Experimental Investigation of Tunnel Face Blowout under Different Cover Ratios and Sand Densities

Face stability is one of the most important issues in shield tunneling. Insufficient support pressure may cause active collapse, whereas excessive support pressure may induce passive-side blowout. Although several studies have investigated face collapse from experimental, theoretical, and numerical perspectives, studies focusing on blowout are limited, and the failure mode and shape of the failure zone have not yet been completely clarified. This study investigated the effects of relative density and soil cover ratio on blowout behavior using three-dimensional half-tunnel tests. Ground deformation was analyzed via particle image velocimetry. The results showed that ground deformation was concentrated ahead of the tunnel face, and the failure zone progressively propagated toward the ground surface as extrusion progressed. Moreover, the width of the failure zone tended to increase with increasing relative density. Face pressure measurements indicated that face pressure increased with increasing relative density and soil cover ratio because of the greater shear resistance and vertical effective stress in the surrounding ground. Furthermore, a larger soil cover reduced surface displacement. Finally, the failure zone was modeled using wedge and rectangular blocks, and theoretical values were derived using the limit equilibrium method. The theoretically calculated values showed reasonable agreement with the experimental results under the shallow-cover, dry-sand conditions investigated in this study.