Zhuojin Pipe Fitting Co., LtdSteel Pipe Frozen Soil Composite Structure Bearing Capacity in the Gongbei Tunnel Project
Literature Overview
The paper by Hu Xiangdong, Deng Shengjun, and Wang Yang (2018), published in Chinese Journal of Geotechnical Engineering (Vol. 40, No. 8), presents model test results on the bearing capacity of steel pipe-frozen soil composite structures used in the Gongbei Tunnel of the Hong Kong-Zhuhai-Macau Bridge Zhuhai Connection. The research was funded by the National Natural Science Foundation of China (51478340) and the Ministry of Transport Construction Science and Technology Project (2013318J11300). The study investigates the relationship between freezing temperature and the composite structure's ultimate bearing capacity and deformation capacity under water-sealing conditions.
Core Technical Analysis
The pipe-curtain freezing method is a novel tunnel pre-support technique that combines pipe-jacking with ground freezing to create a waterproof pre-support structure. Large-diameter jacked pipes are installed around the tunnel perimeter, and the soil between the pipes is frozen to form a continuous frozen soil curtain. The composite structure of steel pipes and frozen soil must simultaneously satisfy two requirements: water-tightness and structural bearing capacity.
Experimental Parameters and Results
| Freezing Temperature | Frozen Soil Strength (MPa) | Deformation Compatibility | Ultimate Bearing Capacity | Deformation Capacity |
|---|---|---|---|---|
| -5°C | Low | Good | Low | Moderate |
| -10°C | Moderate | Good | High | High |
| -15°C | High | Moderate | Moderate | Low |
| -20°C | Very High | Poor | Low | Very Low |
The experimental results reveal a clear optimization window for freezing temperature. At higher temperatures (closer to zero), the frozen soil has good deformation compatibility with the steel pipes but insufficient strength. At lower temperatures (further from zero), the frozen soil has high strength but poor deformation compatibility, leading to premature failure of the composite structure. The optimal freezing temperature of approximately minus 10 degrees Celsius was identified as providing the best balance between strength and deformation compatibility.
Limit State Criteria
The study proposed a limit state criterion for water-sealing conditions that considers both the displacement and the load capacity of the composite structure. The criterion is based on the intersection of the load-displacement curve with a defined deformation threshold, ensuring that the structure can withstand the design loads while maintaining water-tightness.
Steel Pipe Manufacturing and Installation Considerations
The steel pipes used in the pipe-curtain freezing method must meet stringent manufacturing and installation requirements to ensure the integrity of the composite structure. The following table summarizes the key requirements:
| Requirement | Specification | Rationale |
|---|---|---|
| Pipe grade | Q345 or equivalent | Adequate strength for composite action |
| Wall thickness | 14 to 20 mm | Sufficient for pipe-jacking and structural loads |
| Pipe straightness | Within 1/1000 of length | Ensures uniform contact with frozen soil |
| Butt weld quality | Full-penetration, 100 percent UT | Prevents water leakage at joints |
| Surface finish | Clean, free of scale | Promotes bonding with frozen soil |
| Dimensional tolerance | Within ±1 percent of nominal | Ensures uniform gap between pipes |
The welding of the steel pipe joints is critical to the overall performance of the composite structure. Any leakage at the pipe joints would compromise the water-tightness of the frozen soil curtain, rendering the entire pre-support system ineffective. The butt welds must be inspected using ultrasonic testing to ensure full penetration and absence of internal defects such as lack of fusion, porosity, and slag inclusions.
Welding Procedure for Pipe Joints
The welding procedure for the steel pipe joints in the pipe-curtain freezing method should follow these principles:
- Preheat the pipe ends to 50 to 80 degrees Celsius to prevent hydrogen-induced cracking in the heat-affected zone.
- Use GTAW for the root pass to ensure full penetration and a clean, defect-free root.
- Use SMAW or GMAW for fill and cap passes with low-hydrogen consumables.
- Maintain interpass temperature below 200 degrees Celsius to limit HAZ softening.
- Apply post-weld heat treatment for pipes thicker than 16 mm to relieve residual stresses.
- Inspect 100 percent of butt welds using ultrasonic testing to Level II or higher.
Engineering Practice Case Study
The Gongbei Tunnel project provided a unique opportunity to apply the pipe-curtain freezing method in a marine environment with high groundwater pressure. The steel pipes were manufactured to API 5L Grade X65 specifications with a nominal diameter of 3.6 meters and wall thickness of 18 mm. The pipe joints were welded using a multi-pass procedure with GTAW root and SMAW fill and cap passes. The welding procedure was qualified according to NB/T 47014 and the welders were certified according to NB/T 47013.
During installation, the pipes were jacked into the ground using hydraulic jacks with a maximum thrust of 12000 kN. The freezing process was initiated after the pipe installation was completed, with brine circulated through a network of freezing pipes installed between the jacked pipes. The freezing temperature was controlled to maintain the frozen soil at approximately minus 10 degrees Celsius, as recommended by the experimental study. The actual construction experience confirmed that this temperature provided optimal performance in terms of both water-tightness and structural stability.
Study Insights and Reflections
This research provides valuable guidance for the design and construction of steel pipe-frozen soil composite structures in tunnel engineering. The identification of an optimal freezing temperature of minus 10 degrees Celsius is a significant finding that has direct practical implications for construction planning and cost management. From a steel pipe manufacturing perspective, the study highlights the importance of dimensional accuracy and weld quality in ensuring the composite action between the steel pipes and the frozen soil. Engineers should recognize that the performance of the composite structure is not solely determined by the properties of the individual components but by the interaction between the steel pipes and the frozen soil, which is highly sensitive to the freezing temperature. Future research should focus on long-term performance monitoring and the development of predictive models for the composite structure behavior under varying environmental conditions.