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| Experimental study on the influence of cobble blocking on surface settlement in shield tunneling through cobble-boulder stratum |
| DONG Ruixing1, LI Yang1,2*, ZHANG Dong3, WANG Limin3, MA Qianli3, ZHAO Hongyan3, DONG Hongyu3
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1. Changsha University of Science and Technology, Changsha 410001, Hunan, China;
2. Industry Key Laboratory of Traffic Infrastructure Security Risk Management, Changsha 410001, Hunan, China;
3. Beijing Construction Engineering Group Civil Engineering Co., Ltd., Beijing 100055, China |
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Abstract Shield tunneling in cobble-boulder stratum was frequently accompanied by significant ground disturbance and a high risk of surface collapse. For the prediction and mitigation of this problem, empirical methods were primarily relied upon in current practice, leaving a notable gap in quantitative early-warning research on stratum instability. Based on the slurry shield tunneling project of the Beijing Subway Line 1 Branch, laboratory model tests were conducted to investigate the impact of cobble blocking in front of the cutterhead on surface settlement patterns and the evolutionary process of stratum collapse. The results indicated that, influenced by the large-particle granular skeleton and discontinuous medium characteristics, the cobble-boulder stratum exhibited a significant delayed surface settlement phenomenon. This evolutionary process could be broadly divided into four stages: slow development, rapid increase, temporary stability, and collapse failure. A ground loss ratio of approximately 7% to 10% was observed in the cobble-boulder stratum, significantly exceeding that of conventional sandy strata. Furthermore, this ratio continuously increased as the volume of stagnant boulders increased. When the boulder stagnation rate exceeded a critical threshold of 30%, surface collapse was highly likely to be triggered, typically occurring after the shield machine passed the corresponding monitoring point by a distance of approximately 0.5 D (where D is the tunnel diameter). The accumulation of large-diameter boulders in front of the cutterhead profoundly affected the shield's disturbance range; stagnant boulders exacerbated over-excavation, causing prominent cavities directly above the excavation face and in the surrounding stratum, and substantially reducing overall stability. These findings provided quantitative early-warning indicators and theoretical support for mitigating surface collapse induced by cobble blocking in similar engineering projects.
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Published: 07 July 2026
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