Transmission And Substation Foundations - Technical Design Manual (TD06088E)

DESIGN METHODOLOGY

Individual Bearing and Skin Friction Models for Helical Piles with Round (Pipe) Shafts Figure 4-9

L = Length of Round Shaft in Contact with Soil

The design line given by the American Petroleum Institute (API) shown Figure 4-10 may also be used in which:

For s u < 500 psf; α = 1.0 For s u > 1500 psf; α = 0.5 For 500 psf < su < 1500 psf; α varies linearly between 1.0 and 0.5

The shaft resistance should only be calculated for that portion of the shaft length that is in full contact with the soil. This will depend on the length of the lead section, the design of the shaft couplings that connect the pile sections, and the type of soil. For example, flanged and bolted connections generally create an annulus between the shaft and the soil as the pile or anchor is installed as shown in Figure 4-9. This is because the coupling, being larger than the shaft, displaces and compacts soil. Generally, the length of the central shaft between couplings is not considered to develop shaft resistance unless the disturbed soil moves back against the shaft, or sufficient time is allowed for the soil to recover. In this situation, reduced shear strength should be used for shaft resistance capacity. On the other hand, in the case of true flush connections between extension sections, the entire shaft may

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