EN
EN
**Houman Rouhani, Kourosh Shahriyar, Dariush Mohammadi**
**14th Iran Tunnelling Conference**
**October 31, 2022**
The construction of underground metro stations is typically subject to significant time and financial constraints, particularly when the progress of a **Tunnel Boring Machine (TBM)** is ahead of schedule while the station is not yet ready for the TBM to enter. A notable example of this situation occurred at **Station S05 on Tabriz Metro Line 2**, where the early arrival of the TBM placed the station excavation and support works under severe time pressure. To address this situation, the designers were required to implement part of the station’s **excavation support system** in an unconventional manner, whereby one row of **struts** was omitted and the foundation at the bottom elevation of the excavation was constructed as a replacement. This modification resulted in **multi-stage excavation behavior**, with a significant difference in elevation between the bracing members, requiring the excavation to be analyzed accordingly.
In this study, several numerical models were developed in **PLAXIS 2D** to accurately simulate the actual behavior of the excavation and determine the appropriate level of **stress relief** during the excavation stages. The numerical results were compared with **instrumentation data and project monitoring observations**. The primary objective was to determine a reliable value for the **“stress relief parameter”** in the modeling of staged excavation.Comparison of the numerical models showed that a value of **0.46** provided the best agreement with the actual project data and the most accurate prediction of **horizontal pile displacement**. The results also demonstrated a precise and reliable **quadratic relationship between the degree of stress relief and the maximum horizontal displacement of the piles**.
The findings of this study emphasize that, in projects with conditions similar to **Station S05**, omitting one row of struts and replacing it with the **foundation** can significantly reduce the construction time and cost of the station, provided that the numerical analyses are performed using appropriate parameters. Furthermore, the high modeling accuracy achieved with a **stress relief value of 0.46** demonstrates that selecting an appropriate **staged design strategy** plays a key role in controlling displacements and maintaining the safety of **excavation support systems** in the **cut-and-cover method**.
