Abstract <p>Mud cake formation, which impacts construction efficiency, is prone to occur when a slurry shield passes through viscous formations. In this study, the viscosity reduction effect of dispersants was investigated, with a focus on the stability of the excavation face. The viscosity reduction mechanism was revealed through interactions with the double electric layer/solute/apparent structure of the clay. The presence of polycarboxylate (PCAC) at a concentration of 1.5% increased the risk of mud cake formation because of its viscosity-enhancing properties. Additionally, the inclusion of tetrasodium pyrophosphate (TSPP) increased the risk of excavation face instability because it led to an excessive reduction in viscosity. The mechanism by which dispersants reduced viscosity was revealed on the basis of two factors. (1) The effectiveness of viscosity reduction was directly proportional to the repulsion strength of the double electric layer, whereas its intensity was determined by the energy barrier. For example, the S-type reagent exhibited superior viscosity reduction owing to the increased concentrations of Na + and increased pH values in the solute, thereby increasing the thickness of the double electric layer and the maximum repulsion force. (2) Changes in apparent structure would not necessarily lead to a decrease in viscosity. For instance, the presence of PCAC could increase the particle size distribution to more than 10&#xa0;μm, whereas the irregular entanglement of large particles could increase the viscosity. This research provided valuable insights into the development of new dispersants and mud cake treatments for engineering applications.</p> Graphical abstract <p></p>