Retaining walls: Difference between sigma (soil pressure) and P (soil force)

[meaty_matt P.E.]

I know this is probably something very simple I am overlooking, but I am doing some problems and keep confusing sigma=KHgamma and P=0.5KH^2gamma

They both seem to produce the same units lb/sf or kn/m2

I am confused because alot of questions asked for a "force" or a "load" I use sigma=KHgamma

And then they ask for a "pressure" use P=0.5KH^2gamma

But then in the FE manual it says sigma is a load, and P is a force

confused.....

 

[civilized_naah]

With unit weight gamma, the vertical STRESS at a depth of H is (gamma H). This is a STRESS. It has units such as ksf

The lateral (horizontal) STRESS/PRESSURE is (K gamma H). This is a STRESS. It has units such as ksf

Since the pressure profile is triangular, the resultant horizontal force (per unit length of wall) = area of triangle with height H and base K gamma H is equal to (1/2 K gamma H^2). This is a FORCE (per unit length) and has units of klf