Zhuojin Pipe Fitting Co., LtdField Pullout Bearing Characteristics of Thin-Walled Conical-End Grouted Micro Steel Pipe Piles
Literature Overview
This paper, published in Engineering Science and Technology (2021, Vol. 53, No. 5, pp. 21-31) by Cui Qiang, Guo Chaoyi, Zhang Kai, and Zhang Chuanqing, presents a comprehensive field investigation into the pullout bearing behavior of an improved thin-walled conical-end grouted micro steel pipe pile. The research was conducted under the National Grid Corporation of China Science and Technology Project (GCB11201902118) and applied to the Shuanglou-Jiaohe 220 kV transmission line project. The study addresses the urgent need for environmentally friendly transmission tower foundations that meet the requirements of "green construction" in the new era of power grid development.
Core Technical Content
The authors developed and tested 18 steel pipe piles with two diameters (159 mm and 203 mm), two embedment depths (5 m and 8 m), and two conditions (grouted and non-grouted). The primary objective was to characterize the pullout bearing capacity, load-displacement behavior, and soil failure mechanisms around these piles through in-situ pullout tests, CT scanning, and finite element simulation.
Key Experimental Parameters
| Parameter | Configuration |
|---|---|
| Pipe diameters | 159 mm, 203 mm |
| Embedment depths | 5 m, 8 m |
| Grouting condition | Grouted / Non-grouted |
| Total test piles | 18 |
| Test type | In-situ pullout |
| Application | 220 kV transmission line foundation |
Critical Findings
The most significant result is that grouting improved the pullout bearing capacity by 59% to 148%, with the improvement inversely correlated to the slenderness ratio of the pile. This is a remarkably high enhancement that warrants careful engineering interpretation. The load-displacement curve transitioned from a steep failure type (non-grouted) to a gradual failure type (grouted), indicating enhanced plastic deformation characteristics after grouting.
The failure modes differed substantially between grouted and non-grouted piles. Non-grouted piles showed almost no surface cracking with the pile nearly extracted intact from the soil. Grouted piles, however, exhibited radial and circumferential surface cracks upon failure, indicating a different failure mechanism involving the grout-soil interface.
Mechanism Analysis
The grouting process involves cement slurry penetrating into soil voids, filling large quantities of pores, and ultimately forming a cement solidified body around the steel pipe. This creates a shear plane between the cement solidified body and the surrounding soil. The numerical simulation corroborates the field-observed failure mode at the pile-soil interface.
The mechanism of grouting's effect on pullout capacity is attributed to two aspects:
- Increased effective shear area due to the cement solidified body formation
- Enhanced lateral friction resistance of the soil around the pile
Engineering Practice Integration
From a welding and pipe engineering perspective, several observations are particularly relevant. The thin-walled nature of these micro steel pipe piles raises concerns about welding integrity at the conical end and any connections. The conical end geometry introduces stress concentrations that must be carefully managed during fabrication and installation.
Quality Control Considerations
| Quality Aspect | Requirement | Risk |
|---|---|---|
| Weld quality at conical end | Full-penetration weld, NDT verification | Stress concentration, premature failure |
| Wall thickness uniformity | Tolerances per GB/T standards | Reduced capacity at thin sections |
| Grout-pipe bond | Adequate surface preparation | Delamination, reduced friction |
| Corrosion protection | Coating or galvanizing | Long-term capacity degradation |
Practical Implications
The rapid construction cycle and minimal environmental disturbance make this pile type attractive for transmission line foundations. However, the high bearing capacity improvement from grouting (up to 148%) must be validated through long-term performance monitoring, as the grout-soil bond may degrade over time due to environmental factors.
Key Questions and Reflections
The inverse correlation between bearing capacity improvement and slenderness ratio is noteworthy. For longer, slimmer piles, the grouting effect diminishes proportionally, suggesting that the cement solidified body's contribution to friction resistance is more significant for shorter, stockier piles where the grout-soil interaction zone is more critical relative to the total pile length.
The CT scanning technique used to examine the microstructure of grouted soil provides valuable insight into the grout penetration mechanism. This is analogous to techniques used in welding metallurgy to examine microstructural changes in heat-affected zones.
Study Insights and Implications
This research demonstrates that grouting is not merely a passive treatment but actively transforms the pile-soil interaction from a simple friction mechanism to a composite shear-friction mechanism. For engineers designing transmission tower foundations, the selection between grouted and non-grouted micro steel pipe piles should be based on required pullout capacity, soil conditions, and environmental constraints. The finding that grouted piles exhibit a more gradual failure mode is particularly valuable for seismic design, as it provides greater energy dissipation capacity before ultimate failure.