Introduction
Urban metros are vital arteries of modern cities, but their long-term performance can be challenged by large-scale ground works nearby. A growing body of research examines how unilateral excavation—where the soil is removed on one side of an underground structure—affects the stability and serviceability of adjacent metro stations. This article synthesizes experimental findings and numerical analyses that shed light on failure risk, ground movement, and mitigation strategies for existing metro stations during unilateral large-scale excavation.
Background and Relevance
Metro stations built below city streets encounter complex geotechnical environments. Excavations undertaken near these structures can cause asymmetric deformations, soil migration, and increased stress concentrations in support systems. The risk of wall or lining instability, settlement, or excessive deformations is particularly pronounced when excavation pits extend beneath shallow or poorly compacted strata. The literature combines experimental studies with finite element analyses to understand the interplay between excavation geometry, soil stiffness, rock mass behavior, and structural response.
Methodology: Experimental and Numerical Approaches
Researchers often employ a dual approach. First, controlled laboratory or field experiments simulate the excavation process to observe ground response and potential failure modes. Key metrics include lateral earth pressures, settlement patterns, and deformation of lining segments. Second, finite element modeling (FEM) replicates unilateral excavation to evaluate stress redistribution, potential yielding, and stability factors for metro station linings and surrounding soils. Calibration against real-world case studies, such as deep excavations near urban rail structures, enhances confidence in predictions.
In practice, the experimental setup varies from scaled physical models to in-situ monitoring during actual construction. Instrumentation may include surface and borehole extensometers, pore pressure sensors, inclinometers, and strain gauges on lining elements. FEM studies typically adopt material models that capture nonlinear soil behavior, such as elastoplastic or rate-dependent constitutive laws, to accurately reflect post-yield response under asymmetric loading.
Key Findings and Risk Indicators
Across multiple studies, several risk indicators emerge for unilateral large-scale excavation near metro stations:
- Asymmetric ground movement tends to increase near the excavation face, aggravating differential settlement of the station and adjacent tunnels.
- Stiff, well-compacted soils tend to delay the onset of damage, while loosening or soft layers promote earlier yielding of lining segments.
- Soil-structure interaction governs the distribution of stresses in the lining; localized overstress can trigger crack initiation or progressive failure if reinforcement or supports are inadequate.
- Support systems rated beyond minimum design criteria, combined with adaptive excavation sequencing, reduce the likelihood of unexpected failures.
Unilateral excavation can produce distinct failure mechanisms, including leaning, bending, and shear failure of lining segments, along with ground surface settlement and potential pore-pressure buildup in saturated layers. These phenomena underscore the need for careful risk assessment before, during, and after excavation works near metro stations.
<h2Implications for Design and Mitigation
Practical implications center on prevention, monitoring, and rapid response planning. Recommendations commonly include:
- Enhanced ground investigation to characterize soil stiffness, layering, and groundwater conditions along the excavation corridor.
- Conservative load path assessment in FEM, incorporating worst-case scenarios of asymmetrical loading and potential soil weakness.
- Incremental excavation sequences with continuous monitoring to detect early signs of distress in the metro lining or surrounding soils.
- Reinforcement strategies such as ground improvement, temporary supports, or jet-grouting to stabilize the near-field ground and reduce differential movements.
Adopting these measures can help city authorities and project teams minimize disruption, extend the service life of existing metro assets, and safeguard passengers during nearby large-scale excavation activities.
Case Context: Bangkok MRT and Global Insights
Case studies referencing Bangkok MRT and related deep excavation projects illustrate how high-stress environments interact with deep-stored infrastructural elements. While local geology and construction practices differ, the underlying principles of perimeter stabilization, soil-structure interaction, and prudent sequencing translate across contexts. Integrating experimental observations with robust numerical simulations enables engineers to quantify failure risk more accurately and design more resilient station linings.
Conclusion
Unilateral large-scale excavation near existing metro stations poses measurable failure risks that manifest through asymmetric ground movements and reinforcement stresses. A combined experimental and FEM approach offers actionable insights for predicting, mitigating, and managing these risks. With careful site characterization, conservative modeling, and proactive monitoring, cities can uphold the safety and reliability of metro systems while allowing essential urban excavation work to proceed.
