Assessment of Twin Metro Tunnels Settlement
- Oct 13, 2023
- 4 min read
Updated: 6 days ago
Evaluating Excavation Impact and Mitigation Measures with DeepEX
Introduction
Excavating in close proximity to existing infrastructure is one of the most critical challenges in urban geotechnical engineering.
This becomes even more demanding when the adjacent structures are operational metro tunnels, where even small deformations can affect safety, serviceability, and project timelines.
In such cases, the key question is not only whether the excavation is stable, but:
How will it affect the existing tunnels — and how can that impact be controlled?
Numerical analysis, combined with a detailed representation of construction stages, provides a powerful approach to predicting and mitigating these effects.
In this case study, DeepEX is used to evaluate the behavior of existing twin metro tunnels in Shanghai, affected by a nearby deep excavation for a commercial development. The analysis focuses on tunnel response, soil deformation, and the effectiveness of implemented ground improvement measures.
Figure 1: a)Top view of the excavation site b)section I-I and c) II-II as presented in [1]
Figure 2: Table of soil properties as presented in [1]
Project Conditions and Challenges
The excavation is performed in weak, compressible soils, while the twin metro tunnels remain fully operational throughout construction.
This creates a high-risk scenario where:
Ground movements must be strictly controlled
Tunnel deformation must remain within acceptable limits
Construction must proceed without disrupting metro operation
To address these challenges, ground improvement measures were implemented prior to excavation.
These include:
Pumping consolidation (precompression) to reduce pore water pressure and improve soil stiffness
Soil–cement mixing piles around the tunnels and near the diaphragm walls
These measures aim to increase soil strength and reduce deformation, effectively stabilizing the ground before excavation begins.
Figure 3: a) Section with strengthening affected areas according to [1] b) layout of soil-cement mix piles [1]
Numerical Modeling in DeepEX
The excavation and tunnel system are modeled in DeepEX using the Finite Element Method (FEM) to capture the interaction between soil, retaining structures, and existing tunnels.
A key aspect of the model is the accurate simulation of the strengthening measures. This is achieved using the DeepEX soil zone functionality, where improved ground regions are represented with equivalent material properties.
The model follows the actual construction sequence:
Initial conditions prior to excavation
Implementation of ground improvement measures
Progressive excavation and support installation stages
This staged approach allows the analysis to reflect the real evolution of stresses and deformations throughout the project.
Figure 4: a) Stage 1- Site prior to the excavation b) Stage 2 – Strengthening measures
The DeepEX soil zone option is used for the simulation of the strengthening measures. In Stage 2, prior to the initiation of the excavation, two equivalent soil zones of the soil mix piles and pumping strengthening zones are defined with the deepex soil zone tool, and equivalent Mohr coulomb properties are assigned to the soil zones. The remaining stages follow the construction sequence described in [1].
Analysis Results
The results provide a detailed picture of how the excavation affects both the retaining system and the existing tunnels.
The FEM analysis captures:
Stress distribution in diaphragm walls and tunnels
Axial forces and bending moments along tunnel linings
Horizontal and vertical soil displacements
Displacement contours clearly illustrate how ground movements develop and propagate toward the tunnels. The influence of the strengthening measures is evident, as they reduce both the magnitude and extent of deformation.
A key part of the study is the comparison between numerical predictions and field measurements. The calculated wall deflections show strong agreement with monitored data, confirming the reliability of the modeling approach.
Figure 5: Moment, axial force and displacement results for the tunnels and retaining walls
Figure 6: Horizontal displacement contour ux
Figure 7: Vertical displacement contour uz
Figure 8: Analysis result comparison with wall deflection measurements
Key Insights
This case study highlights several important observations for similar urban excavation projects.
First, excavation-induced movements can significantly affect nearby tunnels, particularly in weak soils. Without proper mitigation, even small deformations can become critical.
Second, ground improvement plays a decisive role. Techniques such as precompression and soil mixing can substantially reduce deformation and improve system stability when properly modeled and implemented.
Finally, staged FEM analysis is essential. Simplified approaches cannot capture the complex interaction between excavation, soil, and existing infrastructure. Accurate prediction requires modeling both the construction sequence and the improved soil conditions.
Conclusion
Excavations near existing metro tunnels require a careful balance between construction progress and infrastructure protection.
This study demonstrates that, with proper modeling and the inclusion of ground improvement measures, it is possible to predict and control tunnel response effectively. DeepEX provides a powerful platform for simulating these complex interactions, enabling engineers to assess risks, validate design decisions, and ensure safe construction in challenging urban environments.
References
[1] Z. F. Hu, Z. Q. Yue, J. Zhou, and L. G. Tham, “Design and construction of a deep excavation in soft soils adjacent to the Shanghai Metro tunnels,” Can. Geotech. J., vol. 40, no. 5, pp. 933–948, 2003.
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