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Geotechnical Problem Solving by John C. Lommler

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2.3

Soil Shear Strength

2.3.1 Introduction to Soil Shear Strength

“When I was in school, the professor said clays have cohesion and sands have friction … sounded simple enough to me.”

“Now I am confused, I’m told that clay has cohesion and friction and that sand has cohesion and friction, or that sand has cohesion and clay has friction.”

These are comments I heard from practicing geotechnical engineers at a recent conference after a young professor’s talk on soil shear strength and dilation. He really riled up the crowd when he concluded that there is no such thing as cohesion. What does clay and sand look like (see Figure 2.3.1)? What does cohesion look like? Please read on.

2.3.2 Soil Cohesion and Friction

The problem of characterizing soil shear strength is a perennial issue that confuses engineers at all levels of training from BS to PhD The initial problem with describing soil strength is a plain and simple mix up of terms. The words “cohesion” and “friction” describe physical properties. When a substance is cohesive or sticky like glue or chewing gum, you know it because the material sticks to your fingers or to the bottom of your shoe. You also know when a material has friction such as fine sandpaper or coarse sandpaper by the abrasion of your fingers while running your hand over its rough surface.

Figure 2.3.1 Topsoil and gray-red clay over bedrock and a jar of “running” sand

The terms “c” and “” used in soil shear strength equations are not these physical properties ...

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