Zhuojin Pipe Fitting Co., LtdSteel Tube Concrete Support System for Extremely Soft Rock Roadways
Literature Overview
Gao Yanfa and colleagues from the China University of Mining and Technology (Beijing) published this study in China Coal (2014, Vol. 40, No. 11), addressing the challenging support problem in extremely soft rock roadways within the marine coal-bearing strata of Beizao Coal Mine. The research was supported by the National Natural Science Foundation Key Project (51134025). The paper presents a comprehensive composite support scheme based on steel tube concrete arches, combining field measurements, laboratory testing, and numerical monitoring.
Geological Conditions and Problem Definition
The marine coal-bearing strata of the Beizao Coal Mine present unique challenges: the surrounding rock exhibits extremely low strength, high water content, rapid deformation rates, and large deformation magnitudes that are difficult to control with conventional support methods. The original in-situ stress was measured on site, and laboratory tests characterized the rock mechanical parameters and hydrological properties. These data formed the basis for the support design.
The fundamental engineering challenge is that the rock mass undergoes rapid and continuous convergence deformation after excavation, often exceeding the design capacity of standard steel arch supports. Traditional bolt-mesh-concrete lining systems fail because the rock pressure continues to increase beyond the lining's capacity, leading to progressive failure of the support structure.
Support Design Parameters
| Component | Specification | Purpose |
|---|---|---|
| Main steel tube | 194 mm diameter x 8 mm wall thickness, 20# seamless steel pipe | Primary load-bearing arch |
| Concrete infill | C40 grade | Confine steel tube, distribute load |
| Arch cross-section | Circular | Optimize stress distribution under multi-directional pressure |
| Auxiliary support | Wire mesh + 400 mm thick concrete lining | Secondary containment and water control |
| Support type | Composite (steel tube concrete + lining) | Multi-stage load resistance |
The selection of 20# carbon structural steel seamless pipe with 194 mm outer diameter and 8 mm wall thickness represents a carefully considered balance between structural capacity, manufacturability, and economic feasibility. The seamless construction eliminates the weld seam weakness that would be present in ERW or HFW tubes under the extreme and sustained loading conditions expected in these roadways.
Monitoring Results and Performance Verification
Post-installation monitoring tracked both the stress state of the support arches and the roadway convergence deformation over time. The results demonstrated that the composite support system based on steel tube concrete arches effectively controlled roadway convergence, maintaining long-term stability. The steel tube concrete arch serves as the primary load-bearing element, while the surrounding concrete lining provides secondary containment and waterproofing.
The design philosophy follows a "strong support, early installation" principle adapted for extremely soft rock conditions. The steel tube concrete arch provides immediate and high-capacity support upon installation, while the concrete lining is applied after the initial rapid deformation phase, when the rock pressure has partially stabilized.
Engineering Practice Considerations
From a steel pipe manufacturing perspective, several quality requirements are critical for this application. The 20# seamless steel pipe must meet strict dimensional tolerances to ensure proper assembly and load distribution. The 8 mm wall thickness requires precise control during the hot rolling and cold drawing processes to avoid wall thickness variation that could lead to localized buckling. Hydrostatic testing per GB/T 24511 or equivalent standards is essential to verify the absence of internal defects that could propagate under sustained loading.
The concrete infill process requires careful attention to ensure complete filling without voids. The high strength C40 concrete must be placed in layers with adequate compaction to achieve uniform density. Any voids in the concrete infill would create stress concentrations in the steel tube, potentially leading to premature local buckling. The interface between the steel tube and concrete infill must achieve adequate bond strength, which may require surface treatment of the tube interior.
Study Insights and Reflections
This case study illustrates the effective application of steel tube concrete composite structures in extreme geotechnical environments. The seamless steel pipe selection is particularly appropriate given the sustained and potentially cyclic loading conditions. Engineers designing similar support systems should consider the long-term creep behavior of the steel tube under sustained compressive and bending loads, as well as the potential for corrosion in the wet marine coal-bearing environment. The composite approach—combining the high strength-to-weight ratio of steel tubes with the load-distributing capacity of concrete infill—represents an elegant solution to the challenges of extremely soft rock roadways. Future work should address the long-term durability and maintenance requirements of such systems in aggressive groundwater conditions.