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Calculate Slip Plane Angle And Depth of Crack For Soil:
Use the calculation below to perform trial and error calculations to find the slip plane angle (SPAng) and depth of crack (dc) for cohesive soil with an inclined slope.  For cohesive soil with a horizontal slope or non-cohesive soil, SPAng will be equal to the first run of the calculation below.
(Note: All input Metric units are converted to English units for the equations below and then the output English units are converted back to Metric)
The following calculation will calculate the slip plane angle for soil and the depth of crack for cohesive soil.
For the first trial enter 0.0 for SPAngEst below:
Angle of backfill slope, (AngB - degrees)
Estimated Slip Plane Angle, (SPAngEst - deg)
SPAngEst = 45deg + AIFd/2
Height of retaining wall (H - ft or m)
Density of soil (d - pcf or kg/m^3)
Developed angle of internal friction (AIFd - degrees)*
*AIFd = Arctan(SMF * tan AIF)  
Cohesion of the soil * SMF (cd - psf or kPa)
Soil surcharge (V - lb/ft or N/m)
Typically the Strength Mobilization Factor (SMF) is 2/3  

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Depth of crack, dc =
Calculated slip plane angle, SPAng =

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