Transmission And Substation Foundations - Technical Design Manual

SECTION 3: PRODUCT FEASIBILITY

Feasibility of Using Chance® Helical Products

Power Systems, Inc. recommends an appropriate Factor of Safety (FS) when using Chance® Helical foundation products. Generally, the minimum FS is 2 on all permanent loading conditions and 1.5 for any temporary load situation. Some applications may require more stringent Factors of Safety on certain projects. Site Access The proximity to other structures, rights-of-way and obstructions are some of the first considerations for any construction or improvement. Equipment access may be restricted due to overhead limits and safety issues. The designer needs to consider all the possible limitations when selecting a foundation system. Chance® helical piles/anchors can generally be used anywhere a soil boring can be taken and are virtually the most access-problem-free foundation systems available today. Restricted access and similar concerns should be shown on the bid documents with the usual notes concerning site conditions. Vibration and noise can be another limitation to conventional deep foundations (i.e., driven piles, drilled piers). Chance® helical piles/anchors have been installed inside office buildings, restaurants, retail shops and hospitals without interrupting their normal routines. Chance® helical pile certified installers can assist the designer in determining the best type of product for the application. Working Loads Helical piles have been used in compression to working (design) loads of 200 kip, in the form of the Chance Helical Pulldown® Micropile which is detailed later in this manual. In a “normal consolidated” soil, the working load per foundation is typically less than 100 kip, but special cases may apply. Working tension loads are typically 100 kip or less. The soil is generally the limiting factor as the number and size of helical piles/anchors can be varied to suit the application. The designer should determine the shaft series of products to use from the information provided in Section 6 – Product Drawings and Ratings. Soils Soil may be defined for engineering purposes as the unconsolidated material in the upper mantle of the earth. Soil is variable by the nature of its weathering and/or deposition. The more accurately one can define the soil at a particular site; the better one can predict the behavior of any deep foundation, such as a Chance® helical pile, or Helical Pulldown® Micropile. In the absence of sufficient soil data, assumptions can be made by the designer. The field engineer or responsible person needs to be prepared to make changes in the field based on the soil conditions encountered during construction. Chance® helical piles can be installed into residual soil and virgin or undisturbed soils other than rock, herein defined as having a SPT “N-value” less than 80 to 100 blows per foot per ASTM D1586. This implies that the correct shaft series of helical piles must be chosen to match to the soil density. For example, a standard 1-1/2” shaft, Type SS helical pile with a total helix area of 1 square foot may require so much installing

Hubbell Power Systems, Inc. manufactures steel foundation products that can be designed for a wide range of soil conditions. In order to assist the designer/user in selecting the proper product for the application, Figure 3-1 shows the product type suitable for various soils and rock conditions. When reviewing Figure 3-1, the designer/user should note the following items: • The most common selection of soil parameters for design is from field testing using the ASTM D1586 Standard Penetration Test (SPT) and field or laboratory testing of shear strength (cohesion “c” and friction angle “φ”). Refer to Section 2 in this manual for a detailed discussion of geotechnical investigation requirements. • A range is indicated for use of the helical piles (compression) and helical anchors (tension). As noted on the chart, there are certain conditions for weathered rock and cemented sands where an initial predrilling will permit the installation of helical plates under relatively high installing torque (generally above 10,000 ft-lbs). Helical piles/anchors have been successfully installed on projects where the target depth is not homogenous or consists of hard clays, cemented sands or weathered rock. These factors must be considered and evaluated before a design can be finalized. Modifications may have to be made to the design to be able to accomplish embedment into the target stratum such as: • Cutting a “sea shell” shape into the leading edge of one or more of the helical plates. • Predrilling prior to the installation of a helical pile/anchor. • Using a shaft configuration that provides adequate torques and resistance to “spikes” during installation. The product selection chart shown in Figure 3-1 is intended for use on a preliminary basis. Hubbell assumes no responsibility for the accuracy of design when based solely on Figure 3-1. A Preliminary Design Request Form is provided at the end of this section. This form can be copied and then completed with the required information to request a preliminary design (application) by the Hubbell engineering department. The completed form can be sent to Hubbell or directly to your local Chance® Distributor. NOTE: All foundation systems should be designed under the direct supervision of a Registered Professional Engineer knowledgeable in product selection and application. Hubbell steel foundation products offer simplicity in design and flexibility in adapting to the project. The design for ultimate and allowable bearing capacities, or anchor loads for helical products, is established using classical geotechnical theory and analysis, and supplemented by empirical relationships developed from field load tests. In order to conduct the design, geotechnical information is required at the site. The design and data shown in this manual are not intended for use in actual design situations. Each project and application is different as to soils, structure, and all other related factors.

Factors of Safety To recognize the variability of soil conditions that may exist at a site, as well as the varied nature of loading on structures and how these loads are transferred through foundations, Hubbell

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