Bearing Capacity Analysis of Straight Wall Semi-Circular Arch Steel Tube Concrete Supports in Deep Roadways

Literature Overview

This paper by Chen Bin and colleagues from Guizhou University and Central South University, published in the journal "Mining and Metallurgical Engineering" in 2021 (Vol. 41, No. 1, pp. 11-15), investigates the bearing performance of steel tube concrete (CFST) supports for deep mining roadways. The research was supported by the National Natural Science Foundation of China (Grants No. 51964007 and 51774101), Guizhou Provincial High-Level Innovative Talent Cultivation Project, and Guizhou Provincial Mining Dynamic Disaster Early Warning and Control Technology Innovation Talent Team Project. The study uses the 270 sublevel roadway in a Guizhou mine as the background, comparing conventional U36 steel supports with CFST supports through ANSYS finite element analysis, with particular focus on horizontal stress conditions.

Technical Context and Problem Statement

Deep mining roadways face increasingly severe ground pressure conditions as mining depth increases, particularly in the Guizhou karst region where complex geological conditions and high horizontal stress are common. Traditional U-shaped steel supports, while widely used, have limited bearing capacity and are prone to large deformations under high horizontal stress conditions. The steel tube concrete support represents an innovative structural solution that combines the advantages of steel tube confinement and concrete infill: the steel tube provides tensile strength and confinement, while the concrete provides compressive strength and mass. This composite action significantly enhances the overall bearing capacity compared to either material alone.

The specific geometry studied is a straight wall with semi-circular arch configuration, which is a common cross-section for mining roadway supports. This shape provides good load distribution characteristics for both vertical and horizontal stress conditions, with the arch portion effectively transferring horizontal pressure to the walls and the straight wall portion providing lateral support.

Numerical Analysis and Key Findings

The ANSYS three-dimensional finite element model was constructed to simulate the deep roadway support system under realistic stress conditions, including high horizontal stress ratios. The model compared two support types: conventional U36 steel supports and CFST supports with equivalent cross-sectional dimensions. The horizontal stress coefficient (the ratio of horizontal to vertical stress) is a critical parameter in deep mining geomechanics, as it determines the dominant loading mode on the support structure.

The key finding is that the CFST support has a bearing capacity far exceeding that of the U36 steel support. More importantly, when the horizontal stress coefficient exceeds 1 (indicating horizontal stress dominance), the CFST support exhibits smaller deformation at the base angle compared to the U-shaped steel support. This is particularly significant because base angle deformation is a primary indicator of support failure in deep roadways, and excessive base angle deformation can lead to progressive failure of the entire support system.

Engineering Practice Implications

For mining engineers and ground control specialists, this research provides strong evidence for adopting CFST supports in deep roadway applications where high horizontal stress is expected. The selection of support type should be based on the stress ratio and the required safety factor, with CFST supports preferred when the horizontal stress coefficient exceeds unity.

From a fabrication and installation standpoint, CFST supports require a different construction approach compared to conventional U-shaped steel supports. The steel tube sections must be fabricated and erected first, followed by concrete placement and curing. In underground mining conditions, this sequential construction process presents practical challenges: concrete placement in confined underground spaces requires careful attention to workability, vibration, and curing conditions. The steel tube must be clean and free of debris before concrete placement to ensure proper bond between the steel and concrete, which is essential for achieving the composite action that provides the enhanced bearing capacity.

The welding of steel tube segments in the support fabrication process must follow appropriate welding procedures to ensure joint integrity. For CFST support applications, the welds should be designed to achieve full strength, with attention to HAZ toughness and residual stress control. Standards such as NB/T 47015 for pressure vessel welding or relevant mining industry standards should be referenced for weld procedure qualification and inspection requirements.

Study Insights and Reflections

From a structural engineering perspective, the enhanced performance of CFST supports can be attributed to the composite action mechanism: the steel tube confines the concrete under triaxial compression, which increases the concrete's effective compressive strength and ductility, while the concrete prevents local buckling of the steel tube. This mutual confinement effect is well-established in the literature on CFST columns and has been successfully applied to mining support applications in this study.

However, several practical considerations should be noted. First, the finite element model assumes perfect bond between steel and concrete, which may not be fully achieved in underground construction conditions. Surface preparation of the steel tube interior, including cleaning and possibly roughening, is essential to ensure adequate bond. Second, the curing conditions for concrete in underground roadways may differ significantly from laboratory conditions, potentially affecting the achieved concrete strength and bond quality. Third, the long-term durability of CFST supports in the aggressive underground environment, which may include moisture, chemical exposure, and mechanical impact, requires further investigation.

The research also highlights the importance of considering horizontal stress effects in support design. Traditional support design methods often focus primarily on vertical load, but in deep mining conditions, horizontal stress can be the dominant loading factor. Engineers should ensure that their design methodology adequately accounts for the actual in-situ stress state, including the stress ratio, and select support types accordingly.

Summary

This paper provides compelling evidence through finite element analysis that steel tube concrete supports offer substantially superior bearing capacity and deformation control compared to conventional U36 steel supports in deep mining roadways subject to high horizontal stress. The composite action mechanism between the steel tube and concrete infill is the fundamental reason for this enhanced performance, and the practical application of CFST supports in deep roadway ground control is strongly supported by the findings. Mining engineers should consider CFST supports as a preferred solution for deep roadway support design, while paying attention to the practical challenges of underground concrete placement, steel tube surface preparation, and long-term durability in the mining environment. The research contributes valuable technical guidance for the safe and efficient support of deep mining roadways in complex geological conditions.