Coulomb Earth Pressure Theory in Retaining Wall Design

Coulomb Earth Pressure theory is a classical approach for calculating lateral earth pressures exerted on retaining walls. It extends Rankine's method by incorporating soil friction, wall friction, wall inclination, and backfill slope angle, thus providing a more realistic estimation of earth pressure under various conditions. The theory assumes a planar failure surface behind the wall where the soil wedge slides, and calculates the active earth pressure coefficient $K_a$ by minimizing the resultant lateral force acting on the wall. Key parameters include the soil friction angle $\phi$, wall friction angle $\delta$ (ranging typically from 0 to $\displaystyle \frac{2}{3}\phi$), backfill slope angle , and wall inclination angle. The active earth pressure force is then derived from , where is soil unit weight and is the wall height. This approach allows for design considerations of sloped backfills and inclined walls, common in practical civil engineering projects. However, Coulomb's classical theory assumes planar failure and ignores soil cohesion, limiting direct application to cohesive soils without modification. Understanding and applying Coulomb earth pressure principles is critical for ensuring the stability and safety of earth retaining structures while optimizing material use and construction costs.