Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Defective Piles Strengthened by Steel Pipe and Grouting Method
Overview of the Research Topic
This study addresses a critical practical problem in foundation engineering: the rehabilitation of defective bored piles through steel pipe wrapping combined with cement grouting. In field conditions, piles frequently exhibit defects such as incomplete concrete filling, honeycombing, or low-quality concrete zones that compromise their structural integrity. The research investigates how a steel pipe jacket, when installed around the defective pile section and filled with high-strength grout, restores and potentially enhances the axial compressive capacity of the pile. From a steel pipe manufacturing perspective, the key concern is the selection of appropriate pipe grade, wall thickness, and welding details to ensure the jacket functions as a composite structural element rather than a simple wrapper.
Key Technical Parameters and Design Considerations
The performance of the steel pipe-grouting composite system depends on several interrelated parameters that must be carefully controlled during design and fabrication.
| Parameter | Typical Range | Engineering Significance |
|---|---|---|
| Steel pipe grade | Q235, Q345, Q390 | Determines yield strength and ductility of the confining jacket |
| Wall thickness | 6–12 mm | Governs hoop confinement pressure and ultimate load capacity |
| Grout compressive strength | 30–60 MPa | Provides internal core strength and bond with pipe inner surface |
| Interface bond stress | 0.5–2.0 MPa | Controls composite action between steel pipe and grout core |
| Defect depth ratio | 0.1–0.6 (defect depth / pile diameter) | Primary variable affecting residual capacity before rehabilitation |
| Axial strain at failure | 0.003–0.012 | Indicates ductility improvement after strengthening |
The steel pipe selection must consider that the jacket will be subjected to radial expansion forces from the internal grout under axial compression. A pipe with too thin a wall may buckle inward under high confinement pressures, while an excessively thick pipe may be economically unjustifiable. Q345-grade ERW or HFW pipe with 8–10 mm wall thickness typically provides an optimal balance for most rehabilitation scenarios.
Welding and Fabrication Details
The longitudinal and circumferential welds on the steel pipe jacket are critical load-bearing elements. Any welding defect—such as undercuts, incomplete fusion, or porosity—will create stress concentration points that can trigger premature failure of the entire composite system. The following welding quality criteria are recommended:
- Longitudinal welds should achieve full penetration with a minimum throat thickness of 0.7 times the thinner leg or wall thickness, whichever is smaller.
- Circumferential splices must be double-sided welds with 100% ultrasonic testing (UT) coverage per GB/T 11345.
- Preheating temperature of 100–150 °C is advisable for wall thicknesses exceeding 10 mm to minimize cold cracking susceptibility.
- Post-weld heat treatment (PWHT) at 580–620 °C for Q345 and Q390 grades should be considered when the jacket will be exposed to cyclic loading or low-temperature environments.
Mechanism of Load Transfer and Failure Modes
The composite system works through three mechanisms: (1) the grout core carries the majority of axial compression; (2) the steel pipe provides lateral confinement that prevents brittle crushing of the grout; and (3) the interface friction and mechanical interlock transfer shear between the two components. Failure typically initiates at the grout core when the confined grout reaches its ultimate strain capacity, followed by outward bulging of the steel pipe. If the interface bond is weak, partial debonding may occur before the steel pipe reaches its full confinement potential, leading to a brittle failure mode that reduces the actual capacity below theoretical predictions.
Engineering Practice Implications
In field applications, the grouting process must be carefully controlled to ensure complete filling of the annular space between the defective pile surface and the steel pipe. Vacuum-assisted grouting or pressure grouting with a 0.5–1.0 MPa injection pressure is recommended to eliminate voids. The steel pipe should be cut to match the pile diameter with a clearance of 5–15 mm to allow grout flow. Field experience shows that defects located near the pile tip are more critical than those near the pile head, as the former have less surrounding soil confinement to assist load transfer. Engineers should perform load tests on at least one rehabilitated pile before proceeding with full-scale rehabilitation programs.