<p>The instability mechanism of the tunnel face in sand-clay composite strata remains unclear, and reliable design standards for support pressure are lacking. This study therefore aims to investigate the failure modes of the tunnel face under such strata conditions. The investigation employs centrifuge model tests and numerical simulations, with the stratum interface positioned at the horizontal centerline of the tunnel face. The centrifuge tests are conducted using an apparatus equipped with a high-speed imaging system and stress measurement devices. The limit support pressure, instability zone, and variation of earth pressure at the tunnel face are examined under different strata combinations and cover-to-diameter ratios. A refined finite element model was established through numerical simulation to validate the experimental results. The results indicate that tunnel face instability in sand-clay composite strata first occurs in the sand layer. The limit support pressure ratio is controlled by the measurement points within the sand. As the cover-to-diameter ratio increases, the limit support pressure ratio decreases, while the corresponding limit support pressure increases. The influence of strata combinations on the failure mode is significant. In the sand-over-clay combination, the instability zone is larger and accompanied by noticeable surface settlement. In contrast, in the clay-over-sand combination, a stable soil arch can form, which suppresses the propagation of failure. The earth pressure analysis indicates that earth pressure release is more pronounced in the sand-over-clay combination, with more significant variations observed at monitoring points closer to the tunnel face. The results of numerical simulations are consistent with those of centrifuge tests. However, near the tunnel face, the differences in displacement and stress between the two methods increase. This discrepancy becomes more pronounced as the cover-to-diameter ratio increases.</p>