Composite Support Technology Using CFST Braces in Faulted Fractured Zone Roadways

Literature Overview

This study by Wang Jun, Wang Zhengze, Ding Hougang, Guo Bijun, and Gu Xueqing, published in the Tunnel Construction journal in 2018, addresses a challenging ground support problem in underground mining engineering: the design of composite support systems for roadways passing through faulted fractured zones. The research was conducted at Shandong Jianzhu University in collaboration with Shandong Yingcai University and Shandong Shenbo Roadway Support Technology Co., Ltd., and was supported by multiple National Natural Science Foundation grants (Nos. 51704176, 51474218, 51404105) and a doctoral fund project (XNBS1403). The study focuses on the -650 South Wing Track Roadway in Yangcheng Coal Mine, where significant ground deformation and prolonged deformation duration posed serious challenges to roadway stability.

Core Technical Findings

The research employed a combined approach of indoor testing, theoretical calculation, and support practice to develop an effective support system for faulted fractured zone roadways. The key innovation is the "bearing ring reinforced support" design method based on CFST braces, which provides a composite support system with enhanced load-bearing capacity.

The experimental and theoretical analysis demonstrated that the CFST brace support reaction of 0.92 MPa exceeds the estimated ground pressure, and the composite support system's total bearing capacity of 1.37 MPa provides an adequate safety margin. The support practice confirmed that the φ194 mm × 8 mm shallow arch circular CFST brace composite support scheme effectively controlled roadway deformation, suppressed floor heave, and ensured long-term stability of the -650 South Wing Track Roadway.

Interpretation of Technical Points

The research addresses a fundamental challenge in underground support engineering: how to provide adequate resistance against large and prolonged ground deformation in weak, fractured ground conditions. Faulted fractured zones present unique challenges because the ground pressure is not only higher than in intact rock but also continues to develop over extended periods as the fractured rock mass gradually yields and compacts.

The CFST brace provides several advantages over conventional steel arch supports in this application. First, the concrete fill provides additional stiffness and load-bearing capacity, allowing for thinner steel tubes that are more economical. Second, the composite action between steel tube and concrete provides better resistance to local deformation and punching, which is critical in fractured ground where point loads may develop. Third, the circular cross-section provides uniform resistance in all directions, which is advantageous for the multi-directional ground pressure typical of faulted zones.

The "bearing ring" concept is particularly innovative. Rather than treating the support as a series of independent arches, the design creates a continuous load-bearing ring that distributes ground pressure around the entire roadway perimeter. This approach is more effective in fractured ground where pressure distribution is irregular and may shift over time. The CFST braces function as the primary load-bearing elements, while the surrounding concrete and rock provide secondary support through confinement and friction.

Process and Standards Analysis

The fabrication of CFST braces for underground support requires careful attention to welding quality and dimensional accuracy. The circular steel tubes are typically fabricated from steel plate through rolling and welding, with the longitudinal seam weld being the critical quality control point.

The welding of the longitudinal seam in the CFST tube should follow applicable standards such as GB/T 985 for welding preparation and AWS D1.1 for structural welding. The weld should be designed for full penetration to ensure uniform strength around the circumference. Non-destructive testing (NDT) of the weld should include visual inspection (VT) and radiographic testing (RT) or ultrasonic testing (UT) depending on accessibility.

The concrete placement inside the steel tube requires special consideration for underground conditions. The concrete must be pumpable to accommodate the typical pumping distances in mining operations, while maintaining adequate workability for placement and vibration. Self-compacting concrete (SCC) may be preferred to ensure uniform fill without the need for internal vibration, which is difficult to achieve in confined spaces.

Integration with Engineering Practice

The proposed support system has direct application in mining roadways, tunnel construction, and underground infrastructure projects in weak or fractured ground conditions. The CFST brace composite support method offers several practical advantages over conventional support systems:

• Higher load-bearing capacity per unit weight compared to hollow steel arches
• Better resistance to local deformation and punching
• Improved durability due to concrete protection of steel tube
• Potential for prefabrication and rapid installation
• Adaptability to varying ground conditions through design modification

However, practical implementation requires careful consideration of several factors. The installation of CFST braces in underground conditions may require specialized equipment for handling and positioning. The concrete placement operation must be carefully controlled to ensure complete fill and avoid voids. The connection between CFST braces and the surrounding ground (through grouting or mechanical anchors) is critical to the overall system performance.

From a cost perspective, the CFST brace system may initially appear more expensive than conventional steel arches, but the reduced need for additional support measures and the improved long-term stability may result in lower total life-cycle costs. The decision should be based on a comprehensive cost-benefit analysis that includes construction costs, maintenance costs, and the cost of potential roadway failure.

Key Questions and Reflections

A significant question raised by this research is the long-term performance of the CFST brace composite support system under sustained ground pressure and environmental exposure. While the initial support capacity is adequate, the long-term behavior under creep and fatigue loading requires further investigation. The concrete fill may degrade over time due to moisture ingress and chemical attack, potentially affecting the composite action between steel tube and concrete.

Another consideration is the effect of dynamic loading from mining activities on the support system performance. In mining environments, blasting and equipment operation generate dynamic loads that may affect the support system differently from static ground pressure. The design should account for dynamic amplification factors and ensure adequate ductility to accommodate dynamic deformation.

The research also raises questions about the optimal CFST brace spacing and configuration for different ground conditions. While the study focuses on a specific application, the design principles should be adaptable to varying ground pressure magnitudes and directions. Engineers should develop design guidelines that allow for systematic selection of CFST brace specifications and spacing based on ground classification.

Study Insights and Implications

This research demonstrates the effectiveness of CFST brace composite support systems in challenging underground support applications. The combination of steel tube strength and concrete fill stiffness provides a robust support solution for faulted fractured zones where conventional support methods may be inadequate.

For welding and fabrication professionals, this study highlights the importance of ensuring high-quality steel tube fabrication, particularly the longitudinal seam weld, as the overall support performance depends on the integrity of the steel tube. Any weld defects or dimensional inaccuracies can compromise the load-bearing capacity of the CFST brace. The study reinforces the principle that composite structural performance is only as good as its weakest component.

The research underscores the importance of integrated design approaches that combine structural analysis, material science, and construction technology to develop effective support systems for challenging ground conditions. The success of the proposed system in practice validates the theoretical design approach and provides confidence for its application in similar projects.

Reference Value and Outlook

The research provides valuable guidance for the design of underground support systems in faulted fractured zones, an area of increasing importance as mining operations extend into more challenging ground conditions. Future research should investigate the long-term performance of CFST brace support systems under various environmental conditions, develop simplified design procedures for practical engineering applications, and extend the study to different roadway geometries and ground conditions. The development of performance monitoring systems for CFST brace support would further enhance the reliability and safety of underground support systems.