Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Saturated Clays φ ’ = 0; c > 0 In saturated clays with φ ’ = 0, the term N γ = 0 and Nq = 1.0. The bearing capacity equation becomes: Q H = A H (cN C + γ ’D) Equation 4-9 where: Q H = Ultimate Bearing Capacity A H = Projected Helix Area c = “cohesion”; for φ ’ = 0; c = undrained shear strength = s u N C = Bearing Capacity Factor for φ ’ = 0; for round plates N C = 6.0(1 + 0.2D/B) ≤ 9 γ ’ = effective unit weight of soil above screw-pile D = Depth Note: The term γ ’D is sometimes ignored because it is very small. Sands φ ’ > 0; c’= 0 In clean sands with zero cohesion, the cohesion term of the bearing capacity equation drops out and only two terms remain: Q H = A H (q’N q + 0.5 γ ’BN γ ) Equation 4-10 where: q’ = effective surcharge (overburden pressure) = γ ’D N q and N γ are evaluated from the Table of Bearing Capacity Factors Note: The term 0.5 γ ’BN γ is typically ignored for helical piles because the helix plate is small Mixed Soils φ ’ > 0; c’ > 0 Many soils, such as mixed-grain silty sands, sandy silts, clayey sands, etc., have both a frictional and cohesive component of strength. In these cases, the bearing capacity equation includes all three terms: Q H = A H (c’N C + q’N q + 0.5 γ ’BN γ ) Equation 4-11 Note: The term 0.5 γ ’BN γ is typically ignored for helical piles because the helix plate is small. Tension Loading - Axial Uplift (Shallow Single Helix) Under tension loading in axial uplift, the behavior of a shallow single-helix helical anchor is currently approached more-or-less as an “inverse” bearing capacity problem and the concern is for the failure surface to reach the ground surface, producing “breakout” of the helical plate. Helical anchors should not be installed at vertical depths less than 5 ft. for tension loading. The design approach is similar to that under compression loading, except that instead of using a Bearing Capacity Factor, N C , a Breakout Factor, F C , is used.

DESIGN METHODOLOGY

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