Introduction
Urban rail systems are lifelines in modern cities, yet their safety can be challenged when nearby large-scale excavations unfold. This article synthesizes experimental insights and finite element (FE) analyses to understand the failure risk of existing metro stations during unilateral, large-scale excavation. Drawing on established research in soil-structure interaction and case studies from major metro projects, we outline how engineers evaluate stability, identify critical parameters, and implement mitigation strategies to safeguard underground infrastructure.
Background: Why Excavation Proximity Matters
Unilateral excavation—where only one side of a soil mass is excavated—creates asymmetric ground movements and stress redistributions. For shallow to deep metro stations, such ground disturbances can induce excessive lateral earth pressures, foundation settlements, rocking moments, and potential failure modes in surrounding structures. Prior studies, including finite element analyses of deep excavations in dense urban soils, show that the interaction between excavation geometry, soil type, and structural stiffness governs the response of underground assets.
Experimental Approaches to Risk Assessment
Experimental investigations typically combine field measurements, laboratory tests, and scaled tests to quantify ground settlement, pore pressure development, and the dynamic response of nearby structures. Key data include:
- Ground reaction curves and stiffness degradation under excavation loads
- Settlement and tilt profiles at and beyond the excavation edge
- Interaction forces between retaining structures and soil mass
These data feed into calibration of constitutive models for soils and boundaries for FE simulations. In practice, researchers evaluate the risk of plastic yielding, excessive sway, or soil liquefaction, all of which could compromise a metro station’s safety envelope during unbalanced excavation operations.
Finite Element Analysis: A Key Tool
FE analysis provides a controlled environment to explore various excavation geometries, soil profiles, and structural configurations. A representative approach includes:
- Modeling the station envelope using nonlinear, elastoplastic soil models with appropriate density, friction angles, and cohesion values.
- Simulating unilateral excavation by applying boundary conditions that reflect asymmetric loading and ground movement.
- Assessing foundation response: settlements, rotations, bending moments, and seat-out risks for columns and deck structures.
- Conducting parametric studies to identify critical parameters such as wall stiffness, excavation depth, and relative distance to the station box.
Case studies—such as FE analyses of deep excavations near Bangkok MRT align with practical experiences in dense urban centers—illustrate how excavation sequencing and support systems influence risk trajectories. The goal is to predict failure precursors and design mitigations that keep loads within the station’s allowable capacity.
What the Experimental and FE Insights Imply for Design
Engineers can leverage these insights in several practical ways:
- Define safe excavation envelopes by combining experimental settlement limits with FE-predicted stress states.
- Specify robust retaining structures and diaphragm walls to resist asymmetrical ground pressures.
- Adopt monitoring strategies that track deviations from predicted behavior, enabling early intervention.
- Incorporate adaptive mitigation, such as staged excavation, grouting, or ground improvement, in response to observed responses.
These steps help ensure that nearby metro stations maintain structural integrity and service continuity throughout adjacent construction activities.
Conclusion
Unilateral large-scale excavation presents meaningful risk to existing metro stations if not properly analyzed and mitigated. By integrating experimental observations with finite element simulations, engineers can quantify potential failure modes, determine critical design thresholds, and implement strategies that safeguard urban rail infrastructure. This integrated approach—grounded in field data and robust modeling—offers a practical pathway to maintaining safety while enabling necessary urban development.
Further Reading
For readers seeking deeper technical detail, investigations into deep excavation case studies and related FE methodologies provide a solid foundation for understanding ground-structure interaction in metropolitan contexts. Reference works include experimental studies of soil-structure failure near urban underground facilities and FE analyses of Bangkok MRT-like projects.
