Designing a Nine-story Residential Building above Subway Tunnels in West Bucharest: Finalist of Bentley Geotechnical Engineering YII 2020 awards Featured

*Nine-Story Residential Building project from SAIDEL Engineering presentation on Bentley's YII 2020 Conference*

In western Bucharest, Romania, a residential building is being constructed over existing subway tunnels. The structure, which will cost an estimated EUR 2.5 million, includes nine floors and a basement. However, for the first time a structure was being built over subway tunnels within the city. The project will unlock potential future investments at other areas over tunnels in Bucharest, a city where buildable grounds are becoming a rarity.

Nine-Story Residential Building project from SAIDEL Engineering presentation during Bentley's YII 2020 Conference

SAIDEL Engineering was tasked with providing structural and geotechnical design for the residential building. The geotechnical design had to be approved by the subway operator. The objective was to reduce the overall effect of the building on the tunnel lining, by providing a safe and cost-efficient foundation solution.

For the geotechnical complex analysis, SAIDEL Engineering selected PLAXIS software. Initially, a plane strain analysis was performed regarding feasible solutions for both excavation and foundation. The excavation solution was accepted at that stage by the subway operator. In order for the geotechnical design to be approved as well, 3D models were developed for improved accuracy and optimization, and the results were compared against the plane strain analyses to provide further confidence in the project’s foundation.

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In order to approve the project, the subway operator set some requirements regarding tunnel displacements and structural forces. Various solutions were analyzed, such as a simple raft or raft on piles with various diameters and lengths, with or without base enlargement. The case of using a 15cm thick layer of Storyfoam, under the raft and between the pylons, in order to encase the subway tunnels, was also analyzed.

For the soil structure interaction, non-linear analysis was employed, using the advanced constitutive soil model "Hardening Soil with Small-Strain Stiffness", a unique feature of PLAXIS.

Given the significance of the project, SAIDEL Engineering increased the safety of the solution, employing PLAXIS 3D analysis. Due to the size of the model and the desired increased accuracy, several modeling related issues arose. Modeling the piles and pier loads were some of the obstacles to overcome. For the piles, the prevailing option was the usage of hexagonal prisms. For the pier loads, the rigid bodies feature of PLAXIS was used, to ensure convergence. The submitted analysis provided confidence to the subway operator, that the 10-story building project will have minimal influence on the tunnel lining.

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The high accuracy analysis provided by SAIDEL engineering using PLAXIS software lead to the approval of the project. The investor gained confidence by the analysis, resulting to additional investments aimed at reducing the overall influence on the tunnel lining. That way, the irregularity of the footprint was reduced. The analyses results led to a decision on reduction of the number of stories from 10 to 9, thus reducing the seismic loads acting on the tunnels as well. Furthermore, it allowed the reduction of the thickness of the raft from 2m to 1.5m.

Source: Bentley Systems Announces Finalists in the Year in Infrastructure 2020 Awards Program