Dottorato in INGEGNERIA STRUTTURALE E GEOTECNICA

Titolo della tesi: Impact of a large-diameter shallow twin-tunnel excavation on adjacent high speed rail bridges and related protective measures

Tunnel construction and its impact on free-field and adjacent structures is a very important topic in geotechnical engineering. Recently, shield tunneling technology has advanced, leading to the development of tunnels with larger diameters and shallower depths. At the same time, the rapid expansion of high-speed railways (HSR) has resulted in a substantial increase in the number and length of HSR bridges. As a result, a growing number of newly constructed shield tunnels face the challenge of crossing existing HSR bridges. Shallow depths and large diameters are associated to significant impacts related to tunnel construction, while HSR bridges impose strict displacement control requirements. Notably, the project involving the Suzhou Tongjing Road Tunnel crossing the Shanghai-Nanjing Intercity HSR Bridge in Suzhou represents the first example of a large-diameter twin-line shield tunnel crossing an existing HSR bridge in China and the world. Although scholars have examined the issue of shield tunnels crossing HSR bridges, their focus has primarily been on small-diameter shield tunnels, particularly subway tunnels. Large-diameter tunnels exhibit distinct characteristics due to increased soil loss and disturbance, as well as reduced spacing between the tunnel and the bridge. These factors may result in different impacts of tunnel construction on existing HSR bridges, still not investigated. Additionally, there is a lack of sufficient case histories and research outcomes to establish the necessity of protecting existing bridges by appropriate protective measures. To address these gaps, a systematic investigation on the impact and possible protection measures of twin-line shallow-depth large-diameter shield tunnels crossing existing HSR bridges is carried out, using the Suzhou Tongjing Road Tunnel crossing the Shanghai-Nanjing Intercity High-Speed Railway Bridge as a case study. Additionally, an improved tunnel deformation pattern is proposed to obtain more realistic soil displacements, particularly for shallow and large tunnels. The primary methodology employed in this paper is three-dimensional finite element analysis, utilizing the Hardening Soil Model with Small-Strain (HSsmall) to simulate soil behavior. The main contents of each chapter of the paper are as follows: Chapter 2: provides the research background and a comprehensive literature review relevant to the topic discussed in this thesis; Chapter 3: introduces the case history of Tongjing road Tunnel project and a novel construction technique for steel casing concrete injection piles under low clearance conditions. The technique is specifically developed to protect high-speed railway bridge piles with limited pier height; Chapter 4: establishes a three-dimensional numerical model for simulating the construction process of large-diameter shallow-depth shield tunnels. The accuracy of the model is verified, and various parameters, including tunnel diameter, depth, excavation pressure, synchronous grouting pressure, volume loss rate, and tunnel spacing, are analyzed to understand the characteristics of tunnel construction impacts in open space; Chapter 5: presents a new non-uniform tunnel convergence model that considers the independent and asymmetric contraction of the upper and lower parts of the tunnel. The simulation results obtained from Chapter 4 are compared and validated using this convergence model, enabling the determination of appropriate parameters for shallow-depth large-diameter tunnels; Chapter 6: establishes a comprehensive three-dimensional finite element model that incorporates both bridge pile foundations and isolation piles. The model considers the solid elements representing the bridge piles and isolation piles, as well as the soil-structure interface. Parameter analyses are conducted to investigate the effects of tunnel crossing on high-speed railway bridges, including tunnel parameters, construction parameters, bridge parameters, and isolation pile parameters. The aim is to optimize the isolation pile protection scheme; Chapter 7: focuses on the analysis of field monitoring results from the engineering project of Suzhou Tongjing Road Tunnel crossing the Shanghai-Nanjing Intercity High-Speed Railway Bridge. The aim is to validate the effectiveness of the isolation scheme during the construction of isolation piles and the crossing of the shield tunnel; Chapter 8: summarizes the main research findings of the paper.