The watr tabble is 9' below the surface wthin a sandy gravel layer 30' thick. the sandy gravel layer overlays a 15' thick clay layer.

Sandy gravel (Dray Unit weight=110pcf, n=0.40 Gs=2.65
Clay(saturated Unit weight=112pcf)
C'(cohesion)=3.5psi phi(angle internal Friction)=27
The shear strength at the middle of caly layer=? psi

Here is the only way I see to work the problem unless n and Gs are for the clay and not the sand, you need the dry unit weight to do the calc properly

effective stress
9(110) + 21(110-62.4) + 7.5(112-62.4) = 2316.6

shear strength
3.5(144)+ {30(110) + 7.5(112)}tan(27) = 1100 lb/sqft

Gamma sat for the sand layer = (Gs*Gamma water + e*Gamma water) / (1+e)
Porosity = e/(1+e)
e = 0.67
Gamma sat = 124 pcf

Effective stress at the middle of clay layer = 110 pcf * 9 ft + (124-62.4) pcf * 21 ft + (112 - 62.4) pcf * 7.5 ft = 2655.6 psf = 18.44 psi

Shear Stress = C + Effective Stress * Tan phi
Shear Stress = 3.5 psi + 18.44 psi * Tan 27
Shear Stress = 12.9 psi

Thanks, This procedure make sense to me!