Optimizing Tunnel Design with DIANA: A Cost-Effective Approach with Advanced Analysis

The design of precast segments plays a crucial role in ensuring structural integrity and safety of tunnel segments. Traditionally, engineers have relied on classical and experimental methods grounded in conventional material behavior and recommendations from established model codes. While these approaches provide reliability and safety, they often incorporate conservative assumptions, leading to overdesigned structures, inefficient material usage, and increased costs. As a result, there is a growing need for more advanced and optimized design methodologies that leverage modern computational tools, data-driven approaches, and material innovations to enhance both performance and cost-effectiveness.

A recent study conducted by members of AECOM and DIANA, presented at the ITA-AITES World Tunnel Congress 2023 demonstrates that taking a more detailed approach to analysis can yield significant benefits in terms of cost reduction and optimized design.

The study focuses on the design of fiber-reinforced concrete (FRC) precast segments for fire resistance in light-rail tunnels. By employing computational fluid dynamics (CFD) simulations and coupled thermo-mechanical finite element analysis (FEA), the researchers were able to develop a more realistic and optimized design approach. The full paper can be accessed through Taylor and Francis.

The Value of Detailed Analysis

Investing time into performing detailed analysis can lead to substantial cost savings and improved design efficiency. Specifically, for tunnel segment design, the traditional method of using standard fire curves can often result in an overestimation of fire loads, particularly for light-rail tunnels where the heat release rate is typically lower than in heavy rail or road tunnels.

Computations Fluid Dynamics

Computational Fluid Dynamics software (ANSYS-CFX) was used to simulate a fire event within a 6-meter diameter light-rail tunnel, which involved creating a detailed computer model of the tunnel and a light-rail train. The CFD simulation considered critical factors such as: ventilation system performance (airflow), tunnel and train dimensions as well as fire duration and heat release rate (HRR)

The CFD simulation provided a temperature-time curve (fire curve) showing how the lining temperature changes over time at the hottest location in the tunnel. The maximum lining temperature reached was 735.6°C. This fire curve is then used as input for the structural analysis.

Coupled Thermo-Mechanical Non-Linear Finite Element Analysis

DIANA was used to perform the coupled thermo-mechanical finite element analysis to assess the structural behavior of the FRC tunnel lining when exposed to the fire curve obtained from the CFD simulation. The fire curve was uniformly applied to the lining intrados, considering a 20mm reduction in the 250mm lining thickness to account for potential spalling. Temperature-dependent properties of the FRC were incorporated, including decayed compressive and tensile properties, based on Eurocode 2 and experimental results.

The lining was modeled as a non-jointed solid ring using 45,742 high-order quadrilateral plane-strain elements, and the interaction with the ground was simulated using high-order boundary interface elements. This analysis provided detailed insights into temperature distribution, stress levels, axial forces, and bending moments within the lining.

Lining Capacity & Design Check: Ensuring Structural Integrity

The final step involves verifying the structural adequacy of the FRC lining under fire conditions. This was achieved by comparing the linings capacity to resist combined axial forces and bending moments with the demands calculated from the FEA, considering both fire loads and embedment loads. A bending moment-axial force (M-N) interaction diagram was constructed using temperature-dependent material properties of the FRC, and the design was confirmed to be valid, as the combined loads fell within the capacity limits defined by the M-N diagram.

This optimized design, based on the realistic fire curve obtained from CFD analysis, avoids the overestimation and potential overdesign that would result from using standard fire curves, which predict significantly higher temperatures and lead to unjustified higher demands.

DIANA: Enabling Efficient and Comprehensive Analysis

With an emphasis on efficient model setup process and geometry import capabilities, DIANA offers a range of features that make it particularly well-suited for tunnel segment design optimization:

1. Integrated Material Models and Library: DIANA provides a dedicated material library for tunnel applications, including conventional, advanced soil, rock, and structural material models aligned with design codes. It also supports fiber-reinforced concrete (FRC) as an integrated material model.
2. Advanced Concrete Crack and Degradation Modeling: The software features advanced crack models for predicting tunnel segment performance under fire conditions (e.g., cracking and spalling) and simulates material degradation due to temperature effects based on the Model Code.
3. Coupled Thermo-Mechanical Analysis: DIANA performs coupled thermo-stress analysis to assess temperature and fire effects on concrete’s mechanical behavior.
4. Non-Linear Joint and Soil-Structure Interaction Analysis: The software enables non-linear joint modeling between segments and rings for accurate segmental tunnel lining representation under flexural loading. It also includes interface elements for realistic soil-structure interaction.
5. Comprehensive Output and Post-Processing: DIANA offers intuitive and extensive output and post-processing tools, including tabular data, contour plots, diagrams, sectional views, and temperature gradients.

What does this mean for the infrastructure projects?

The findings of this study are significantly important for tunnel engineering. By performing detailed analysis using DIANA, engineers can:

1. Optimize Segment Design: More accurate fire load assessment allows for optimization of segment thickness and reinforcement, potentially reducing material usage and costs.
2. Increase Structural Resiliency: Advanced analysis enables a better understanding of structure behavior under fire conditions, leading to more resilient designs.
3. Enhance Safety and Risk Management: Detailed analysis provides a more comprehensive understanding of fire risks, allowing for improved safety measures.
4. Meet Regulatory Requirements: As infrastructure authorities increasingly require contractors to prove code compliance, advanced analysis tools become essential for demonstrating design adequacy.

Conclusion

Performing detailed analysis using DIANA can lead to significant benefits in tunnel segment design. By moving beyond conservative standard methods and embracing more sophisticated analysis techniques, engineers can efficiently optimize their designs.

As the industry continues to evolve, the adoption of advanced analysis methods and tools is quickly becoming standard. DIANA with its comprehensive capabilities backed by a team of industry experts, is designed to help you improve upon your tunnel engineering design practices.