2.6
Static Equilibrium and Limit States
2.6.1 Introduction to Static Equilibrium and Limit States
Static equilibrium is a basic principle of physics that much of civil engineering is based upon. If an object is not moving relative to the earth, it is in a state of static equilibrium. To be in a state of static equilibrium an object must not have any unbalanced forces or moments acting on it. The principle of static equilibrium is the basic principle of the study of statics, which is often the first engineering course that most engineers study. I used to tell my statics students that non-engineers do not carry their arithmetic books around with them for reference because they need to know how to add and subtract wherever they go, and engineers don't carry their statics books with them because they need to know it wherever they go!
Analysis of retaining walls, spread footings, piles, and slope stability, to name a few geotechnical topics, all require the summation of forces in the x-, y- and z- directions equal to zero and the summation of moments about the x-, y- and z-axes also be equal to zero. The problem that we have to keep in mind is that we must not forget any of the applied forces or resistances acting on our geotechnical structure.
Figure 2.6.1 (a) Vertical and lateral stresses at point 1; (b) Mohr's circle of stresses for point 1; (c) Mohr's circle of stresses for point 1 after excavation lateral stress relief causes shear failure
2.6.2 What Are Limit States?
If ...
