In Retaining Wall design, we commonly rely on specific equations to calculate the Active and Passive Pressure factors. These formulas, listed below in the order of Active Pressure followed by Passive Pressure, are fundamental to the design process:

These factors play a crucial role in Retaining Wall design. The angle in the formula represents the Angle of internal friction of the soil behind and in front of the wall. While these formulas are straightforward, discrepancies often arise when attempting a quick hand-calculation check in Retaining Wall design software. You may find that the values you obtain and the K_a & K_p values calculated by the program do not match up. This discrepancy stems from the program’s utilization of Coulomb’s theory for design.

Coulomb’s theory employs different formulas to calculate the K_a & K_p factors:

As illustrated, three additional variables are utilized beyond the soil’s Angle of internal friction. These variables include:

All of these factors are visible within the General Parameters section of the Inputs Table, except for α, which you will need to calculate unless it equals 90°.

In conclusion, the calculation of K_a & K_p depends on the applied theory. By utilizing the provided formulas, you should be able to reconcile your hand-calculated factors with those generated by the program.

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