Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Table C-9 – SS175 P3 Helix Strength

The allowable helix strength (P3) must equal or exceed the end-bearing capacity (P4) of the of the helix plates. It is possible for the bearing capacity of a helix plate to exceed the structural strength of the helix plate For example, an SS175 10” diameter helix plate has an allowable strength of 33.1 kip per Table C-9 above. If the maximum allowable torque based capacity of an SS175 helical pile (52.5 kip) is needed, then more than one 10” helix is required to meet structural strength requirements since 33.1 kip is less than 52.5 kip. A twin-helix or triple-helix configuration will work. This is an example where the designer may want to specify a minimum number of helix plates in the project plans. As helix plate diameter increases, the helix strength (P3) generally decreases. This is because the line of bearing (average radius) increases with increasing diameter, which in turn increases the moment arm distance. The increased distance increases the bending forces at the helix/shaft welded connection. Load tests [Method 3] are used to verify the feasibility and capacity of helical piles/anchors and are de- scribed in detail in Appendix B of the TDM. They can be part of a pre-production test program where at least one helical pile is installed and tested to determine the ultimate resistance and the load/deflection response. Project requirements may also require production tests on a specified number of helical piles/ anchors to ensure capacity and performance requirements are being met. It is VERY IMPORTANT that the performance requirements be clearly specified BEFORE the start of work. It should be part of the data gathering process and feasibility assessment for helical piles. Helical piles are primarily end-bearing foundation elements, meaning they derive most of their resistance with the helix plates transferring load to the soil at the pile tip. Therefore, the load/deflection response of a helical pile at a particular load (serviceability) must take into account the section modulus and length of the shaft. The designer must understand that long end-bearing piles will displace more than short end-bearing piles because of the pile length. The recommended acceptance criteria for the allowable capacity of helical piles/anchors is 50% of the applied test load causing a net displacement equal to 10% of the average helix diameter. This means that total displacement of the pile/anchor may exceed 1 inch in order to fully mobilize the bearing capacity of the helix plates. This is the acceptance criteria used in ICC-ES Acceptance Criteria AC358 for Helical Sys- tems and Devices, per Section 4.4.1.2. It can be expressed mathematically as PL/AE + 0.10Dave, where “PL/AE” is the elastic shortening or lengthening of the pile shaft under load. As mentioned previously, the net displacement of the helix plates at allowable loads will average 0.25 in (6.4mm) ± 0.12 in when us- ing a geotechnical factor of safety of two.

HELICAL PILES AND ANCHORS

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