Transmission And Substation Foundations - Technical Design Manual (TD06088E)

Engineers might ask “Why should the SPT N-value correlate to pile side resistance?” Other than being purely coincidental, there must be a rational and logical explanation for such observations. The range in reported values of α given in Table 4-12 is quite large and the results might seem of limited use. Nonetheless, we can make some general observations and summarize these observations: 1) For most of these correlations, the value of β is very low and for practical purposes may be reasonably taken as zero with little effect on the correlation, which simplifies Eq. 5-29 to: fs = α N Equation 4-30 2) The value of α ranges from 0.3 to 12.5; 3) The observations presented in Table 4-12 generally suggest higher values of α for fine-grained soils as compared to coarse-grained soils; and 4) Values of α are generally higher for driven piles as compared to bored piles. The values of α vary considerably for a number of obvious reasons, deriving from both the pile data as well as the SPT data. In regard to the pile data: 1) The data represent a wide range of pile types, i.e., different geometry, such as open and closed end pipe, H-Piles and construction practices; such as dry bored vs. wet bored as well as pile size, pile plugging, L/d, and other factors; 2) Different methods may have been used to interpret the ultimate capacity and to isolate the side resistance from end bearing; 3) The unit side resistance from pile tests is typically averaged over the length of the pile except in the case of well instrumented piles. Regarding the SPT data:1) The results most likely represent a wide range in field practice including a wide range in energy or hammer efficiency; 2) It is likely that other variations in field practice or equipment such as spoon geometry are not consistent among the various studies and may affect results. Engineers should use the correlations in Table 4-12 with caution. In fact, Equation 4-30 is similar to Equation 4-21, suggesting a correlation between SPT N-values and undrained shear strength (s u ) in fine-grained soils. FACTOR OF SAFETY The equations discussed above are used to obtain the ultimate capacity of a helical anchor/pile. For working, or allowable stress design (ASD), an appropriate Factor of Safety must be applied to reduce the ultimate capacity to an acceptable design (or working) capacity. The designer determines the Factor of Safety to be used. In general, a minimum Factor of Safety of 2 is recommended. For tieback applications, the Factor of Safety typically ranges between 1.25 and 2. Design or working loads are sometimes referred to as unfactored loads and do not include any Factor of Safety. They may arise from dead loads, live loads, snow loads and/or earthquake loads for bearing (compression) loading conditions; from dead loads, live loads, snow loads and/or wind loads for anchor loading conditions; and earth pressure, water pressure and surcharge loads (from buildings, etc.) for helical tieback or Soil Srew ® earth retention conditions. Ultimate loads, sometimes referred to as fully factored loads, already fully incorporate a Factor of Safety for the loading conditions described above. Hubbell Power Systems, Inc. recommends a minimum Factor of Safety of 2.0 for permanent loading conditions and 1.5 for temporary loading conditions. This Factor of Safety is applied to the design or working loads as defined above to achieve the ultimate load requirement. National and local building code regulations may require more stringent Factors of Safety on certain projects. Most current structural design standards in Canada use a Limit States Design (LSD) approach for the structural design of helical piles/anchors rather than working or allowable stress design (ASD). All specified loads (dead, live, snow, wind, seismic, etc.) are factored in accordance with appropriate load factors and load combinations should be considered. In addition, the geotechnical resistance of the helical pile/anchor must be factored. Geotechnical resistance factors for helical piles/anchors are not yet clearly defined. Therefore, a rational approach should be taken by the designer and resistance factors should be considered that are suitable to specific requirements.

DESIGN METHODOLOGY

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