Zhuojin Pipe Fitting Co., LtdMicro Steel Pipe Piles for Underpinning Existing Pile Raft Foundations
Literature Overview
This paper by Zhu Yanpeng, Lv Xiangxiang, Yang Kuibin, Wu Linping, Wei Zhengde, and Shi Duobang, published in the Journal of Architecture and Civil Engineering (2023, Vol. 40, No. 4, pp. 171-178), presents an application study of micro steel pipe piles for underpinning an existing 16-story building with a pile raft foundation in Sanmenxia, China. The building had developed significant tilt, and the authors conducted a cause analysis, compared various underpinning methods, and ultimately adopted a combined scheme of micro steel pipe pile underpinning and raft foundation extension. The research was supported by the Changjiang Scholars Innovation Team Support Program.
Cause Analysis and Method Selection
The 16-story building experienced differential settlement and tilt, likely caused by uneven ground conditions in the fill area. The authors systematically evaluated various underpinning methods, considering factors such as construction period, structural disturbance, cost, and effectiveness. The comparison of underpinning methods is summarized below:
| Underpinning Method | Construction Period | Structural Disturbance | Effectiveness | Applicability |
|---|---|---|---|---|
| Micro steel pipe piles + raft extension | Short | Low | High | Fill area foundations |
| Jet grouting columns | Medium | Low | Moderate | Soft soil areas |
| Underpinning piles | Long | High | High | Load-bearing capacity improvement |
| Foundation grouting | Short | Medium | Moderate | Settlement control |
| Raft extension alone | Medium | Medium | Moderate | Edge settlement |
The micro steel pipe pile method was selected because it offers a short construction period, clear load transfer mechanism, minimal disturbance to the existing building during construction, and significant overall reinforcement effectiveness.
Technical Design and Construction Parameters
The micro steel pipe pile underpinning system involves driving small-diameter steel pipes (typically 57-114 mm in outer diameter) through the existing foundation into competent bearing strata, followed by grouting or concrete filling to develop pile capacity. The key design parameters include:
- Single pile vertical bearing capacity: Calculated based on soil parameters obtained from in-situ testing, including skin friction along the pile shaft and end bearing resistance.
- Stability analysis: The combined system of micro piles and extended raft must satisfy stability requirements against overturning and sliding under the full building load.
- In-situ pile load testing: Conducted to verify the calculated bearing capacity and determine the appropriate number and spacing of micro piles.
- Construction monitoring: Settlement and displacement monitoring at designated points throughout the construction period to assess the effectiveness of the underpinning scheme.
Monitoring Results and Performance Evaluation
The monitoring data reveal important insights into the underpinning performance. The settlement monitoring points DX1, DX2, and DX7 exhibited relatively larger cumulative settlement, with DX1 recording a maximum cumulative settlement displacement of -81.1 mm and DX7 recording a maximum daily average settlement of -1.23 mm. After approximately 15 days following the completion of micro steel pipe pile installation, the daily average settlement displacement stabilized within the range of 0 to 0.05 mm per day, indicating that the underpinning system had effectively arrested further differential settlement.
The monitoring data also demonstrate that the micro steel pipe pile system provides rapid load transfer to deeper bearing strata, effectively bypassing the weak fill layer that was causing the differential settlement. The combination with raft foundation extension provides additional load spreading capacity, reducing the contact pressure on the underlying soil and further minimizing settlement.
Quality Control and Engineering Practice
From a steel pipe fabrication and welding standpoint, the micro steel pipe piles require careful attention to the following quality aspects:
- Pipe straightness and dimensional accuracy: The micro pipes must be manufactured to tight tolerances to ensure smooth driving through the soil and proper grout-steel bond.
- Weld quality of pipe segments: If the piles are constructed from multiple pipe segments, the butt welds must be full-penetration welds with appropriate heat input control to avoid excessive distortion or residual stress.
- Grout quality: The grout used for filling the micro pipes or the annular space between the pipe and the soil must have appropriate fluidity, strength, and shrinkage characteristics to ensure adequate load transfer.
- Driving control: The driving process must be monitored to prevent pipe deformation, damage to adjacent structures, or excessive vibration that could affect the stability of the existing building.
Study Insights and Conclusions
This case study provides compelling evidence for the effectiveness of micro steel pipe pile underpinning combined with raft foundation extension for addressing differential settlement in existing buildings founded on fill areas. The short construction period, minimal structural disturbance, and significant reinforcement effectiveness make this method particularly attractive for urban environments where construction space and time are limited. For engineers working on pipeline, structural, or foundation projects involving existing structures, the key takeaway is that micro steel pipe piles offer a practical and efficient solution for foundation underpinning, provided that the design is based on thorough site investigation and the construction is executed with careful monitoring and quality control. The successful application of this method in Sanmenxia serves as a valuable reference for similar projects in other regions with comparable ground conditions.