Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Sands φ ’ > 0; c’ = 0 HeliCAP ® determines the ultimate bearing capacity in a non-cohesive sand or gravel soil with Equation 4-32 in which the fine-grain (clay) term has been eliminated. The bearing capacity factor N q is dependent on the angle of internal friction ( φ ’) of the non-cohesive sand or gravel soil. When a value is provided for the friction angle, HeliCAP uses Figure 4-7 (Nq vs φ ’ ) and Equation 4-19 to determine the value for N q . When the angle of internal friction is not known, HeliCAP estimates it (and Nq) by using blow counts obtained from the Standard Penetration Test per ASTM D 1586. Equation 4-33 allows an estimate of the angle of internal friction from SPT blow count data. This equation is based on empirical data given by Bowles (1968) and its results should be used with caution. The graph in Figure 4-7 allows the determination of N q for a specific angle of internal friction when measured in degrees. This curve was adapted from work by Meyerhof (1976). Equation 4-19 was written for the curve shown in Figure 4-7, which is Myerhof’s N q values divided by 2 for long term applications. Note the correlated φ ’ and Nq values determined by HeliCAP® can be overridden. This is encouraged when more reliable soil data are available. Q h = A h q’N q = Ah γ ‘DN q Equation 4-32 where: A h = Projected helix area D = Vertical depth to helix plate N q = Bearing capacity factor for non-cohesive component of soil γ ‘ = Effective unit weight of the soil

DESIGN METHODOLOGY

Equation 4-33

φ ’ = 0.28 N + 27.4

where: φ ’ = Angle of internal friction N = Blow count per ASTM D 1586 Standard Penetration Test

Fine-Grain Cohesive Soil, φ ’ = 0; c’ > 0 HeliCAP determines the ultimate bearing capacity in a cohesive or fine-grained soil with Equation 4-17 with the overburden term not used. The Nc factor is 9, provided the installation depth below grade is greater than five times the diameter of the top most helix.

Equation 4-34

Qh = A h cN c = A H [(9)(s u )]

where: A h = Projected helix area c = “cohesion”; for φ ’ = 0; c = undrained shear strength = s u N c = Bearing Capacity Factor for Deep Failure = 9 (minimum depth ≥ 5D)

Page 4-35 | Hubbell Power Systems, Inc. | All Rights Reserved | Copyright © 2017