Constructing large-scale underground infrastructure is a complex challenge, especially in areas with abundant water and sandy soil. This study explores the intricate surface settlement patterns caused by building a quasi-rectangular shield tunnel in the rich water-sand layers of Zhengzhou, China. The findings provide valuable insights into managing the impacts of such tunneling projects, which are crucial for preserving the stability and safety of the surrounding environment. Tunnel construction is a crucial aspect of modern urban development, and this research offers practical solutions for engineers tackling similar challenges worldwide.
Tackling the Complexities of Quasi-Rectangular Tunnel Construction
Building large-scale underground infrastructure, such as metro systems, is a significant engineering feat, especially in areas with abundant water and sandy soil conditions. The construction of quasi-rectangular shield tunnels, which allows the formation of dual-line tunnels in a single pass, is a groundbreaking approach that aims to maximize space utilization and minimize disruption to the surrounding environment.
Monitoring Surface Settlement Patterns
Researchers from Henan University of Technology and Zhengzhou University of Technology conducted a detailed study on the Zhengzhou Metro Line 8 project, which involved constructing a large-section quasi-rectangular shield tunnel in the rich water-sand layers of the city. By closely monitoring the surface settlement patterns during different stages of the construction process, the team was able to uncover valuable insights.
Four Distinct Stages of Surface Settlement
The study revealed that the surface settlement caused by the quasi-rectangular shield tunnel construction exhibited a distinct temporal pattern, with four distinct stages:
1. Slow Settlement (Stage I): Before the shield machine arrived, the surface experienced gradual settlement due to changes in pore water pressure and soil consolidation.
2. Rapid Settlement (Stage II and III): During the shield’s passage and when the shield tail exited the construction area (14.4 – 18 m), the surface experienced rapid settlement, caused by factors such as over-excavation, soil shear, and compression.
3. Stable Settlement (Stage IV): In the later stages, the surface settlement gradually stabilized as the disturbance caused by the shield’s advancement dissipated, leading to soil consolidation and creep settlement.
Effective Settlement Control Measures
To mitigate the surface settlement challenges, the researchers proposed several control measures:
1. Balancing Excavation Rates: Maintaining a balance between the cutter face pressure and soil pressure by adjusting the excavation rate can help control the settlement in front of the shield.
2. Reducing Soil Friction: Injecting low-friction lubricating mud between the shield shell and the sand layer can help reduce soil friction and shear slip, minimizing surface settlement during the shield’s passage.
3. Optimizing Grouting: Increasing the synchronous grouting volume at the shield tail and adjusting the grouting pressure can help fill the construction gaps more effectively, reducing settlement after the shield’s exit.
4. Controlling Shield Attitude: Paying close attention to the shield machine’s rotation and maintaining proper attitude can help mitigate the squeezing effect on one side of the soil, reducing uneven surface settlement.
Predicting Surface Settlement with Peck Formula
The study also demonstrated the feasibility of using the Peck formula to predict surface settlement troughs caused by the construction of large-section quasi-rectangular shield tunnels in the Zhengzhou Fushui sand layer. The researchers achieved a high linear correlation coefficient (R2 = 0.983) when fitting the Peck formula to the observed settlement data.
Paving the Way for Sustainable Urban Development
This comprehensive research on the surface settlement patterns and effective control measures during quasi-rectangular shield tunnel construction provides valuable insights for engineers and urban planners. By understanding the complex dynamics of soil-structure interactions, they can better navigate the challenges of building large-scale underground infrastructure in areas with challenging geological conditions, ultimately contributing to the sustainable development of modern cities.
Author credit: This article is based on research by Yong-gang Ding, Cheng Huang, Shi-ju Ma, Kai-rong Hong, Qi-keng Xu, Miao-jun Yan.
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