Transmission And Substation Foundations - Technical Design Manual (TD06088E)

SINGLE-HELIX SCREW-PILES AND SCREW-ANCHORS – DEEP INSTALLATION Deep installations of screw-piles and helical anchors are generally more common than shallow installations, provided there is sufficient soil depth to actually perform the installation. The reason is simply that higher load capacities are generally developed from a deeper installation in the same soil so it makes more sense economically to go for a deep installation when possible. Figure 5.5 below demonstrates the single-helix plate capacity model, where the soil failure mechanism will follow the theory of general bearing plate capacity. Compression capacity is mobilized from soil below the helix plate and tension capacity from soil above the helix plate. Compression Loading (Deep Single-Helix) A deep installation, like a deep foundation, is one in which the ratio of depth (D) of the helix to diameter (B) of the helix is greater than 5 - 7, i.e., D/B > 5 - 7. In this case, the design is very analogous to compression loading of deep end bearing foundation.

DESIGN METHODOLOGY

Figure 4-5 Single-Helix Plate Bearing Capacity Model – Helical Piles with Slender Shafts

Saturated Clays φ ’ = 0; c’ > 0 Under compression loading, the ultimate capacity of a single-helix screw-pile in clay is calculated from Equation 5-9 as: Q H = A H [(N C )(s u ) + γ ’D]] where: N C = Bearing Capacity Factor for Deep Failure = 9 Which gives: Q H = A H [(9)(s u ) + γ ’D] Equation 4-17

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