Actually you could use the Cali Trench and shoring manual method where you apply an effective width adjustment factor.
Basically your soil below the excavation is a certain percentage based of your soil density above the excavation (bottom grade).
Example:
Similar to Brom's method they suggest increasing the effective width of the pile due to passive arching capability of the soil call this factor weff=0.08*phi , MAX=3.0. So if you have a 14" wide pile and phi=28 then your design width would be 0.08*28*14/12=2.61' for you passive resistance. Then say your pile spacing is 8'-0" oc, you would divide you design width by 8'. Call this f which is the factor you apply to your soil below the excavation.
f=weff/S = 2.61'/8'=0.326
or
f=min(0.08*phi,3)*width/spacing
Create top soil layer with normal soil density say g=120pcf
Create lower soil layer below excavation with soil density g=f*120pcf.
This is for simple cases.
or you could create a spreadsheat based off the California Trenching and shoring manual as I did. They give similar results for coulomb and FS=1.5.