重庆某公路隧道为双向5车道城市隧道，洞口段穿越由城市建设回填土组成的松散岩堆体，左洞在施工过程中发生了冒顶事故，并出现了大变形、支护开裂、地表裂缝等不良现象。为探究事故发生原因，并验证变更后方案的合理性，首先，对原方案现场监测结果进行分析； 在此基础上，采用FLAC3D软件进行数值模拟计算，并与现场监测结果进行对比，验证结果的可靠性； 最后，分析不同施工方案所产生的洞周位移、支撑结构应力与塑性区等内容，得出以下结论： 1）原方案采用三台阶七步法施工，支护未能及时封闭成环，变更后的施工方案能够有效控制围岩位移，且能够提高隧道洞周变形协调性。2）原施工方案中的初期支护存在明显的应力集中现象，最大拉压力超过结构容许值，同时围岩应力较大。通过对比发现，增大超前加固范围有助于减小初期支护所受的围岩压力、改善支护结构所受应力状态。3）变更后施工方案的土体受破坏区域小，变更后的施工方案能够减小隧道施工对周围环境的影响。该研究成果适用于松散岩堆体地层条件下隧道洞口段的开挖。

A highway tunnel in Chongqing is a twoway fivelane urban tunnel. The portal section of the tunnel passes through the loose deposits composed of urban construction backfill. During the construction of the left tunnel, the tunnel crown collapsed, and there are some adverse phenomena such as large deformation, support cracking, and surface crack. To explore the causes of the accident and verify the rationality of the modified scheme, the field monitoring results of the original scheme are analyzed. On this basis, the numerical simulation software FLAC3D is used, and the calculation results are compared with the field monitoring data to verify the reliability of the results. Finally, the displacement around the tunnel, the stress of the support structure, and the plastic zone induced by different construction schemes are analyzed. Some conclusions are drawn as follows. (1) In original construction scheme, threebench and sevenstep construction method is adopted, and the support cannot be closed to form a ring in time; whereas the modified construction scheme can effectively control the surrounding rock displacement, and improve the coordination of deformation around the tunnel. (2) In original construction scheme, the primary support shows obvious stress concentration phenomenon, the maximum tensile pressure exceeds the allowable value of the structure, and the surrounding rock stress is large; whereas the advance reinforcement scope is enlarged in the modified scheme, which can reduce the surrounding rock pressure of the primary support and improve the stress state of the support structure. (3) The plastic area of the modified construction scheme is obviously smaller than that of the original one, and the damaged area of the soil is small; the modified construction scheme can reduce the impact of tunnel construction on the surrounding environment. The research results are applicable to the excavation of tunnel portal section under the condition of loose deposits.