I have a Geotechnical Lab class and it is my last class in the geotech series, yay! We’re testing all the interesting things I’ve been learning all year! We just finished the direct shear strength testing where we used a shear testing device like this one below to apply normal stress that would be applied by a footing to see what kind of shear would be created.

If a building were to fail under direct shear it would look something like the picture below. The area of soil that should be tested for shear is going to be below the footing about the same distance down from the footing that the footing is from the surface of the soil.

The results were predictable for the sample I had. It was a loose sand and compacted nicely as well as withstanding a large amount of shear stress. For the test a small ring sample was inserted into the direct shear machine, compressed and sheared. This is what a typical sample would look like after the test is completed.

Below is a graph taken from the computer calculated results applying 1000 lb of normal force. The graph shows the shear stress vs shear displacement and axial displacement vs shear displacement. This test was done on 3 different undisturbed samples of the same soil.


Three samples were tested and under different normal loadings which would simulate different applied footing loads and direct shear was measure for all of them. Once these were tested I should be able to construct a Mohr-Coulomb Failure envelope slope for normal and shear stress. Max shear and cohesion will be obtainable from the graph. It is also always key to note cohesion is generally apparent unless chemical properties change or cementation occurs.

Only a couple more weeks of class and the quarter will be done! Only to have a week off before starting summer school! Wahoo! Anyways, fluid mechanics is calling my name and I have a midterm next Wednesday. Hopefully I will master Reynolds Transport Theorem. Details to come soon!

xoxo

Diane

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