Transmission And Substation Foundations - Technical Design Manual (TD06088E)

4.2 CHANCE® HELICAL PILE/ANCHOR ULTIMATE BEARING CAPACITY The capacity of a helical pile/anchor is dependent on the strength of the soil, the projected area of the helix plate(s), and the depth of the helix plate(s) below grade. The soil strength can be evaluated by use of various field and lab techniques. The projected area is controlled by the size and number of helix plates. Helical anchors and screw piles may be used for a variety of applications involving both tension loading (helical anchors) and compression loading (screw piles or helical piles). Screw piles and helical anchors are generally classified as either “shallow” or “deep” depending on the depth of installation of the top helix below the ground surface, usually with respect to the helix diameter. There are some situations in which the installation may be considered partway between “shallow” and “deep”, or “intermediate”. In this Manual, only design procedures for “shallow” and “deep” installations will be described. Table 1 gives a summary of the most common design situations involving screw-piles and helical anchors that might be encountered. Note that the use of “shallow” multi-helix anchors for either compression or tension loading is not a typical application and is not covered in this Technical Design Manual. The dividing line between shallow and deep foundations has been reported by various researchers to be between three and eight times the foundation diameter. To avoid problems with shallow installations, the minimum recommended embedment depth of helical piles and anchors is five helix diameters (5D). The 5D depth is the vertical distance from the surface to the top-most helix. Whenever a CHANCE ® Helical Pile/Anchor is considered for a project, it should be applied as a deep foundation for the following reasons: 1. A deep bearing plate provides an increased ultimate capacity both in uplift and compression. 2. The failure at ultimate capacity will be progressive with no sudden decrease in load resistance after the ultimate capacity has been achieved. The approach taken herein for single-helix piles/anchors assumes that the soil failure mechanism will follow the theory of general bearing capacity failure. For multi-helix helical piles and anchors, two possible modes of failure are considered in design, depending on the relative spacing of the helix plates. For wide helix spacing (s/B ≥ 3), the Individual Plate Bearing Method is used; for close helix spacing (s/B < 3), the Perimeter Shear Method is used. Technical Design Assistance The engineers at Hubbell Power Systems, Inc. have the knowledge and understand all of the elements of design and installation of CHANCE ® Helical Piles/Anchors, Tiebacks, Soil Screw ® Anchors. Hubbell Power Systems, Inc. will prepare a complimentary product selection (“Preliminary Design”) on a particular project Dynamic Loads Dynamic or cyclic loads are encountered when supporting certain types of equipment or conditions involving repetitive impact loads. They are also encountered during seismic events and variable wind events. These loads can prove destructive in some soil conditions and inconsequential in others. The designer must take steps to account for these possibilities. Research has shown that multi-helix anchors and piles are better suited to resist dynamic or cyclic loads. Higher factors of safety should be considered when designing for dynamic loads. Codes And Standards The minimum load conditions, especially live loads for buildings are usually specified in the governing building codes. There are municipal, state and regional as well as model codes that are proposed for general usage. The designer must adhere to the codes for the project location. Chapter 18 of the IBC 2009 and 2012 contain Code sections for helical piles, as well as sections for general design of deep foundations. Section 4 of ICC-ES ESR-2794 provides guidelines for the design and installation of helical piles.

DESIGN METHODOLOGY

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