My office mate’s post-class assessment of the demonstration was positive.  The class was jolted out of “take notes” mode into “something different is happening” mode.  They appeared to pay more attention and really grasp the concept more fully than if a blackboard description had been used.  Score one for the Crumpled Paper.

Bonus demonstration to explain hydrometer analysis and Stokes Law.  Prior to Step 4 above:

  1. Hold the ball and sheet the same distance above the ground.
  2. Drop both at the same time.  The ball will fall to the ground while the sheet floats back and forth, impeded by drag forces that are large in comparison to particle weight.
  3. Ask what is different about this demonstration and the assumptions of Stokes’ Law.
  4. Repeat Step 4 above.