Methods and DeepFND software
A. Introduction
The installation torque for helical piles stands as a critical parameter in assessing their capacity and integrity during installation. This measure represents the resistance encountered as the helical pile screws into the soil, a process influenced by various factors, including soil type, helix configuration, pile diameter, depth, and the chosen equipment and installation method.
B. Factors Influencing Installation Torque
Soil Type: Distinct soil types offer varying levels of resistance, with harder and more compact soils generally demanding higher torque for installation.
Helix Configuration: The size, shape, and number of helices on the pile significantly impact installation torque. Larger or multiple helices may elevate torque requirements.
Pile Diameter: The diameter of the helical pile is a pivotal factor; larger-diameter piles typically necessitate more torque for installation.
Pile Depth: The depth of installation plays a crucial role, as deeper installations encounter more resistance, requiring higher torque.
Equipment and Installation Method: The choice of machinery and installation technique, whether handheld or hydraulic equipment, can influence torque requirements.
Site-Specific Relationships:
The relationship between installation torque and the ultimate capacity of helical piles is inherently site-specific. Engineers establish this connection through empirical correlations derived from field testing. Installation torque data is pivotal for assessing the load-bearing capacity of the helical pile, ensuring it aligns with the design requirements for the specific project.
Challenges in Calculation:
Calculating installation torque is a complex process, often relying on the expertise of engineers who incorporate experience, field testing, and empirical correlations. This multifaceted approach is crucial in determining the appropriate torque values tailored to the unique characteristics of a given project.
C. DeepFND’s Approach
DeepFND software serves as a comprehensive tool for engineers and designers, streamlining the estimation of installation torque for helical piles. DeepFND’s user-friendly interface and robust calculation methods make it a powerful asset in the field of deep foundation design.
A crucial feature of DeepFND is the ability to compare the maximum torque with the pile's torsional capacity. This step ensures that the designed helical pile can withstand the applied torsional forces during installation and throughout its service life. This capability enhances the reliability and safety of helical pile foundations.
DeepFND software offers two approaches for the estimation of the installation torque of helical piles:
Empirical Approach with Torque Correlation Factors (kt):
DeepFND utilizes an empirical approach to estimate installation torque through torque correlation factors (kt). The software features two torque installation profiles – the AC-358 for piles up to 3.5 inches in diameter and a profile for larger piles. Users have the flexibility to customize these profiles or manually add additional ones to align with project-specific needs.
The torque correlation factor is determined by dividing the ultimate resistance by the estimated installation torque, offering a practical and efficient method for engineers in the design process.
Figure 1: AC-358 profile – Kt factors in DeepFND software
Figure 2: Installation torque for a 3.5’’ diameter pile with helixes in DeepFND, estimated with the correlation factors approach
Advanced Torque Calculation Method:
DeepFND takes a step further by incorporating an advanced torque calculation method. This approach considers both shaft resistance and blade resistance, with the first blade taking additional resistance as a cutting plate. By factoring in these detailed considerations, DeepFND provides a more nuanced and accurate estimate of the installation torque.
Figure 3: Installation torque for a 3.5’’ diameter pile with helixes in DeepFND, estimated with the advanced torque calculation approach
D. Installation Torque Estimation in DeepFND with and without SPT Records
DeepFND provides user-friendly dialogs to define desired soil type properties and stratigraphies. Engineers can input specific soil properties or choose from published values based on soil types and descriptions. Additionally, the software supports the import of SPT records and Cone Penetration Test (CPT) log data, facilitating the estimation of soil properties with depth.
Let's consider a scenario involving a 26-foot deep helical pile (PP3.5x0.336 pipe with 3 helical plates). We'll explore two cases to illustrate the calculated bearing capacities and estimated installation torque:
Case 1: Model with specific soil properties for each soil layer:
In this scenario, we assume two frictional soil layers with predefined properties. The table below outlines the assumed soil properties and stratigraphy:
Table 1: Assumed soil properties and stratigraphy
Soil | Top El. | Soil Type | γ' | Φ’ | c' | E | exp |
(-) | (ft) | (-) | (pcf) | (deg) | (psf) | (ksf) | (-) |
F | 0 | Sand | 120 | 30 | 0 | 300 | 0.5 |
S1 | 0 | Sand | 125 | 34 | 0 | 400 | 0.5 |
Figure 4: Calculated maximum bearing capacities and installation torque in DeepFND – Case 1
Case 2: Model with SPT Record:
In this scenario, we maintain the same soil types and stratigraphy as Case 1 but incorporate an NSPT record into the model. The applied SPT record values and the resulting calculated results are depicted in the following figures.
Figure 5: SPT Record – Blow counts applied in DeepFND
Figure 6: Calculated maximum bearing capacities and installation torque in DeepFND – Case 2
E. Conclusion
Accurate estimation of installation torque is fundamental to the success of helical pile projects. DeepFND, with its blend of empirical approaches and advanced calculation methods, emerges as a valuable tool for engineers navigating the complexities of deep foundation design. As the demand for precision in deep foundation solutions continues to grow, DeepFND stands ready as a reliable ally for those seeking optimal outcomes in helical pile installations.
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