Pull-Out Bearing Capacity of Pressure-Grouting Spiral Steel Pipe Piles

Literature Overview

This paper by Zong Zhongling, Zhuang Xiaoxuan, Huang Yunhan, Cao Bo, and Zhu Jianguo (2022, Ocean Engineering, Vol. 40, No. 1, pp. 160-166) presents a field pull-out test investigation of a novel pressure-grouting spiral steel pipe pile designed for marine soft soil regions. The research was funded by the Jiangsu Provincial Key R&D Program (Grant BE2021681) and the State Grid Jiangsu Electric Power Co., Ltd. Science and Technology Program (Grant J2019113). Five full-scale test piles were constructed and subjected to pull-out loading to evaluate the influence of spiral blade diameter and arrangement on the formed pile diameter and pull-out bearing capacity.

Core Technical Findings

The study addresses two critical challenges in marine foundation engineering: the low bearing capacity of steel pipe piles in soft marine soils and the corrosion susceptibility of steel piles in aggressive marine environments. The proposed pressure-grouting spiral steel pipe pile combines the advantages of spiral reinforcement with pressure grouting to create a reinforced cement-soil column that enhances the pull-out resistance.

The positive correlation between spiral blade diameter and formed pile diameter is physically intuitive: larger blades displace more soil and create a larger cylindrical zone of grout-soil mixture. The finding that placing spiral blades at the end of each extension section improves the integrity of the cement-soil column is particularly significant, as it suggests that the spiral reinforcement prevents slumping and segregation of the grout during the pressure grouting process.

Pressure Grouting Mechanism and Pull-Out Resistance

The pressure grouting process involves injecting cement slurry into the soil surrounding the steel pipe pile at controlled pressures. The spiral blades act as mechanical barriers that prevent the grout from flowing upward or outward uncontrollably, creating a well-defined cylindrical column of cement-soil mixture. The pull-out resistance is then provided by the skin friction between the cement-soil column and the surrounding soil, as well as the end bearing at the pile tip.

The comparison between the calculated and measured pull-out capacities reveals that the current code calculation method provides a slightly conservative estimate, with calculated values averaging approximately 94% of the measured values. This suggests that the existing calculation parameters may be slightly conservative for pressure-grouting spiral steel pipe piles, and the authors propose corrections to the calculation parameters to improve the accuracy of capacity predictions.

Engineering Practice Implications

The proposed pressure-grouting spiral steel pipe pile offers a practical solution for foundation design in marine soft soil regions, where conventional steel pipe piles often have insufficient bearing capacity. The spiral blade configuration is relatively simple to fabricate and install, and the pressure grouting process is well-established in the industry. The key design parameters include the spiral blade diameter, the spacing between blades, the grout pressure, and the grout mix design.

From a welding and fabrication perspective, the spiral blades must be welded to the steel pipe pile with high-quality welds to withstand the driving and grouting pressures. The weld geometry and procedure should be carefully selected to ensure full penetration and adequate throat thickness, particularly at the blade-to-pipe junction where stress concentrations are expected.

Key Questions and Reflections

A significant question is the long-term durability of the cement-soil column in the marine environment, where sulfate attack and chloride-induced corrosion can degrade the grout over time. The study focuses on short-term pull-out capacity but does not address the long-term performance under cyclic loading and environmental degradation. Additionally, the interaction between multiple piles in a group and the group efficiency factor should be investigated for practical foundation design.

Another consideration is the effect of grout pressure on the pull-out capacity. Higher grout pressures may create a more compact cement-soil column with higher skin friction, but excessively high pressures can cause soil heave and damage to adjacent piles. The optimal grout pressure range should be determined through parametric studies.

Summary and Conclusions

This paper presents a practical and innovative solution for improving the pull-out bearing capacity of steel pipe piles in marine soft soil regions. The pressure-grouting spiral steel pipe pile concept effectively combines mechanical reinforcement with soil improvement, and the field test results demonstrate its effectiveness. The proposed corrections to the calculation parameters provide a more accurate basis for design, and the study contributes to the advancement of marine foundation engineering practices. The spiral blade arrangement at the end of extension sections is a particularly valuable finding, as it provides a simple and effective method for improving grout column integrity.