复合地层对于隧道工程施工开挖面的稳定性控制具有重要影响。针对复合地层隧道开挖面变形规律问题，利用中小型电机加载试验系统，通过相似模型试验，采用数字图像相关（digital image correlation, DIC）方法，进行围岩变形时空演变过程分析，并建立力学模型，探究上软下硬隧道围岩及开挖面的变形失稳机制。结果表明：上软下硬复合地层隧道围岩变形空间分布特征是隧道上方围岩首先发生变形，拱顶产生沉降，内部产生水平裂缝并不断扩展，在荷载作用下隧道开挖面产生应力集中，导致开挖面及内部发生坍塌，隧道最终失稳破坏；根据试验现象观测，提出将不同破坏阶段的隧道围岩简化为相应模型，揭示了上软下硬复合地层隧道开挖面变形破裂演化机制；上软下硬复合地层隧道开挖时，软岩强度起到决定性作用，软岩开挖面破裂大于硬岩，拱顶沉降大于拱底沉降，因此在选择支护方式时应重点考虑软岩部分。

Composite strata have an important effect on the stability control of excavation face in tunnel engineering. Aiming at the deformation regularity of tunnel excavation face in composite strata, the spatio-temporal evolution process of surrounding rock deformation was analyzed through similarity model test, digital image correlation(DIC) measurement method and medium-sized loading test system, and mechanical model was established to explore the deformation and instability mechanism of the surrounding rock and excavation face of the tunnel with soft upper and hard lower. The results show that the spatial distribution characteristics of surrounding rock deformation of tunnel in the composite strata is that deformation first occur at above surrounding rock of tunnel, the arch produces settlement, the horizontal cracks occur the inside and continue to expand, and the stress concentration occurs on the tunnel excavation face under load, which lead to collapse of excavation face and inside, and final instability failure of tunnel. Based on experimental observation, the model of tunnel surrounding rock in different failure stages is simplified to reveal the deformation and failure evolution mechanism of tunnel excavation face in soft upper and hard lower strata. The strength of soft rock plays a key role in tunnel excavation in soft upper and hard lower strata. The failure of soft rock excavation surface is greater than that of hard rock, and the settlement of arch roof is greater than that of arch bottom. Therefore, the soft rock part should be considered when choosing the support method.