Design Aspects of Under-Passing a City’s Heritage Landmark with EPB Machines Under Low Overburden; The Case of Chandpole Gate in Jaipur Metro, India

UG1B is the first underground Metro Line under construction in Jaipur which is the capital of Rajastan (India). The twin bore tunnel route pass through the historical centre of the old city of Jaipur called “Pink City”. Significant historical monuments of world’s heritage, such as Hawa Mahal, Jantar Mantar, Isarlat tower, Tripolia Gate (main entrance to the city palace) and Chandpole Gate, are within the greater influence zone of the project. Specifically, the twin bore TBM tunnel passed 4.5 m under the foundation of Chandpole Gate which is one of the main significant historical entrances to the old city, almost 300 years old. The paper addresses the main design aspects of the under-passing of Chandpole Gate in Jaipur by two EPB TBM machines, and introduces the new method which is used for designing these shallow tunnels, by combining the usage of the analytical solution of Massinas and Sakellariou (Géotechnique 59:691–701, 2009) with detailed numerical analysis; the innovative closed form solution has been applied to assess the required support pressure, whereas its results have been used as input to 3-D numerical analyses of tunnel excavation below the Gate to predict induced settlements. The route from the evaluation of the geotechnical model, to the study of the structural and foundation system of the monument, the approach to the necessary tunnel face retaining pressure to be maintained by the EPB machines during under passing and the prediction of the surface and Gate settlement due to tunneling operations is introduced and analyzed. Monitoring of TBM operation parameters and settlement measurements during construction are presented in the form of comparative diagrams and graphs, to illustrate the application of the main design recommendations and verify the design predictions, in terms of applied face pressure and resulting settlements of the Chandpole Gate. Comparison has shown that main recommendations, as derived from the new proposed method of design and followed during the tunnelling operations, resulted to a successful under-passing of Chandpole Gate, with induced settlements within the predicted, permissible limits.