Transmission And Substation Foundations - Technical Design Manual (TD06088E)

• Project site factors such as equipment access, overhead clearance, right-of-way restrictions, spoils disposal, noise restrictions, etc. must be considered. This is often where helical piles turn out to be the most cost effective deep foundation. Small equipment results in low mobilization cost and easy access. • Manageable schedule must be considered as well. Helical piles and anchors can be loaded immedi- ately after installation, which can save time compared to waiting for concrete or grout to cure. C. It is convenient to break down the geotechnical capacity and the structural strength into subcategories or groups. For helical piles and anchors the groups are: • P1 – bracket or connection to structure • P2 – shaft, including couplings • P3 – Helix(s) • P4 – Soil (geotechnical) capacity, including resistance to both axial and lateral loads We recommend the design sequence be inverted – start with P4 – soil (geotechnical) capacity because it usually will control the ultimate resistance. IV.P4 – GEOTECHNICAL CAPACITY: The axial and lateral capacity is determined per the methods detailed in Section 2 and Section 5 of the TDM. Installation torque requirements can be estimated at this point. If a geotechnical report is available, use HeliCAP® v2.0 Helical Capacity Design Software to determine the axial capacity (tension, compression, or both) via bearing capacity on the helix plates and side resistance on the shaft [Method 1] . HeliCAP® will help determine the shaft type (square shaft, pipe shaft, Combo Pile, or grouted PULLDOWN Pile), shaft size (diameter), pile depth, helix configuration (number and size of helix plates), and estimate the torque required to install the pile. If a geotechnical report is not available, then axial capacity must be determined by other methods. Heli- cal piles have the advantage of being installed (screwed) into the ground and then removed (unscrewed) quickly. A “probe” helical pile can be installed to assess the relative shear strength of the soil profile using torque correlation relationships per TDM Section 6. Well documented correlations with torque are used to estimate helical pile capacity based on the torque measured with the probe pile [Method 2] . The shaft type, shaft size (diameter), pile depth, helix configuration can be determined based on the probe pile. The axial capacity can also be determined from full-scale load tests per Appendix B of the TDM [Method 3] . Full-scale tests are often used to verify Method 1 capacity and Method 2 torque correlation. If a geotechnical report is available, the lateral capacity of a vertical shaft can be determined with vari- ous methods including the Finite Difference method (LPILE & GROUP by Ensoft®) and the Broms’ Method (1964a) and (1964b) as detailed in Section 5 of the TDM [Method 1] . Each of these methods may be ap- plied to Round Shaft helical piles or PULLDOWN® Micropiles. Lateral resistance can also be provided by passive earth pressure against the structural elements of the foundation. The resisting elements of the structure include the pile cap, grade beams and stem walls. The passive earth pressure against the struc- tural elements can be calculated using the Rankine Method. Battered or inclined piles can be used to resist lateral loads by components of the axial capacity on the battered pile. The induced shear and moment in battered piles often dictates the shaft size and batter angle. If a geotechnical report is not available, the lateral capacity of a vertical shaft must be determined from load tests per Appendix B of the TDM [Method 3] . P4 SHAFT Type and Size: The shaft type/size is critical to both the axial and lateral capacity – especially for compression in soft/ loose overburden soils where lateral stability of the shaft must be considered. The following is a brief sum- mary of the 4 different shaft types for helical piles.

HELICAL PILES AND ANCHORS

Page C-6 | Hubbell Power Systems, Inc. | All Rights Reserved | Copyright © 2019