Explain Rankine's earth pressure theory and its assumptions.

Rankine's theory gives lateral earth pressure coefficients: Ka = (1-sin(phi))/(1+sin(phi)) for active, Kp = (1+sin(phi))/(1-sin(phi)) for passive. Assumptions include: homogeneous semi-infinite soil mass, smooth vertical wall, horizontal ground surface, and soil at plastic equilibrium. For cohesive soils: pa = Ka*gamma*z - 2c*sqrt(Ka). Theory predicts tension zone depth where negative pressure exists (ignored in design). Surcharge loads add Ka*q to active pressure. Sloping backfill and wall friction require Coulomb's theory.