# Abutment Earth Pressure \[SIG]

The Abutment Earth Pressure workflow item applies lateral earth pressure or a live-load surcharge against the back wall (and optionally the footing) of a selected abutment. The object builds a triangular (earth pressure) or uniform (live-load surcharge) surface load on the abutment FE group using an input active earth-pressure coefficient and backfill unit weight, and activates it at the specified construction stage.

## General

**Abutment:** Pick the abutment on which earth pressure is applied. The back-wall geometry, footing, and alignment are pulled from the selected abutment.

**Stage:** Construction Stage at which the generated surface load becomes active.

**Load Type \[Earth Pressure/Live Load Surcharge]:** Choose the load to generate:

* **Earth Pressure (0, default):** Produces a triangular pressure distribution that is zero at the top of the back wall, equals `K_a * unit weight * back-wall height` at the base of the back wall, and (if a footing exists) continues down the footing face to `K_a * unit weight * total height`.
* **Live Load Surcharge (1):** Produces a uniform horizontal pressure equal to `K_a * unit weight * heel width` applied over the back wall and footing. Use this to model the effect of a live-load surcharge on the backfill.

**Active Lateral Earth Pressure Coefficient (Ka):** Active earth-pressure coefficient used to convert vertical soil pressure into horizontal pressure (`K_a`). The default value is 0.523599. For a level backfill with no wall friction this can be estimated from `(1 - sin(phi)) / (1 + sin(phi))`; refer to AASHTO LRFD 3.11.5 for guidance.

**Backfill Soil Unit Weight:** Unit weight of the retained backfill soil used to compute the vertical effective stress (typical US units pcf, SI kN/m³). The generated load is only produced when this value is greater than zero.

**Heel Side Width:** Horizontal heel width used only for the Live Load Surcharge case to convert the specified Ka and unit weight into a uniform back-wall pressure. Only visible/editable when `Load Type` is set to **Live Load Surcharge**.


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