Shear Strength

I think it would be prudent to understand the concept of shear strength before starting lateral earth pressures and consolidation.

Here are a few excerpts from my graduate course (I attribute a lot of my current abilities as a geotechnical engineer to my graduate advisor) on shear strength:

What is "failure" and "strength"?

We can answer the question is several steps:

1. If the load or stress in a foundation or an earth slope is increased until the deformations become unacceptably large, we say the soil has "failed."

2. In case 1, we are referring to the "strength" of the soil. This is the maximum or ultimate stress the soil can sustain.

3. In geotechnical engineering, we are generally concerned about shear strength, since most of our problems in foundations and earthwork engineering see failure resulting from excessive applied shear forces.

Most of the relationships that are used for characterization of strength and stress-deformation properties of soils are empirical. The Mohr-Coulomb theory is by far the most widely used:

shear resistance (on failure plane at failure)  = cohesion + normal stress (on failure plane at failure) x tan(phi)
phi = angle of internal friction

In reality the shearing resistance of the soil depends on many factors and a complete equation might be of the form:

shearing resistance = f(e,phi,c,effective normal stress, c',strain, strain rate,T,S,H)  where
e = void ratio
phi = angle of internal friction
c' = effective cohesion
c = cohesion
T = temperature
S = soil structure
H = stress history

There are a variety of types of friction angles, cohesion, etc:
-Total stress
-Effective stress
-Drained
-Undrained
-Peak Strength
-Residual Strength

I have inserted a typical stress vs strain plot (to the best of my drawing ability) for you to have an idea.

The values of cohesion and friction angle applicable in practice depend on such factors as
- whether the problem is one of loading or unloading
- whether short or long term stability is of interest (drainage).

A variety of test types are employed:
- Direct shear
- Triaxial compression
- Triaxial extension
- Simple shear
- Vane shear

I hope this helps form the basis of some of the further discussions and problem solving sessions we'll have from tomorrow.