To investigate the stability of the excavation face during tunnel traversal through an upper-sand-lower-clay composite stratum

centrifugal model tests and numerical simulations were combined to analyze the displacement variation in instability zones

profile characteristics of final instability zones

earth pressure evolution patterns

and ultimate support pressure under different stratigraphic boundary positions and burial depth ratios. Test results indicate: Significant instability occurs when the stratigraphic boundary is at the tunnel face center

while stability is maintained when the boundary is at the tunnel crown. Displacements concentrate in the upper sandy layer with negligible changes in the clay layer

demonstrating that initial instability disturbance influences subsequent instability zone development. Analysis of normalized vertical earth pressure and excavation face retreat displacement curves reveals that increased burial depth ratios and clay layer thickness enhance formation resistance to disturbances. Support pressure ratio-displacement curves for two instability cases exhibit three distinct stages

with the upper side central point of the excavation face reaching ultimate support pressure first. When the burial depth ratio increases from 1.0 to 1.5

the ultimate support pressure shows minimal change. 3D finite element simulations of the excavation process validate the ultimate support pressure

failure patterns in instability zones

and earth pressure evolution

with numerical results showing good agreement with experimental data.