Transmission And Substation Foundations - Technical Design Manual

A Basic Guideline For Designers APPENDIX C: HELICAL PILES & ANCHORS

A. Data Gathering: The loads applied to the foundation. Section 4 of the TDM is a brief review of structural loads and provides several tables that can be used to estimate dead and live loads for various residential & commercial structures. If applicable, lateral loads must be included. • The description and strength characteristics of the project soils. See Section 2 of the TDM for a brief review of soil mechanics and the procedures used for site investigations, which are typically summarized in the geotechnical report. Information needed in the geotechnical report includes: soil profile, Nspt values per ASTM D1586, depth to ground water, the presence of fill, debris, or cobbles, and bedrock. • The designer must determine load resistance requirements and serviceability based on the application. This includes choosing either ASD with a deterministic factor of safety, or LRFD with probabilistic load and resistance factors. Section 5 for the TDM provides guidelines to evaluate soil properties for foundation design, and also gives estimates of helical pile displacement at working loads. Section 5 also provides the design methodology used with HeliCAP®, which is the design software most often used to determine the axial capacity of helical piles. • The applicability of local, regional, or national building codes. The designer must comply with code requirements depending on the jurisdiction. For example, some codes require helical piles to be tested for every project. Others only require load tests if the pile capacity is above a certain limit. Codes often dictate acceptance criteria in terms of allowable displacement for deep foundations, such as the City of Chicago and New York building codes. • Location tolerances. The helical pile designer must understand the location tolerances for the piles. For example, most Chance helical piles can be installed to a location tolerance of 1 inch or less, and an elevation tolerance of 1/8 inch. Angular tolerances are typically less than 2°. B. Feasibility: • Helical piles are designed to transfer load to soil or bedrock with a reasonable displacement. However, they are not designed to drill into solid rock. Table 7-4 is a quick reference guide for feasibility. It lists helical pile type based on the upper limit Nspt range of soils that pile type can be installed into, along with the typical upper limit of ultimate resistance. It’s a good place to start for helical pile feasibility. For example, Type RS2875.276 2-7/8” OD pipe shaft helical piles can be installed into soils with Nspt blow counts up to 35 bpf. • The size (diameter) of the helical pile shaft should be closely tied to its application. Chance offers small displacement (up to 4 in.), medium displacement (4 in., to 8 in.), and large displacement (> 8 in) helical piles. The pile shaft should be large enough to transfer the axial and lateral loads to the soil. However, it is detrimental to oversize the helical pile shaft. This is because of torque correlation – the relationship between the amount of torque energy required to install a helical pile and its load capacity. Smaller diameter helical piles more easily advance like a screw, which minimizes soil disturbance and increases capacity efficiency. More information about shaft type and size will be presented later. • 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 immediately 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® 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. Helical 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 various 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 applied 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 structural elements can be calculated using the Rankine Method. Battered or inclined piles can

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