When using TCP information, consider both skin friction and point bearing for drilled shaft capacity. Calculate total allowable skin friction by multiplying the perimeter of the shaft by the unit value for allowable skin friction derived from Figure A1-2, Figure A1-4, or laboratory data. For drilled shafts, apply a reduction factor of 0.7 to allowable skin friction values derived from Figure A1-2 or from laboratory testing to account for disturbance of the soil during drilling. Do not apply the reduction factor to allowable skin friction values obtained from Figure A1-4. Accumulate skin friction along the length of the shaft beginning at the previously defined disregard depth and continuing down to the tip of the shaft. Calculate total allowable point bearing by multiplying the area of the drilled shaft times the unit value for allowable point bearing derived from Figure A1-3, Figure A1-5, or laboratory data. If softer layers exist within two shaft diameters of the proposed tip, use allowable point bearing values for the softer layers. If drilled shafts are to be tipped in very hard material that is overlain by soft strata, the skin friction contribution of the softer strata may be disregarded in design. However, do not ignore the contribution of significant amounts of competent material to tip in rock. In many areas of the state, rock is overlain by thick layers of material that can support considerable loads.

When using TCP information for driven piles, designs generally rely solely on skin friction capacity and partial to no capacity is generated by end bearing.

Application of the (0.7) reduction factor to the design of driven piling is not necessary

as these are displacement piles and do not remove subsurface soils as with drilled shafts.

Calculate total allowable skin friction by multiplying the perimeter of the pile by the unit value for allowable skin friction derived from Figure A1-2, Figure A1-4, or laboratory data or a combination thereof. The maximum recommended value for allowable skin friction for piling is 1.4 tons per square foot (TSF). Accumulate skin friction along the length of the pile beginning at the previously defined disregard depth and continuing down to the tip of the pile. If using point bearing, calculate total allowable point bearing by multiplying the area of the pile times the unit value for allowable point bearing derived from Figure A1-3, Figure A1-5, or laboratory data. If softer layers exist within two diameters of the proposed tip, use allowable point bearing values based on the softer layers.

Displacement piling typically refuses to advance once material with TCP values harder than 100 blows/12 in are encountered.

Use Figure A1-2 to determine allowable skin friction capacity for soil softer than 100 blows/12 in. Select the curve based on the description of the soil type.

Use Figure A1-3 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 A1-4 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.

Use Figure A1-5 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.

If additional strength data is available from triaxial or direct shear testing, use this data with TCP results. 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 shafts, 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.