The Significance of Bentonite in Achieving Optimal Conditioning Design for EPB tunneling through rock mass Under Groundwater

Document Type : Research Paper

Authors

1 School of Geology, College of Science, University of Tehran, Tehran, Iran

2 DIATI, Politecnico di Torino, Turin, Italy

3 RahsazTarh Consulting Engineers, Tehran, Iran

4 Colorado School of Mines, Golden, Colorado, United States of America

Abstract

Earth pressure balancing (EPB) technology was initially developed for mechanized tunneling in soft soils rich in fines. Recently, it has been successfully applied in coarse-grained soils and even rock formations, reducing the need for slurry shield machines and overcoming challenges related to hydraulic transportation and separation plants. EPB tunnel boring machine (TBM) performance depends significantly on the proper conditioning of the excavated material, whether soil or rock, by adding appropriate additives to achieve the desired properties. However, varying geological conditions and excavated material characteristics present challenges, necessitating different conditioning strategies. This paper explores the role of bentonite in transforming excavated rock material into a suitable support medium, particularly in groundwater environments. The study employs slump tests and pressurized permeability cell tests (PPCT) to assess bentonite's effectiveness in improving the workability and permeability of conditioned mixtures. Results from 400 slump tests reveal that bentonite can convert loose, cohesionless material into a uniform plastic paste. However, accurately adding dry bentonite in a lab setting is challenging and nearly impossible during actual excavation. Conditioning with bentonite in pre-made suspension form mitigates issues related to powder handling, although the conditioned material still exhibits a relatively high unit weight. In 22 PPCT trials, bentonite-conditioned mixtures showed strong resistance to water pressure up to 5 bar. Notably, the mix becomes impermeable before fully transforming into a plastic paste. A statistical analysis of slump values led to the development of an empirical model demonstrating bentonite's performance in conditioning excavated rock material.

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Article Title [Persian]

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  • Receive Date: 03 September 2024
  • Revise Date: 23 October 2024
  • Accept Date: 21 December 2024

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