为探明三台阶开挖中半揭露溶洞对大断面公路隧道稳定性的影响，以李洞隧道为研究对象，通过模型试验和理论分析，研究不同位置、不同尺寸溶洞对隧道围岩变形及初期支护结构受力特征的影响。结果表明： 1）不同位置、不同尺寸溶洞存在下隧道拱顶沉降及周边收敛时程曲线均呈“S”形变化，大致分为初期缓慢增长—突变—增长减缓—稳定4个阶段。2）掌子面挤出随溶洞内压变化呈现“指数型”增长，具有明显的内压突变点，仰拱位置处溶洞突变内压最低、边墙位置处溶洞突变内压最高，直径越小的溶洞对应的突变内压越高。3）溶洞位置对近侧的支护结构影响较大，对于远侧的支护结构影响偏小。改变溶洞位置，隧道钢拱架左侧的轴力呈现一定的波动，而右侧的轴力趋于稳定。

In order to learn the influence of the halfdeep cave in the threebench excavation on the stability of the highway tunnel, the influence of locations and dimensions of caverns on surrounding rock deformation of primary support structure stress characteristics of Lidong Tunnel is studied by model test and theoretical analysis. The results show that: (1) The timehistory curves of settlement of the tunnel crown and the surrounding convergence show a Sshaped variation under different locations and dimensions of caverns, which can divided into 4 stages, i.e. initial slowly growing stage, mutation stage, slowly growing stage, and stable stage. (2) The extrusion of the working face changing with the internal pressure of the cavern shows an “exponential” growth, with obvious internal pressure abrupt points; the internal pressure of the cavern is the lowest at the position of the invert, and the internal pressure of the cavern is the highest at the position of the sidewall; the smaller the karst hole, the higher the internal pressure of the mutation. (3) The position of the cavern has a great influence on the support structure on the near side, and the influence on the support structure on the far side gets smaller; by changing the position of the cavern, the axial force on the left side of the tunnel steel arch shows a certain fluctuation, while the axial force on the right side tends to be stable.