Micro Steel Pipe Pile Underpinning of Existing Pile Raft Foundations

Literature Overview

The paper by Zhu Yanpeng, Lv Xiangxiang, Yang Kuibin, Wu Linping, Wei Zhengde, and Shi Duobang (2023), published in the Journal of Architecture and Civil Engineering, presents a case study on the application of micro steel pipe piles for underpinning an existing 16-story building with a pile raft foundation that had developed significant tilting. The study includes cause analysis, method selection, design calculations, construction monitoring, and effectiveness evaluation. This work addresses a critical challenge in geotechnical and structural engineering: the safe and effective reinforcement of existing buildings experiencing foundation settlement and tilting.

Core Technical Content

The case study involves a 16-story building in Sanmenxia City that had developed significant tilting due to uneven settlement of the fill foundation. The underpinning solution combined micro steel pipe piles with an extended raft foundation. Key aspects of the study include:

Analysis of the cause of building tilting.
Comparison of various underpinning methods.
Selection of micro steel pipe pile underpinning combined with extended raft foundation.
Single pile vertical bearing capacity calculation and stability analysis.
On-site in-situ pile load testing.
Construction monitoring and settlement control.
Evaluation of reinforcement effectiveness.

Monitoring Results

Monitoring Point	Maximum Cumulative Settlement	Maximum Daily Settlement
DX1	-81.1 mm	Not specified
DX2	Relatively large	Not specified
DX7	Not specified	-1.23 mm

After approximately 15 days following the completion of micro steel pipe pile installation, the daily settlement displacement was stabilized and controlled between 0 and 0.05 mm/day.

Technical Interpretation of Underpinning Method

The micro steel pipe pile underpinning method involves installing small-diameter steel pipe piles beneath the existing foundation to transfer structural loads to deeper, more competent soil layers. The method is particularly suitable for situations where:

The existing foundation has experienced significant settlement or tilting.
The surrounding environment restricts the use of large-diameter piles or other underpinning methods.
Rapid construction is required to minimize the period of structural instability.

The combination of micro steel pipe piles with an extended raft foundation provides a dual mechanism of load transfer. The micro piles directly transfer loads to deeper soil layers, while the extended raft foundation distributes loads over a larger area and provides additional stability.

Comparison of Underpinning Methods

Method	Construction Period	Disruption to Building	Load Transfer Mechanism	Applicable Conditions
Micro steel pipe piles	Short	Low	Direct load transfer to deep soil	Fill foundations, restricted access
Jet grouting	Moderate	Moderate	Ground improvement	Cohesive soils, moderate settlement
Underpinning piles	Long	High	Load transfer to new piles	Large settlements, deep foundations
Raft extension	Moderate	Moderate	Load distribution	Shallow foundations, moderate settlement
Chemical grouting	Short	Low	Ground improvement	Fine-grained soils, minor settlement

Engineering Practice Implications

The successful application of this underpinning method provides several practical insights:

Monitoring is critical: The settlement monitoring data shows that the maximum cumulative settlement at DX1 reached -81.1 mm, which is significant. Continuous monitoring during and after construction is essential to ensure the building's safety and to verify the effectiveness of the underpinning.
Construction sequencing: The construction sequence must be carefully planned to avoid excessive disturbance to the existing building. The micro steel pipe piles should be installed in a controlled sequence, with monitoring between each installation stage.
Soil-structure interaction: The behavior of the micro steel pipe piles is influenced by the surrounding soil conditions and the interaction with the existing foundation. The design must account for these interaction effects to ensure reliable performance.
Long-term performance: The stabilization of daily settlement to 0–0.05 mm/day after 15 days indicates that the underpinning is effective. However, long-term monitoring should continue to verify that the settlement remains stable over time.

Key Questions and Reflections

One important question is the long-term durability of the micro steel pipe piles. Steel pipes are susceptible to corrosion, particularly in aggressive soil environments. The design life of the underpinning system should consider corrosion protection measures, such as cathodic protection or the use of corrosion-resistant coatings.

Another reflection concerns the load-sharing between the micro piles and the extended raft foundation. The relative load transfer between these two components depends on their relative stiffness and the soil conditions. Understanding this load-sharing mechanism is important for the design and for predicting the long-term performance of the underpinning system.

Study Insights and Outlook

The micro steel pipe pile underpinning method offers a practical and effective solution for reinforcing existing buildings with pile raft foundations experiencing uneven settlement. The method's advantages—short construction period, low disruption to the building, clear reinforcement mechanism, and significant overall effectiveness—make it suitable for many practical applications.

For future development, I would recommend investigating the use of advanced monitoring technologies, such as fiber optic sensors and inclinometers, to provide more detailed information on the behavior of the underpinning system during and after construction. Additionally, the development of design guidelines and standards for micro steel pipe pile underpinning would facilitate wider adoption of this method in the engineering community. The case study demonstrates that with proper design, construction, and monitoring, micro steel pipe pile underpinning can successfully stabilize existing buildings and provide long-term safety and serviceability.