Transmission And Substation Foundations - Technical Design Manual (TD06088E)

IX. HOW TO SPECIFY HELICAL PILES: A. Minimum Capacity or Installation Torque : Whether using a performance or prescriptive specification, the helical pile/anchor capacity (ultimate resistance) should be specified in order to ensure that the required pile/anchor resistance is achieved. This can be done by specifying the minimum capacity directly or indi- rectly by specifying the required installation torque. The designer can choose either way. A.1: Minimum Capacity: Regardless of the design method used, the ultimate resistance is the same. Ultimate resistance is the limit state based on the structural strength or the geotechnical capacity of the helical pile, defined as the point at which no additional load can be applied without failure. A factor of safety (or a resistance factor) is applied to the ultimate resistance to provide a reserve ca- pacity greater than expected loads. This “normal use” load is commonly referred to as service, design, working, SLS or un-factored load. The safety or resistance factor may be prescribed by building code, but is often left up to the designer. A proper factor of safety/resistance is a combination of economics and statistics. It is not typically economically feasible to design for zero probability of failure. Generally the more uncertainty, the higher the factor of safety/resistance applied. Conversely, the less uncertain- ty, the lower the factor of safety/resistance applied. For ASD design, the industry standard for helical piles is a factor of safety of 2 for permanent applications. For LRFD design, the resistance factor (Ø) recommended for helical piles used in compression range from 0.65 to 0.75. The resistance factor (Ø) recommended for helical piles used in tension range from 0.55 to 0.65. For tieback anchors that are going to be individually post-tensioned and tested, a factor of safety of 1.5 is used. A lower factor of safety is justified since there is less uncertainty (the tieback is tested). One problem with construction documents regarding helical piles/anchors is clearly identifying the capacity required. The best method is to clearly define the ultimate resistance required. If the de- signer chooses to specify the un-factored load, then the loads should be clearly identified as (service/ design/working/SLS/un-factored loads) and clearly state what the required factor of safety/resistance is. A.2: Installation Torque: Installation torque can also be specified as the minimum requirement as it relates to the pile/anchor capacity required. This should only be done for piles/anchors that will not receive a proof test. Installation torque should not be used to specify minimum capacity for helical tieback anchors when each anchor will be post tensioned and proof tested. In that case, passing the proof test is the only criteria that matters and obtaining a minimum torque is really a convenience for the contractor to ensure the anchorage does not fail the proof test. If the installation torque approach is utilized, the designer should be aware that torque capacity corre- lations only apply to helical piles with advancement rate that equals or exceeds 85% of the helix pitch per revolution at the time of final torque measurement. Refer to Section 6 of the TDM for a full discus- sion of torque correlation (Kt) relationships. On-site testing can be used to obtain a site specific Kt, otherwise use the default values listed in Table C-1 above. Also, tension and multi-helix compression capacity should be determined based on the average torque measured over the last three helix diameters of installed length. Most specifications simplify this to 3 feet. The reason this is done is to better predict the bearing capacity of the helix plates as they distrib- ute load to the soil in a passive pressure bulb either below (compression) or above (tension) the helix plate(s). Depending on how fast the torque increases over the last 3 feet of penetration will have a significant impact on the capacity of the helical pile/anchor. Note that it is virtually impossible to av- erage a helical anchor/pile’s maximum torque rating over the last three average helix diameters, which means a shaft with higher torque strength may be needed in very dense soils.

HELICAL PILES AND ANCHORS

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