Zhuojin Pipe Fitting Co., LtdDeformation Analysis of Underground Buried Pressure Steel Pipes During Concrete Placement Without Internal Support
Literature Overview
This study by Wang Jianguo, Wu Hegao, Shi Changzheng, Peng Peng, and Peng Zhixiang (2016), published in Water Power, examines the deformation behavior of ultra-large diameter underground buried pressure steel pipes during the concrete encasement phase, specifically investigating the scenario where internal supports are removed. The research is supported by the National Natural Science Foundation of China (Grant 51409194) and involves collaboration between Wuhan University's State Key Laboratory of Water Resources and Hydropower Engineering Science and Chengdu Alang Technology Co., Ltd.
Finite Element Analysis Methodology
The numerical analysis considers two concrete placement scenarios: simultaneous horizontal rising on both sides of the pipe, and differential placement with a height difference between the two sides. The finite element model incorporates the steel pipe shell elements, concrete solid elements, and the interaction between them through contact algorithms. The deformation patterns are analyzed at various stages of concrete placement height.
Key modeling parameters include:
| Parameter | Typical Value | Notes |
|---|---|---|
| Steel pipe diameter | Ultra-large (project-specific) | Thick-walled design for high pressure |
| Concrete placement rate | Variable | Two scenarios compared |
| Height differential (asymmetric case) | Several meters | Worst-case condition |
| Steel grade | Q235/Q345 | Typical for pressure pipes |
| Concrete grade | C30-C40 | Surrounding cast-in-place |
| Analysis type | 3D nonlinear FEA | Large deformation considered |
Core Findings and Deformation Patterns
The results demonstrate that the maximum deformation occurs at the pipe bottom in both placement scenarios, which is consistent with the asymmetric lateral earth and concrete pressure distribution. The deformation magnitude remains within the limits specified by applicable codes for steel pipe deformation control. Importantly, the arrangement of internal supports and tie rods has minimal influence on the overall deformation pattern of the pipe.
The axial stresses in internal supports and tie rods are found to be relatively small, indicating that these elements do not significantly contribute to deformation control. This finding has substantial implications for construction methodology and project economics.
Engineering Recommendations and Practice Integration
The primary engineering recommendation is to eliminate internal supports while retaining tie rods as positional restraint measures. This approach offers several advantages:
- Facilitation of automated welding: Without internal supports obstructing the welder access, automated welding equipment can operate continuously around the pipe circumference, improving weld quality consistency and reducing labor costs.
- Improved construction efficiency: Removing internal supports eliminates the need for support installation, adjustment, and removal operations, which can consume significant construction time for ultra-large diameter pipes.
- Cost reduction: The elimination of internal support systems, including materials, labor, and equipment for their installation and removal, results in meaningful project savings.
- Reduced risk of interference: Internal supports can interfere with the automated welding process, causing weld defects such as undercut, incomplete fusion, or porosity due to access restrictions.
From a welding engineering perspective, this recommendation is particularly significant. Automated welding of ultra-large diameter pressure pipes requires unobstructed access to the weld seam, precise torch positioning, and continuous travel along the pipe circumference. Internal supports create geometric obstructions that force the welding system to deviate from optimal parameters, potentially compromising weld integrity. The elimination of these supports allows for uninterrupted automated welding cycles, which is essential for maintaining consistent weld bead geometry and penetration depth.
Quality Control Considerations
While the study supports removing internal supports, quality assurance measures must be maintained. The tie rods must be designed to prevent any axial displacement or rotation of the pipe sections during concrete placement. Monitoring of pipe deformation during the concrete placement process should be implemented through strain gauges and displacement sensors at critical locations, particularly at the pipe bottom where maximum deformation occurs. The welding sequence should be coordinated with the concrete placement schedule to ensure that welds are completed before significant concrete pressure develops.
This research provides strong technical justification for streamlined construction methodologies in large-scale hydropower projects, where the integration of fabrication, welding, and installation processes can be optimized to reduce overall project duration and cost while maintaining structural integrity.