Earth Pressure & Retaining Wall

What am I seeing?

• Ka is the fraction of the vertical stress that the soil pushes sideways once it has relaxed enough to reach the active state. At φ = 30 that fraction is exactly one third.
• The pressure diagram is not one shape but several stacked. Soil weight gives the triangle, surcharge gives a rectangle, cohesion cuts the top off and opens a tension crack, and water adds its own triangle on top of everything.
• Water is the wall killer. Its unit weight is only half the soil's, but it pushes with the full weight instead of Ka times it, so letting the table rise to the surface can more than double the thrust. That is what the drainage behind a real wall is for.
• Three checks, three different failures. Sliding is about friction under the base, overturning is about moments around the toe, and bearing is about whether the ground can carry the corner pressure once the resultant moves off centre.
• Keep the resultant inside the middle third and the whole base stays in compression. Once it steps outside, the heel lifts off and the toe pressure jumps.