Some things are outliers.  There is a really good book by Malcolm Gladwell with the same title. 

Some outliers need special attention.  I once worked on a project that involved stiff fissured clay.  By the time I came into the project, all of the lab testing had been completed.  One test report was especially puzzling.  It was the triaxial shear test results of the stiff fissured clay.  All of the Mohr circles lined up very well to form a nicely developed failure envelope.  All the circles except one, that is.  And that circle was very small, and way to the right of the other circles.  

Hmmmm. What gives? 

As it turns out, that was a very important circle.  My theory is that the specimen being tested had a slickenside that was perfectly oriented for failure.  The friction angle for that slickenside was only a few degrees, while the intact material was something like 30 degrees.  The engineer who proceeded me just disregarded that outlying Mohr circle, assuming it was a false point.  And it was probably the most important data.  It may have been the controlling parameter for slope stability.  It was very fortunate that there was adequate time to modify the design.

Beware the outliers!

Leave Your Comments