Shear Strength of Soil

We introduced shear stress before in this book. If you are not an engineer, and forgot what shear stress means, first please go back to section Structural Basics > Forces and look at again, what it means “shear”.

Now,

Imagine the following different situations:

  • Think of water running down a sloped street. It can flow, because it has no shear resistance. Now assume, you put something solid on the same sloped street, such as a wooden cube or volume of soil. It would stay where it is, and would not flow like water. Because it can resist shear stress.
  • Try to walk on water. You can not. Because water can not resist shear stress. But you can walk on soil. Because soil can resist shear stress. (Note: Ships can float on water not because water has shear strength, but because of water pressure, as we have seen in previous section, when an object is submerged into it).
  • Imagine you are to build a retaining wall and in one case you have a very soft weak soil, and in other case you have strong, firm soil. Retaining the firm soil is easier, you can get away with a simpler support system, as the soil itself has more shear strength. For the weaker soil you have to build a stronger support system to retain it.
  • Imagine you build pile foundation, to support a large structure. In first, case the piles are embedded in weak soil. In second case they are embedded in strong soil. Which soil can support piles better, both from its tip and its sides? Of course the firm soil which has greater shear strength.

In all these cases, shear stress strength of the soil is the key. In other words, in geotechnical engineering, as far as strength calculations, the name of the game is “shear”. Strength of a soil is a result of its shear strength (and in case of clays, cohesion too, which we will see shortly).

In the following sections we will see why and how.

In the next post of this series, we discuss “Lateral Earth Pressure”

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