Buckling Characteristics of Lining Steel Pipes in Composite Lining Under Complex Constraints

Literature Overview

This paper by Wang Hanhui, Su Kai, Zhang Cunhui, and Wang Boshi from Yangtze River Survey, Planning and Design Institute, Wuhan University, and Suzhou Science and Technology City Management Committee investigates the buckling instability characteristics of the lining steel pipe in composite lining under external water pressure. Published in the Journal of Hydroelectric Engineering in 2023, the study is supported by the National Natural Science Foundation of China (Grant No. 51879207) and the National Key Research and Development Program (Grant No. 2016YFC0401803). The authors calculate the elastic critical external pressure of the stiffened steel pipe based on the Mises theory, establish a three-dimensional numerical model using ABAQUS considering the differential constraint strength of self-compacting concrete, material nonlinearity, and the non-uniformity of the surrounding medium, and compare the results with the in-tunnel bare pipe scheme and the buried pipe scheme.

Core Technical Findings

The study demonstrates that the constraint provided by self-compacting concrete can enhance the external pressure resistance of the steel pipe, while the presence of drainage boards reduces the external pressure resistance. The Mises theory-based design method recommended by the code is applicable for the external pressure stability design of steel pipes under full peripheral constraint. However, after local drainage board installation in the composite lining, the critical buckling pressure of the steel pipe is approximately consistent with the in-tunnel bare pipe scheme, and both exhibit multi-wave instability. The authors recommend that the constraint effect should not be considered in the external pressure design of the composite lining steel pipe.

Interpretation of Technical Points

The Mises theory is a classical approach for calculating the critical external pressure of circular steel pipes under external pressure. The theory assumes that the pipe is a thin-walled circular cylinder under uniform external pressure and that the material behaves elastically. The critical external pressure is the pressure at which the pipe becomes unstable and undergoes buckling deformation. The Mises theory provides a conservative estimate of the critical external pressure because it does not account for the constraint effect of the surrounding medium.

The composite lining system consists of an inner steel pipe, a self-compacting concrete layer, and an outer drainage board layer. The self-compacting concrete provides a confining pressure to the steel pipe, which enhances the external pressure resistance by delaying the onset of buckling. However, the local installation of drainage boards creates a weak zone in the composite lining, where the constraint effect of the self-compacting concrete is reduced. This weak zone can lead to a significant reduction in the external pressure resistance of the steel pipe, as the constraint effect is not uniform along the pipe circumference.

The multi-wave instability mode observed in the study indicates that the buckling deformation is not a single-wave mode but rather a multi-wave mode with multiple lobes around the pipe circumference. This mode is characteristic of thin-walled circular cylinders under external pressure and is influenced by the boundary conditions, the material properties, and the external pressure distribution. The number of waves in the buckling mode is determined by the balance between the bending stiffness and the membrane stiffness of the pipe wall.

The recommendation not to consider the constraint effect in the external pressure design of the composite lining steel pipe is a conservative approach that ensures safety. By ignoring the constraint effect, the design is based on the bare pipe critical external pressure, which is lower than the actual critical external pressure under constraint. This conservative approach accounts for the uncertainty in the constraint effect and the potential for local weakening due to drainage board installation.

Integration with Engineering Practice

In the design of underground tunnels and pipelines, the external pressure resistance of the lining steel pipe is a critical design parameter. The steel pipe must be able to withstand the external water pressure without undergoing buckling instability. The composite lining system offers the advantage of combining the high strength and ductility of the steel pipe with the confining effect of the self-compacting concrete. However, the local installation of drainage boards for water management can compromise the constraint effect and reduce the external pressure resistance.

The Mises theory-based design method is widely used in engineering practice because of its simplicity and conservatism. However, the study demonstrates that the Mises theory may not be accurate for composite lining systems with local drainage boards, where the constraint effect is not uniform. Engineers should use the Mises theory as a baseline and then apply a safety factor to account for the potential reduction in external pressure resistance due to the local drainage board installation.

The three-dimensional numerical model developed in this study can be used for detailed analysis of the buckling behavior of the composite lining steel pipe. The model accounts for the differential constraint strength of the self-compacting concrete, the material nonlinearity, and the non-uniformity of the surrounding medium. By simulating the buckling process, the model can predict the critical external pressure and the buckling mode, providing valuable information for the design and optimization of the composite lining system.

Key Questions and Reflections

A key question is how the local drainage board installation affects the long-term performance of the composite lining system. The drainage board creates a weak zone in the composite lining, and the long-term performance of this weak zone under cyclic external water pressure loading is uncertain. Engineers should consider the fatigue behavior of the steel pipe and the self-compacting concrete in the weak zone and ensure that the system can withstand the expected number of loading cycles without failure.

Another important consideration is the effect of construction quality on the constraint effect of the self-compacting concrete. The self-compacting concrete must be placed and compacted uniformly to provide a consistent constraint effect. Any voids or defects in the self-compacting concrete layer can reduce the constraint effect and lead to a lower external pressure resistance. Engineers should implement rigorous quality control measures to ensure the uniformity and integrity of the self-compacting concrete layer.

Study Insights and Implications

This paper provides a comprehensive analysis of the buckling characteristics of the lining steel pipe in composite lining under complex constraints. The study demonstrates that the constraint effect of the self-compacting concrete can enhance the external pressure resistance, but the local installation of drainage boards can significantly reduce this effect. The recommendation to ignore the constraint effect in the external pressure design is a conservative approach that ensures safety. Engineers should use the Mises theory as a baseline and apply appropriate safety factors to account for the uncertainty in the constraint effect. The three-dimensional numerical model developed in this study is a valuable tool for detailed analysis and optimization of the composite lining system, and it should be used in conjunction with the Mises theory-based design method to ensure the safety and reliability of the design.