EN
EN
**Behro Comprehensive Consulting Engineers**
**Davood Parsapour, Dariush Mohammadi**
Indian Geotechnical Journal
**August 7, 2024**
This study emphasizes the critical role of **construction method selection** in the success of underground projects and introduces a novel construction method for **two-level metro stations in soft soils**, known as the **Pile and Concrete Arch Pre-Support System (PCAPS)**. Developed through the adaptation and refinement of the **CAPS (Concrete Arch Pre-Support) system**, originally developed for single-level stations, this method aims to control **ground deformation** and ensure **ground stability** during the excavation of large underground spaces.
In the **PCAPS**, structural elements are installed in stages before excavation of the main underground space to improve and stabilize the ground. These elements include **concrete piles** on both sides of the underground opening, **arched ribs** (roof arch beams), and, ultimately, the **final concrete lining** within the excavated space. As a **pre-support system for large underground spans**, the method is primarily implemented through **manual, staged excavation** and is classified into four methods based on different construction scenarios. In terms of the sequence of lining installation, PCAPS is divided into two categories: **“top-down lining”** and **“bottom-up lining.”**
To evaluate its performance, **Molavi Station (I7)** on **Tehran Metro Line 7** was selected as a case study, and a **three-dimensional FDM model** was employed to analyze induced settlements and compare different PCAPS scenarios. The numerical results showed good agreement with field monitoring data, demonstrating that the **structural stiffness of the piles and roof arch frames**, together with an appropriate excavation sequence and staged construction, plays a decisive role in controlling deformations. Among the scenarios, **“PCAPS 3,”** featuring multiple excavation faces operating simultaneously at the platform level, resulted in the **lowest levels of settlement and surface heave**.
The findings can be summarized as follows: **(1)** the use of structural elements prior to excavation of the station space improves and stabilizes the ground; **(2)** continuity and connection stiffness between the piles and their corresponding ribs are critical for reducing settlement and preventing damage to adjacent buildings; **(3)** the **“top-down lining” methods (PCAPS 2 and PCAPS 3)**, in which the upper tube is constructed before the lower tube, result in lower settlement; and **(4)** under the scenario involving simultaneous excavation of the **ticket hall and platform**, varying the distance between the excavation faces had no significant effect on the final ground-surface settlement.These findings provide practical guidance for selecting the **optimum construction scenario for large-scale underground stations in soft ground conditions**.
