Risk assessment of mud cake formation in alternating soft-hard strata using modified soil index classification boundaries: a case study of the Shenzhen Mawan Cross-Sea Tunnel

In subsea shield tunneling projects, cutterhead clogging induced by mud cake formation is a critical factor causing construction interruption, particularly when tunneling through alternating soft–hard strata beneath the seabed. Conventional risk assessment methods based on standard soil index boundaries are often inadequate, as they fail to capture the complex interactions between soil consistency and cutterhead operation, resulting in unreliable early-stage predictions. To overcome this limitation, this study develops a practical and quantitative approach for mud cake formation risk evaluation in subsea environments by integrating revised soil index boundaries with multi-factor analysis. Laboratory Atterberg limit tests and improved mixing tests were conducted on representative clay-sand mixtures to systematically redefine the classification ranges of the plasticity index and liquidity index under alternating soft–hard conditions. By measuring the adhesion of different soil samples under different water contents, the resulting risk classification system defines four mud cake formation risk levels. Building on these revised criteria, a structured assessment system incorporating two geological factors and four construction-related factors was established, and their relative contributions were quantified using the Analytic Hierarchy Process (AHP). The results demonstrate that the modified index boundaries significantly improve the discrimination of mud cake formation risk levels compared with conventional criteria. Among the considered factors, the plasticity index plays a dominant role (49.5%), followed by the no-opening area ratio of the cutterhead center (17.3%) and the liquidity index (16.5%), indicating that clogging is governed by the combined effects of soil plasticity, flow characteristics, and cutterhead structural configuration. This quantitative weighting further reveals the relative sensitivity of mud cake formation risk to both geological and operational controls. The application to the Shenzhen Mawan Cross-Sea Tunnel project—a typical subsea infrastructure—demonstrates the practical value of the proposed approach. Unlike conventional assessments, the method successfully identifies previously undetected medium-risk zones where significant stratum adhesion and reduced excavation efficiency were observed. This case validation confirms that integrating revised soil index boundaries with multi-factor analysis enables a more sensitive and reliable early-stage mud cake formation risk assessment in complex composite strata beneath marine environments.