Transmission And Substation Foundations - Technical Design Manual (TD06088E)

bearing helical anchors/piles, the net displacement of the helix plates at the working loads averaged about 0.25 in (6.4mm). The working load is based on the geotechnical capacity divided by a factor of safety of 2 (deterministic design). CHANCE application engineers have either conducted or reviewed the results of several hundred load tests, which support the findings of Cherry and Perko. Serviceabil- ity limits should also take into account the elastic response of the helical pile material, which can be significant for deep piles with slender shafts. III.DESIGN PROCESS The designer has a specific task to perform, or problem to solve to which helical piles can offer a solution. At the beginning of the design process, it is best to keep all options on the table until circumstances dic- tate one foundation option(s) as being the better choice for the client. The designer should always keep in mind the client is best served with a good solution at a reasonable price, both of which are not always intuitively obvious. As with any deep foundation, helical pile design has several steps. The steps can be summarized as: • 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 D-1586, 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 applica- tion. This includes choosing either ASD with a deterministic factor of safety, or LRFD with probabi- listic load and resistance factors. Section 5 for the TDM provides guidelines to evaluate soil prop- erties 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 founda- tions, 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°. • Helical piles are designed to transfer load to soil or bedrock with a reasonable displacement. How- ever, they are not designed to drill into solid rock. Table 7-4 on page 7-12 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 of- fers 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 mini- mizes soil disturbance and increases capacity efficiency. More information about shaft type and size will be presented later. B. Feasibility: A. Data Gathering:

HELICAL PILES AND ANCHORS

Page C-5 | Hubbell Power Systems, Inc. | All Rights Reserved | Copyright © 2019