Use the following charts to determine skin friction and point-bearing capacity based on Texas Cone Penetration data for drilled shaft and piling designs. Use Figure 5-1 to determine allowable skin friction for soil softer than 100 blows/12 in. Select the curve based on the description of the soil type.

Use the CH curve in clay soil identified as high-plasticity, or fat clay. Use the CL curve in clay soil identified as low-plasticity, or lean clay. In clay soil, use the CL curve if no specific identification is provided regarding plasticity. Use the SC curve for soil described as either sandy clay or clayey sand. Use the OTHER curve for soils described as silt, sand, gravel or any layers not fitting into one of the previous designations.

For drilled shaft designs, multiply the allowable design stress by a reduction factor of 0.7. The reduction factor is used to account for disturbance of the soil during drilling. Application of the reduction factor to the design of driven piling is not necessary.

Use Figure 5-2 to determine allowable point bearing for soil softer than 100 blows/12 in. Select the curve based on the description of the soil type, using the criteria noted for the previous chart.

Use Figure 5-3 to determine allowable skin friction for soil or rock strata harder than 100 blows/12 in. The upper limit of 3.25 tons/ft2 applies for all Texas Cone Penetration values less than 2 in/100 blows. Do not apply skin friction reduction factor to values obtained from this figure because this figure is derived only for use in drilled shaft design. Piling typically cannot be driven into soil of this strength, so this figure is not generally used for piling design.

Use Figure 5-4 to determine allowable point bearing for soil or rock strata harder than 100 blows/12 in. The upper limit of 31 tons/ft2 applies for all Texas Cone Penetration values less than 2 in/100 blows.