Bearing Characteristics of Micro Steel Pipe Piles in Deep Fill Areas

Literature Overview

The study by Zhou Yong and Liu Yong, published in the "Journal of Lanzhou University of Technology" (Vol. 47, No. 3, 2021, pp. 132-138), investigates the bearing characteristics of micro steel pipe piles in deep fill areas through both laboratory and field testing. Funded by the National Natural Science Foundation of China (Grant 51568042), this research addresses a critical geotechnical engineering challenge: the foundation reinforcement of structures built on deep fill deposits, which are common in urban expansion areas, reclaimed land, and infrastructure corridors.

Core Technical Content and Key Points

Research Background and Significance

Deep fill areas, characterized by loose, heterogeneous, and compressible fill materials deposited over natural soil, present significant challenges for foundation engineering. Traditional deep foundation solutions (large-diameter bored piles, driven piles) may be impractical due to cost, space constraints, or the presence of existing structures. Micro steel pipe piles (typically 50–150 mm in diameter) offer a compact, high-capacity alternative for ground improvement and foundation reinforcement in such challenging ground conditions.

Experimental Methodology

The research employs a combined approach:

Test Type	Parameters Measured	Purpose
Laboratory tests	Stress-strain relationship, elastic modulus	Material characterization
Field load tests	Axial force distribution, pile head displacement	Bearing capacity verification
Instrumentation	Strain gauges along pile shaft	Load transfer mechanism

The composite section consists of a steel pipe filled with cement slurry, creating a reinforced column that mobilizes both skin friction and end-bearing resistance.

Key Findings

Composite Section Elastic Modulus:

The confinement effect (hooping action) of the steel pipe on the cement slurry has a relatively small influence on the composite section elastic modulus. The measured value is only 1.2 times the calculated value that does not consider the confinement effect. This is lower than the confinement enhancement observed in large-diameter steel tube concrete columns, likely due to the small diameter-to-thickness ratio and the relatively low stiffness of the cement slurry compared to structural concrete.

Load Distribution Between Steel Pipe and Cement Slurry:

The steel pipe bears approximately 2/3 of the total axial load, while the cement slurry bears the remaining 1/3. This distribution is governed by the relative stiffness of the two components and the load transfer mechanism through the interface. The high load share by the steel pipe is consistent with its significantly higher elastic modulus compared to the cement slurry.

Load-Settlement Characteristics:

The Q-s curve (load-settlement relationship) of micro steel pipe piles in fill areas exhibits a gradual (缓变型) pattern, indicating predominantly frictional resistance with minimal end-bearing contribution. The end resistance approaches zero, confirming the pile behaves as a friction pile rather than an end-bearing pile. This is consistent with the soft, compressible nature of fill deposits, which cannot provide significant end-bearing capacity.

Parameter	Typical Value	Design Implication
Pile diameter	50–150 mm	Compact, suitable for confined spaces
Steel pipe wall thickness	2–5 mm	Adequate for load transfer
Cement slurry water-cement ratio	0.4–0.6	Optimized for pumpability and strength
Ultimate axial capacity	50–200 kN per pile	Depends on fill properties and pile length
Settlement at service load	< 30 mm	Satisfies code limits
Steel pipe load share	~67%	Steel governs design
End-bearing contribution	~0%	Friction pile behavior

Load Transfer Mechanism

The research reveals that the axial force and skin friction along the pile shaft follow a characteristic distribution pattern:

Near the pile head: High axial force, low accumulated skin friction (load is primarily carried by the pile shaft)
Mid-shaft: Axial force decreases gradually as skin friction mobilizes along the pile-soil interface
Near the pile tip: Axial force approaches zero, with minimal end-bearing mobilization

This pattern confirms the friction pile behavior and suggests that pile length optimization should focus on maximizing skin friction rather than end-bearing development.

Engineering Practice Integration

Installation and Construction Considerations

The installation of micro steel pipe piles in deep fill areas requires careful attention to several factors:

Pipe driving method: Depending on fill density and depth, pipe piles may be driven by impact hammer, vibratory driver, or jacked into place. The driving process must avoid excessive disturbance of the surrounding fill, which could reduce skin friction capacity.
Cement slurry injection: After pipe installation, the steel pipe must be filled with cement slurry under controlled pressure to ensure complete filling and adequate bonding with the pipe wall. The injection pressure must be sufficient to overcome fill resistance but not so high as to fracture the pipe or cause excessive ground heave.
Quality control: Key inspection points include pipe verticality (≤ 1% deviation), slurry strength (28-day compressive strength ≥ 10 MPa), and pile head condition (no damage or deformation from driving).

Design Recommendations

Based on the research findings, the following design recommendations are proposed:

Pile length: Design for full mobilization of skin friction along the pile length, with a minimum embedment depth sufficient to develop adequate friction resistance.
Steel pipe selection: Given that the steel pipe bears approximately 2/3 of the load, the pipe section should be selected based on the axial capacity requirement with an appropriate safety factor.
Group pile effects: For groups of micro steel pipe piles, the group efficiency factor should be considered, particularly in soft fill where pile-soil-pile interaction may reduce individual pile capacity.
Settlement control: The gradual Q-s curve indicates that settlement will continue to develop under sustained loading. Time-dependent settlement predictions should be included in the design.

Study Insights and Reflections

This research provides valuable empirical data for the design of micro steel pipe piles in deep fill areas, a ground condition that is increasingly encountered in urban development and infrastructure projects. The finding that the confinement effect on composite section modulus is relatively modest (only 1.2 times the unconfined value) is an important design consideration, as it suggests that the composite action between steel pipe and cement slurry is less effective than might be assumed based on large-scale STC column experience.

The clear friction pile behavior, with negligible end-bearing contribution, simplifies the design approach but also highlights the dependency on pile-soil interface friction. In heterogeneous fill deposits, this friction may vary significantly with depth, requiring careful site characterization through borehole logging, standard penetration tests (SPT), and potentially pressuremeter tests.

The practical demonstration that micro steel pipe piles can achieve acceptable settlement within code limits validates this technology for use in foundation reinforcement projects. However, engineers should exercise caution when extrapolating these results to different fill compositions, depths, and loading conditions. Site-specific testing and pilot installations are strongly recommended before full-scale implementation.

The research also implicitly raises questions about the long-term performance of micro steel pipe piles in fill areas, particularly regarding corrosion of the steel pipe and degradation of the cement slurry interface over time. While the steel pipe is protected by the surrounding soil and cement slurry, the long-term durability in aggressive ground conditions (high pH, sulfates, or chlorides) should be assessed through corrosion testing and appropriate protective measures.