Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Helix strength : The structural strength of an individual helix is dependent on the plate thickness, grade of steel, diameter, and strength of the weld that connects it to the pile/anchor shaft. There must be enough helix plates so that the sum of their individual strengths can share the load that is required of the pile/ anchor. The product family sub-sections in Section 7 of the TDM provide the P3 helix strengths. A perfor- mance-based specification requires a minimum number of helix plates required to share the load. The size of each helix plate is left up to the installation contractor as long as the minimum number plates is provid- ed, and that other requirements are met, such as minimum depth and installation torque. For example, if 60 kip capacity is required, and the individual helix strength is 40 kip, then a minimum of two helix plates are required to share the 60 kip load. A prescriptive-based specification would be explicit on the exact number and size of the helix plates. Helix Size and Configuration : The size (diameter) of helix plates have a significant influence on the installa- tion and performance of a helical pile/anchor. The helical configuration (number and size of helix plates) can change from pile to pile. The designer can choose between a performance based design and a pre- scriptive based design. A performance based design means the helical pile contractor is responsible for some design and construction procedures. A prescriptive based design means the owner or designer has the sole responsibility for all aspects of helical pile design and installation. Hubbell Power Systems, Inc. recommends using a performance based design in most situations. An example of a performance based design for helical piles is minimum number of helix plates, minimum installation torque, and minimum depth. The contractor can then decide the actual number and size of helix plates, depths and torque required to achieve the required resistance; so long as the specified mini- mums are met. A prescriptive based design is the actual number and size of helix plates, actual installation torque, and actual depth. A prescriptive design may be required for comparative bid reasons and is fine as long as a payment mechanism for adjustment is provided. Typically, the denser the soil, the helix plates must be smaller. Alternately, the softer or less dense the bearing soil strata, the helix plates must be larger to generate the required torque/capacity. It is important that the smallest helix plate be the bottom-most helix. A multi-helix pile will then have sub- sequent helices increasing in size. Generally, the same size helix is not repeated until the largest size avail- able is reached. For example, a typical three-helix configuration would be an 8”/10”/12” or 10”/12”/14”. The larger the shaft size, the larger the smallest helix diameter. For example, the smallest helix plate on pipe shaft is typically 10 in or larger. Helical piles with multiple helix plates will drive straighter, and are more likely to advance properly than single helix configurations, and perform better. If too few helical plates are used, the most likely instal- lation problem is “spinning out”. This can be solved by adding more helix plates, larger helix plates, and/ or more crowd pressure (downward force from installing equipment). Increasing crowd pressure may require a larger piece of equipment (excavator, backhoe etc.). Generally, adding more helical plates is more economical compared to upsizing to larger equipment. If too many helical plates are used, the likely installation problem is that the torque capacity of the shaft is reached prior to reaching the required depth. Helical extensions can be removed by unscrewing the pile/anchor, taking them off and reinstalling the pile/ anchor. If helix plates on the lead section need to be removed, it will require the installation contractor to supply a different configuration lead section or remove helical plates in the field with a torch or saw. Removal of helix plates in the field is done quite often, but for cost/time reasons the installing contractor would prefer not having to remove helical plates regardless of the method.

HELICAL PILES AND ANCHORS

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