Axial Bearing Characteristics of Grouting Micro Steel Pipe Piles: Experimental and Theoretical Analysis

Literature Overview

The paper by Si Yu, Xiao Chengzhi, and Wang Zihan, published in the Journal of Hebei University of Technology (2023, Vol. 52, No. 4, pp. 63-70), presents a comprehensive experimental and theoretical study on grouting micro steel pipe piles. Funded by the National Natural Science Foundation of China (Grant No. 41877255), Hebei Provincial Natural Science Foundation (Grant No. E2018202108), and Chengde Science and Technology Support Program (Grant No. 201706A075), this research addresses a significant gap in the understanding of micro steel pipe piles with grout sleeves. The study is based on 21 indoor model tests and aims to provide design guidance for this relatively underexplored pile type.

Research Background and Motivation

Micro steel pipe piles represent an emerging foundation solution in which a small-diameter steel pipe is inserted into a pre-drilled hole and surrounded by grout. Unlike conventional steel pipe piles, the load-bearing mechanism involves a complex interaction between the steel pipe, the grout sleeve, and the surrounding soil. The authors note that existing research on grouting micro steel pipe piles is limited, with most studies focusing on overall pile behavior rather than the specific contribution of the steel pipe under combined internal and external pressure.

Experimental Program and Key Findings

Test Parameters and Variables

The experimental program systematically varied six key parameters to understand their influence on pile performance:

Key Experimental Results

The study reveals three distinct behavioral regimes based on the ratio d/D:

1. Low ratio regime (0.28 ≤ d/D ≤ 0.40): In this range, the grout sleeve bears the majority of the axial load. Failure occurs simultaneously through grout crushing and steel pipe yielding. The steel pipe acts primarily as a reinforcement element within the grout column rather than as the primary load-bearing member.
2. Medium ratio regime (0.50 ≤ d/D ≤ 0.72): In this range, the pile exhibits pronounced ductile behavior. The ultimate load increases approximately linearly with increasing d/D. Both the steel pipe and the core cement grout contribute significantly to load-bearing capacity. The composite action between steel and grout is most effective in this range.
3. Transition regime (0.40 < d/D < 0.50): This intermediate range represents a transition between the grout-dominated and composite-dominated failure modes, where the relative contributions of steel and grout are more balanced.

Critical Design Parameters

The following findings are of direct practical significance:

• Slenderness ratio: When H/D ≥ 4, the pile integrity is optimal with a water-cement ratio of 0.45. This suggests that grout consistency must be carefully controlled for longer piles to ensure adequate bonding with the steel pipe.
• Grouting hole arrangement: The arrangement of grouting holes on the steel pipe surface has the least influence on pile performance among all tested parameters. This is a valuable finding, as it simplifies fabrication requirements for the steel pipe.
• Failure criterion: The study proposes that steel pipe yielding deformation should be adopted as the failure criterion for micro steel pipe piles, rather than grout failure alone. This is a significant departure from conventional design approaches that focus solely on grout strength.

Theoretical Analysis Method

The authors propose a theoretical method for analyzing the steel pipe under combined internal and external pressure. This approach considers:

• The radial pressure exerted by the grout on the pipe inner surface
• The lateral earth pressure acting on the pipe outer surface
• The axial stress distribution along the pile length
• The interaction between grout deformation and pipe deformation

The theoretical model provides a framework for predicting the load-sharing ratio between steel and grout components, which is essential for rational design of composite micro steel pipe piles.

Engineering Practice Integration

Design Recommendations

Based on the experimental and theoretical findings, the following design recommendations can be derived:

1. Optimal d/D ratio: For composite action design, a d/D ratio between 0.50 and 0.72 is recommended to maximize ductility and ensure both steel and grout contribute to load-bearing.
2. Grout mix design: A water-cement ratio of 0.45 is optimal for pile lengths where H/D ≥ 4, ensuring adequate grout strength and bonding.
3. Grouting hole design: Since hole arrangement has minimal influence, designers can simplify hole patterns without compromising performance, reducing fabrication complexity.
4. Failure assessment: Designers should evaluate both grout crushing and steel pipe yielding as potential failure modes, adopting steel pipe yielding as the governing failure criterion for ductility-based design.

Quality Control Considerations

From a quality control perspective, the following points should be emphasized during construction:

• Grout consistency must be verified at the point of placement using the flow cone test or slump test.
• The steel pipe must be straight and free of surface defects before grouting, as pipe deformation during installation can compromise grout-pipe bonding.
• Post-grouting pile integrity should be verified through low-strain dynamic testing or cross-hole sonic logging.

Study Reflections and Implications

This research makes a significant contribution to the understanding of composite micro steel pipe piles, a pile type gaining attention in urban foundation engineering where space constraints and load capacity requirements demand innovative solutions. The identification of three distinct behavioral regimes based on the d/D ratio provides a clear design framework that practitioners can readily apply.

The finding that grouting hole arrangement has minimal influence on performance is particularly valuable from a fabrication and cost perspective. It allows manufacturers to adopt simpler hole patterns without compromising structural performance, potentially reducing production costs.

The proposed theoretical method for analyzing combined internal and external pressure on the steel pipe addresses a gap in existing design codes, which typically do not account for the composite behavior of steel-grout systems. This method should be further validated through full-scale field tests and incorporated into future revisions of relevant design standards.

This study provides a solid foundation for the rational design of grouting micro steel pipe piles, offering both experimental data and theoretical tools that can significantly advance the practical application of this innovative foundation technology.