How to use the earth pressure calculator
Designing a retaining wall or shoring system starts with the force the backfill applies. This calculator uses Rankine theory to get the active coefficient Ka = tan²(45−φ/2) and the passive coefficient Kp = tan²(45+φ/2), then integrates the pressure diagram over the wall height to give the resultant per foot of wall and its point of application.
In cohesive soil the computed pressure is negative near the surface. Because soil cannot take tension, that zone is treated as a crack with zero pressure. Surcharge is applied to the retained (active) side only; surcharge on the passive side is conservatively ignored because it can be excavated away during construction.
Passive resistance is much larger than the active push, but it only mobilizes after significant wall movement, so designers divide it by a safety factor or count only part of it. Water pressure is not included here. An undrained wall carries full hydrostatic pressure on top of the earth pressure, which can more than double the total load, so drainage details are essential.
Results assume a vertical wall face, level backfill and no wall friction. Have a licensed structural or geotechnical engineer confirm the final wall design against site-specific soil data.
Frequently asked questions
Active pressure is what the retained soil pushes with when the wall yields slightly away from it. Passive pressure is the resistance the soil offers when the wall pushes into it. Passive values are far larger, but they only develop after substantial wall movement.
Rankine is simple and conservative when the wall face is vertical, the backfill surface is level and wall friction can be ignored. Use Coulomb theory or numerical analysis when wall friction or a sloping backfill has to be included.
In cohesive soil the theoretical active pressure is negative near the surface. Soil cannot carry tension, so a crack forms and that zone is taken as zero pressure when the resultant is computed. If the crack fills with water, hydrostatic pressure is added, which is why drainage matters.