Transmission And Substation Foundations - Technical Design Manual (TD06088E)

VI. SUMMARY: In summary, helical pile design determines the geotechnical resistance (P4) and structural capacity (P1, P2, & P3), typically in that order. Probe helical piles and load tests are often done before start of work when a geotechnical report is not available or when verification of capacity is required. The geotechnical and structural resistance are separate limit states and whichever one is the lesser will control the design. In most cases, the geotechnical resistance (P4) will be the controlling factor. The designer is encouraged to design helical piles so that the geotechnical resistance (P4) controls to make the most efficient use of the soil’s ability to bear load. This often means choosing the right shaft type/size, end condition, and helix configuration to maximize capacity. VII. RELIABILITY: Reliability is an important aspect of helical pile design. Reliability is defined as the probability of long- term satisfactory performance. The better the capacity prediction method(s) used, the greater the reli- ability. Hubbell Power Systems, Inc. recommends using base plus shaft resistance [Method 1] and torque correlation [Method 2] to determine capacity whenever possible. Perko 2009 did a statistical analysis of helical pile capacity in order to check the reliability of this approach. He used a database of several hun- dred load tests in the analysis and used a factor of safety of 2 to determine a safe allowable load (deter- ministic approach). Using bearing capacity theory, the load test data suggests that 1 out of 10 helical piles will exhibit unsatisfactory performance. That is a 90% success rate, but still means 10% will have unac- ceptable performance. Using torque correlation, load test data suggests that 0.3 out of 10 will exhibit unsatisfactory performance. That’s a 97% success rate which is much better, but still means that 3% will have unacceptable performance. Methods 1 and 2 are independent methods used to determine helical pile capacity. When two independent methods are statistically combined, the result of poor helical pile per- formance drops to only 3 piles out of 1000, or 0.3%. That is a 99.7% success rate, which most engineers agree is acceptable reliability. Loads tests [Method 3] is another independent method of capacity predic- tion which can be used when soil data is lacking or uncertain, or when soil conditions change. VIII. OTHER TOPICS RELATED TO DESIGN: Corrosion Potential : Underground corrosion is discussed in detail in Appendix A of the TDM. In most ground conditions, corrosion is not a practical concern for deep foundations, including helical piles. There is typically little to no oxygen in undisturbed soils, especially below the ground water table. Driven steel piles have been installed with pile hammers for more than a century and are still commonly used today. The vast majority of interstate highway bridges in the Piedmont regions of the southeast United States are bearing on driven steel H-piles. If the geotechnical report declares the corrosion potential is moder- ate to severe for a given project, then a square shaft helical pile is a good choice because of its solid cross section and low perimeter surface area compared to a pipe shaft; which is hollow and has more perimeter surface area relative to the cross-sectional area of steel. Hot-dip galvanization adds a thick coating of zinc to the steel pile. It provides a durable coating that increases service life. Service life calculations based on metal loss rates can be done when corrosion potential data is available. Appendix A of the TDM contains 4 design examples for corrosion design. A Helical PULLDOWN® Micropile with its solid square shaft encased in a very dense grout mixture provides the most resistance to corrosion since the grout acts as an additional layer of protection. Cathodic protec- tion, or adding a corrosion allowance (additional thickness of sacrificial steel) are also options in aggres- sive environments.

HELICAL PILES AND ANCHORS

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