If additional strength data is available from triaxial or direct shear testing, use this data with TCP data in the design of foundations. Determine the ultimate shear strength for each stratum using Coulomb’s formula (Shear Strength = τ = c’ + σ

y

’ (tan φ’). Determine allowable skin friction by applying a factor of safety of at least 2.0 to the ultimate shear strength. For drilled shaft design, reduce the allowable skin friction value by an additional reduction factor of 0.7 to account for soil disturbance. Determine allowable point bearing by multiplying the ultimate shear strength by a bearing capacity factor of 9 and then dividing by a factor of safety of at least 2.0.