By analyzing the large deformation events that occurred in the Zhegushan tunnel of the Wenchuan-Maerkang expressway, the rock failures are divided into three categories:plastic flow of soft rock, bending deforma-tion of the plate girder and slipping of the structural surface. A comparison of the two-step excavation method and three-step excavation methodwas performed for a 50-meter-long test section with a typical rock mass. The results show that for the two-step excavation method, the advance rate and construction space between the left and right side of the lower bench have a significant effect on tunnel stability. Based on a comparison of the tunnel's peripheral displacement, contact pressure between the surrounding rock and the initial support (as well as the internal force of the steel arch) for the two-step and three-step construction methods, it is found that for the two-step excavation method, the tunnel's peripheral deformation, the pressure between surrounding rock and the initial support are rela-tively large since the one-pass excavation section is very large and the pressure of the rock mass increases too quick-ly, and the safety reverse of the steel arch is not enough. For tunnels in complex regions or environments, the re-served deformation volume of the rock mass should be adjusted for the actual situation at the site, a reasonable con-struction method and tunnel advance rate should be chosen and the three-step construction method can effectively control the occurrence of large deformation in cases of unfavourable rock masses.%通过对汶马高速公路鹧鸪山软岩隧道多次出现的大变形现象进行分析,文章将围岩破坏分为3大类,即软岩塑流、板梁弯曲变形及结构面滑移.针对建设过程中的大变形问题,选取了50 m典型围岩区段作为试验段,开展了两台阶开挖参数优化试验及两台阶与三台阶开挖工法的比选试验.两台阶开挖参数优化试验表明,开挖进尺以及下台阶左右侧前后间距对隧道的稳定性影响最大.开挖工法比选试验中,通过对两台阶与三台阶开挖洞周位移、围岩与初期支护间压力及钢拱架受力的对比发现,两台阶开挖方法由于一次开挖断面过大,围岩来压快,导致洞周变形值及围岩与初期支护压力值相对偏大,钢拱架安全储备相对不足.因此,对于处于复杂区域环境中的软岩隧道,应结合现场实际情况适当调整围岩的预留变形量;合理地选择开挖工法与开挖进尺,且在围岩条件很差的情况下,采用三台阶法可有效控制大变形灾害的产生.
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