The application range of the earth pressure balance (EPB) shield has been expanded due to advances in optimization methods, one of which is the application of foam conditioning. This method is widely used in EPB tunnelling owing to its strong applicability in different hydrological and geological conditions. When applying the foam conditioning method under different circumstances, it is necessary to optimize the conditioning parameters. Therefore, it is necessary to propose a test procedure for evaluating foam properties and the conditioning effect. This paper proposes a procedure to assess foaming agents by the mixing test and microscopic observation of the foam and a procedure that combines the slump test, compression test, and shear test to assess the foam-conditioned soil and determine the optimal parameters of conditioning. The test method is introduced and performed on fully weathered granite from Guangzhou Metro Line 21. The test results demonstrate that the foam injection ratio and pressure and type of the foaming agent all influence the performance of the conditioned soil. Moreover, the suggested conditioning scheme is proposed, and the application of the scheme can improve the tunnelling efficiency.
There are severe challenges for slurry pressure balance tunnel boring machine (TBM) tunnelling in sandy cobble soil of Beijing, Chengdu, and Lanzhou in China. And the problems caused by tunnelling from silty clay to sandy cobble stratum are more serious. With the change of stratum, the key parameters and surface settlement will change correspondingly. Controlling the key parameters and predicting the surface settlement accurately and efficiently is important for hazard mitigation and risk management. In this study, based on the Tsinghuayuan Tunnel project in Beijing, the key parameters and surface settlement while tunnelling from silty clay to sandy cobble stratum are studied. Firstly, the difference of key parameters while tunnelling in two different strata is analyzed. The analysis shows that immediate responses to changes in the stratum are recommended in order to ensure construction efficiency. Then, a refined 3D finite difference model is developed to simulate the slurry TBM tunnelling in different strata. For refined simulation, three key parameters obtained from measurement data were applied to the 3D models, and the simulation results were compared with the field data. Results show that the refined model has good performance in terms of the accuracy and efficiency. This study provides a good engineering practice reference for slurry TBM tunnelling in mixed strata.
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