Investigation of the Interface Effect on Cutting Force and Failure Characteristics of Soil–Rock Composite Ground During Shield Tunneling

Soil-rock composite strata are frequently encountered during shield tunneling for underground projects such as metros, railways, and other underground building facilities. The strong heterogeneity and abrupt variations in soil and rock mechanical properties often induce excavation instability, thereby adversely affecting construction safety and efficiency. Based on an urban shield tunneling project, the interface effects on cutting force and the failure characteristics of soil–rock composites under shield cutter penetration are investigated in this study using the finite–discrete element method (FDEM). Firstly, the FDEM input parameters are calibrated, after which the cutting force, crack number, acoustic emission (AE), breaking process, displacement, and stress fields with different interface inclinations (a) during penetration are analyzed. The influence of the interface strength reduction factor (λ) on the cutting force and failure mode is then discussed. The results indicate that the cutting force–penetration curves can be classified into two phases: in stage I, the cutting force grows smoothly, and some cracks are initiated in the sandstone layer with a few AE events; in stage II, the cutting force drops with violent fluctuation, and both tensile and shear cracks develop rapidly in the sandstone layer, accompanied by many AE events. Generally, cracks mainly propagate in the sandstone layer, and less shear failure occurs in the silty clay layer. As a increases, the peak force of cutter A increases, while that of cutter B decreases. Additionally, tensile cracks between the cutters are connected when a is 30° and 45°, resulting in rock bridge breakage. However, a larger λ leads to a greater peak cutting force. The total crack length tends to decrease with increasing α and λ with an a of 30°, indicating that larger interface inclinations and higher interface strengths are associated with less damage to the composite specimens. These findings provide new insights into the fragmentation mechanisms of soft–hard composite ground and can contribute to the efficient construction of underground building and transportation infrastructure.