Transmission And Substation Foundations - Technical Design Manual (TD06088E)

ROUND SHAFT SCREW-PILES AND HELICAL ANCHORS Screw-piles and helical anchors are available with both square shaft and round steel pipe shafts. Square shaft is used for tension applications and also for compression applications when shaft buckling or bracing is not an issue. Pipe shaft helical piles have become increasingly popular for use in compression loading for both new construction and remediation or underpinning of existing structures. They may be either single or multi-helix. Typical round shaft pile diameters range from 2-7/8 inches (73 mm) to 12 inches (305 mm). For the most part, the design is essentially the same as with square shaft screw- piles as previously described with two simple modifications: 1) some provision is usually made to include the additional load capacity developed via skin friction by the round shaft; and 2) in tension loading, the area of the helical plate is reduced to account for the central shaft as shown in Figure 4-9b. In compression loading, the full projected area of the helix plate develops capacity since the pipe generally plugs with soil. Typically, the length of the shaft for about one helix diameter above the helix is not included in calculating shaft resistance due to skin friction. In addition, load capacity due to friction along the pile shaft is generally mobilized only if the shaft diameter is at least 3 inches (89 mm). Shaft Resistance in Clay φ ’ = 0; c’ > 0 In clays, the shaft resistance developed by round shaft screw-piles and helical anchors is considered in much the same way that shaft resistance in a driven pile develops. In this traditional approach that is used for many driven piles in clays and available in most textbooks, the available “adhesion” between the shaft and the clay is obtained as a percentage of the undrained shear strength of the clay. This is the undrained or “Alpha” method in which: α = f S /s u Equation 4-21 where: α = Adhesion Factor f S = Unit Side Resistance s u = Undrained Shear Strength of the Clay

DESIGN METHODOLOGY

Figure 4-6 Variation in Adhesion Factor with Undrained Shear Strength of Clays [from Canadian Foundation Manual (2006)].

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