Transmission And Substation Foundations - Technical Design Manual

A Basic Guideline For Designers APPENDIX C: HELICAL PILES & ANCHORS I. Introduction This Technical Design Manual (TDM) is a comprehensive collection of information for the express purpose to educate the practicing engineer in the art of helical pile design. The amount of information is extensive, and we recognize the need to provide a short length “primer” for the busy professional who does not have the time to read and learn all the comprehensive methods used to design helical piles. The goal of this “How To” is to bring the design and selection of helical piles and anchors into a short easy-to-follow Guideline. This Guideline will provide the design method used every day by the Application Engineering Staff at Hubbell Power Systems, Inc. Citations throughout will direct the designer where to find the required information in the Technical Design Manual. The result is a simple step-by-step process culminating in a helical pile design that can then be correctly written into a project specification. II. Helical Pile Capacity The design method for helical pile capacity is simple. It consists of two limit states criteria; namely the Ultimate Resistance and the Serviceability Limit. Serviceability is the behavior of a helical pile at a particular load that is less than the ultimate resistance. For helical pile design, the Serviceability Limit primarily deals with limiting the deflection or displacement of the pile at a specified service load. 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. For helical pile design, ultimate resistance typically consists of two elements – the geotechnical capacity and the structural capacity, or strength. It is more descriptive to refer to structural “strength” of the helical pile components, which is the approach taken in the TDM. A. According to the International Building Code (IBC) Section 1810.3.3.1.9, there are four ways to determine the ultimate resistance of helical piles. • Method 1: Base resistance plus shaft resistance of the helical pile, where the base resistance is equal to the sum of the areas of the helical bearing plates times the ultimate bearing resistance of the soil or rock comprising the bearing stratum, and shaft resistance is equal to the frictional resistance of the soil times the shaft area above the helix bearing plates. This is commonly referred to as the theoretical geotechnical limit state method. It is described in great detail in Section 5 of the TDM. • Method 2: Ultimate capacity determined from well documented correlations with installation torque. This is commonly referred to as the empirical geotechnical limit state method. The key words are “well documented” which will be discussed later. Torque correlation is described in Section 6 of the TDM. • Method 3: Ultimate capacity determined from load tests. This is the most direct method to determine the geotechnical capacity of any pile, not just helical piles. Load testing of helical anchors and pile is described in Appendix B of the TDM. • Method 4: Resistance of the pile’s structural elements (shaft, helix, couplings, connection to structure). Structural strength is described in Sections 4 & 6 of the TDM.

Of the four methods above, the only one that is unique to helical piles is Method 2, commonly referred to as torque correlation. B. According to IBC Section 1810.3.3.1.9, the geotechnical capacity (Methods 1, 2, or 3 above) shall not exceed the strength of the pile’s structural elements (Method 4); including the pile connection to structure, pile shaft, pile shaft couplings, and the helix bearing plates. The structural strength of Chance brand helical piles is described in Section 7 of the TDM. C. Therefore, both the geotechnical capacity and the structural strength of the helical pile must be determined; and whichever limit state is the lesser, will control the capacity. This is the ultimate resistance of the helical pile. In most cases, the geotechnical capacity will be the limit state, but the structural strength can sometimes control. D. Allowable Strength Design (ASD) or Limits States Design (LRFD). ASD has been used for many years for the geotechnical capacity of deep foundations. It is sometimes referred to as deterministic design since the factor of safety is determined based on standard practice. LRFD is sometimes referred to as probabilistic design. It uses load factors and resistance factors based on statistically based probabilities of uncertainty. In the United States, most geotechnical design is deterministic based (global factor of safety); whereas in Canada most geotechnical design is probabilistic (limit states – ULS, SLS). The TDM includes both LRFD design and ASD allowable strength values, so the design can use either design method. E. The Serviceability Limit may also control. Serviceability is the load/deflection response of a helical pile at a particular load of interest, i.e. a factored load well below the ultimate resistance limit state. There may be strict deflection limits required based on the application; the structure may be sensitive to overall settlement or differential settlement, which may require the helical pile ultimate resistance to be increased. For example, a deflection limit may be specified at the working/design load. Cherry and Perko (2012) reviewed hundreds of tension and compression load tests. They suggested that for end-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. Serviceability 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 dictate 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:

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