Investigating the role of bentonite on the conditioning of muck during tunneling through marly limestone with EPB machines

With advancements in manufacturing technology, Earth pressure balance (EPB) machines are successfully applied in coarse-grained soils and even all types of rocks. The performance of EPB machines, whether in soil or rock, significantly depends on the proper conditioning of the excavated materials. This study aims to investigate the role of bentonite on the optimal conditioning of graded marly limestone grains, considering their mineralogical properties. To achieve this goal, 98 slump tests and 9 under-pressure permeability (UPP) tests were conducted to evaluate the effectiveness of bentonite in improving workability and reducing permeability of the conditioned muck (marly limestone). The slump test results indicate that when the fine content ( C F ) in the samples is 4.5%–8.5%, bentonite slurry concentration ( C B ) is about 7.5%–10%, and bentonite slurry injection ratio is around 30%–40%, conditioned materials turn into an optimal plastic paste. Additionally, the results show that optimally conditioned mixtures obtained by adding bentonite suspension exhibited significant resistance to water infiltration under high pressures (up to 0.7 MPa) during the 9 UPP tests.