1. At-Rest ($K_o$): Wall does not move. $K_o \approx 1 - \sin \phi'$ (Jaky's formula for normally consolidated soil)
2. Active ($K_a$): Wall moves away from the soil (soil expands). Pressure decreases.
3. Passive ($K_p$): Wall moves into the soil (soil compresses). Pressure increases.

For horizontal backfill: $K_a = \tan^2(45 - \phi/2) = \frac{1 - \sin \phi}{1 + \sin \phi}$ $K_p = \tan^2(45 + \phi/2) = \frac{1 + \sin \phi}{1 - \sin \phi}$

Active Force ($P_a$): $P_a = \frac{1}{2} \gamma H^2 K_a - 2c H \sqrt{K_a}$ (Note: Tension cracks may form where stress is negative).