Over the years, my department has asked me to give a review session for my students before they take the FE exam. In this time of COVID, I’ve committed all my other lectures to video, and this one is now no exception:
I mention a few of things in the intro I’d like to elaborate on:
About ten years ago, it was brought to my attention that my students weren’t doing well on the FE Exam geotechnical section. My response to that was simple: “I’ll fix that problem.” I did that by aligning what I taught in class with what was in the FE “cheat sheet” (I’m sure NCEES loves that designation.) I don’t subscribe to the idea that we should only be “teaching to the test” but the FE exam’s geotechnical requirements are pretty basic, so that wasn’t much of a conflict. What has been tricky is that they’ve shifted around what they require over the years. But my students’ performance on the test has improved.
Since COVID I’ve put my lectures online. If you need to investigate some topics in detail, I’ve got them either at my Soil Mechanics or Foundations pages.
Once you’ve digested what’s presented in the video, you can and should solve sample problems. I just don’t recommend that you start your preparation doing that.
There are some things in geotechnical engineering that don’t get really good (if any) coverage in many textbooks, which means that those who go on into that part of civil engineering are blindsided by their appearance. One of these is the “line of optimums” approach for compaction evaluation. The only formal textbook I know of that covers it is Soils in Construction, for which I must credit my co-author, Lee Schroeder. It also appears in the Soils and Foundations Reference Manual.
The line of optimums approach seeks to answer a key question in compaction: how much compactive energy is necessary to effect a given compaction? We have the Standard Proctor and the Modified Proctor test, but when we’re trying to determine a specific compactive effort for a particular soil and project, we need more flexibility.
Compaction Chart with Multiple Compactive Energies and Line of Optimums, from Rebrik (1966)
The lines on the chart are as follows:
Lines 1, 2, 3 and 4 represent compaction curves for a soil, but with a different number of blows (25, 50, 100 and 150, as shown in the chart.) As is customary, the plot is the water content (x-axis) against the dry density (y-axis,) although in American practice this is usually the dry unit weight.
Line 5 is the “zero air voids curve,” i.e., the curve where the degree of saturation S = 100%.
Line 6 is a “trendline” of the peak compaction dry densities, a “line of optimums.”
At this point, we will show a more “contemporary” approach to the line of optimums method than what’s in the book, which dates back to the semilog paper era. The first thing we do is to switch the x-axis from the water content to the number of blows for each compaction. We then plot this against the y-axis of the maximum dry density for each compaction, the result is tabulated as follows:
As it happens the power correlation turns out to have the highest R2 value. The book’s graph implies an exponential fit, but the variance in R2 between them is not great. Using the spreadsheet’s trendline feature gives the designer more flexibility in reducing the data. An explanation of that trendline feature–and curve fitting in general–can be found at Least Squares and Curve Fitting.
At this point we have a problem: we do not have the compactive effort for the differing blow counts. This is easily remedied because Equation 8.5 in Soils in Construction shows that, if the compaction mould, volume of soil, number of lifts and impact energy are all the same (a reasonable assumption in this case,) then the number of compactive blows is directly proportional to the compactive energy.
With that information in hand, we can take an actual compaction method and lift depth and estimate the maximum dry density we can expect. If it is not adequate for the task, we can either use a different compaction method or revise our design for a lower compactive dry density. We also need to determine a reasonable relative compaction, which will reduce this value to one which we can expect to happen during actual compaction.
The line of optimums method is a good one for compaction evaluation, and we hope that this little presentation helps you to understand it.
Reference
Rebrik, B.M (1966) Vibrotekhnika v burenii (Vibro-technology for Drilling.) Moscow, Russia: Nedra.
A salutary reminder from Y. Ryabov’s An Elementary Survey of Celestial Mechanics: There is of course no sense in asking why the planets rotate or why they have motion in general. Everything in the universe, from the smallest dust particle to colossal cosmic bodies, is in constant motion. There is no such thing as matter […]
With the Unified soil classification system, there are many ways of diagramming it. One of those was presented in the last post. With the AASHTO system, there’s generally only one, as shown in the Soils and Foundations Reference Manual. For classification this is pretty much it, but it’s not very informative when it comes to getting a “feel” for what these classifications mean.
Below is a chart from the Ohio Department of Transportation (about the only DOT I know of which uses the AASHTO system for just about everything they do) which describes each AASHTO classification in words and attempts to describe the type of soil for each type in the system.
AASHTO Soil Classification System as Described by the Ohio Department of Transportation
In the course of teaching my Soil Mechanics class, I’ve tried numerous different charts and methods for teaching the Unified system of soil classification. Probably the most success I’ve had is with the one from NAVFAC DM 7, and it’s below. (I’ve included the plasticity chart for completeness.)
This chart is reproduced (with better typography) in my book Soils in Construction. Unfortunately ASTM has been messing with this procedure, and for that reason I have had to shift to it in the last years of teaching Soil Mechanics. I still prefer this NAVFAC chart because it reduces soil classification to a straight-up process of elimination rather than the “decision tree” approach ASTM apparently prefers. NAVFAC DM 7.1, the newer edition, has gone with a narrative description of the system (except for the plasticity chart) which is even harder to follow for those just learning the system.
vulcanhammer.net 