Helical pile installation influence on the pile lateral resistance, Canada

A. INTRODUCTION

The subject of the present case study pertains to the simulation of four test helical piles located approximately 70 kilometers north of Fort McKay, situated in the northern region of Alberta, Canada [1]. The primary focus of the investigation is to assess the impact of p-y multipliers on these models, accounting for the influence of installation and predrilling disturbance. The predrilling technique was utilized for the installation of the ST10 and ST16 piles, whereas ST23 and ST43 were installed using conventional helical pile installation procedures, without pre-drilling. The sides of the full-scale tests exhibit a range of soil stratifications, encompassing cohesive to cohesionless soil materials. The evaluated helical piles exhibit different pipe sections, featuring one or two helical plates. Table 1 illustrates the soil properties and soil layers utilized to simulate Sites 1 to 4, and Table 2 presents the helical pile section properties and helix configurations, included in the original case study. The DeepFND- Deep Foundations Design Software was employed to replicate the full scale tests.

Table 1: Soil Properties and Stratigraphy

Table 2: Helical Pile Section Properties

B. MODEL SIMULATION AND RESULTS

The DeepFND software was used to simulate 4 different helical piles (ST10, ST16, ST23 and ST43). Figure 1 presents the soil properties and soil layers in DeepFND. The following sections describe the analysis assumptions, the model and the results for each examined pile scenario.

Figure 1: Soil properties and soil layers in DeepFND.

Case 1: Site 1 (Boring 1), ST16, Pile type T6.

At Site 1, the diameter of the drill augur utilized for predrilling was similar to that of the pile shaft; however, the predrilled hole was roughly 50 millimeters larger than the augur’s size. As a result, soil disturbance occurred, which impacted the lateral resistance of the tested helical piles on site. Initially, the model was analyzed without accounting for the predrilling disturbance. Subsequently, we repeated the analysis, this time considering a y multiplier of 2.5. The DeepFND model and analysis results are presented in Figure 2, while Figure 3 showcases the Lateral Load/Displacement diagrams, comparing the measured displacements with the pushover analysis’ estimated curves generated by the software. We observe that while the original model with no multipliers predicted considerably smaller displacements, the use of y=2.5 facilitated the software in predicting displacements that closely approximated the measured values.

Figure 2: ST16: DeepFND Model – Analysis results (Bearing capacities, Settlements & Displacements)

Figure 3: ST16: Comparison between measured displacements and DeepFND pushover results.

Case 2: Site 2 (Boring 2), ST23, Pile type T8.

The installation of Pile ST23 followed a conventional screw pile installation procedure, without predrilling, resulting in its lateral resistance exhibiting reasonable conformity with the measured values. The DeepFND model and the analysis results are depicted in Figure 4, while Figure 5 displays the Lateral Load/Displacements diagrams, which compare the measured displacements with the pushover analysis' estimated curves generated by the software.

Figure 4: ST23: DeepFND Model – Analysis results (Bearing capacities, Settlements & Displacements)

Figure 5: ST23: Comparison between measured displacements and DeepFND pushover results.

Case 3: Site 4 (Boring 4), ST43, Pile type T8.

The lateral resistance of Pile ST43, which was installed using the standard screw pile installation method without predrilling, demonstrated reasonable accordance with the measured values. The DeepFND model and analysis results are presented in Figure 6, while Figure 7 showcases the Lateral Load/Displacements diagrams, comparing the measured displacements with the pushover analysis' estimated curves generated by the software.

Figure 6: ST43: DeepFND Model – Analysis results (Bearing capacities, Settlements & Displacements)

Figure 7: ST43: Comparison between measured displacements and DeepFND pushover results.

Case 4: Site 3 (Boring 3), ST10, Pile type T3.

Pile ST10 was installed in a predrilled hole. Initially, we analyzed this model without considering the predrilling disturbance. We repeated the analysis, this time considering a y multiplier of 3.5. Figure 8 below presents the DeepFND model and the analysis results. Figure 9 presents the Lateral Load/Displacements diagrams, comparing the measured displacements with the software-estimated curves from pushover analysis. We notice that while the original model with no multipliers was predicting quite smaller displacements, the use of y=3.5 allowed the software to predict displacements that almost match the measured values.

Figure 8: ST10: DeepFND Model – Analysis results (Bearing capacities, Settlements & Displacements)

Figure 9: ST10: Comparison between measured displacements and DeepFND pushover results.

C. CONCLUSIONS

Utilizing the deep foundation analysis software, DeepFND, a series of full-scale laterally loaded tests of helical piles were simulated [1], wherein the impact of disturbance resulting from the installation method of the piles was approximated by modifying the “y” multipliers of the lateral soil springs. As per the comparison, employing a “y” multiplier coefficient within the range of 2.5 to 3.5 proved to be a sufficient adjustment to account for the installation effect in the piles that were installed with predrilling. Conversely, in cases where installation was carried out without predrilling, the disturbance effect on the lateral resistance of the pile appeared to be negligible in this case. Our experience suggests that installation effects are more pronounced in smaller diameter piles installed in clay soils.

D. REFERENCES

[1] Mohammed Sakr, "Lateral Resistance of High Capacity Helical Piles – Case Study.", GEO 2010, Calgary, Alberta, Canada.

 

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