Abstract: The maximum buried-depth of Bamiao tunnel on Taoyuan-Bazhong highway is 312 m, the rock stratum is nearly of horizontal form and it presents as interbed of sand and mudstone. Unconventional rockburst events occurred during the tunnel construction, and a series of diseases were found at these sections after operation. The causes of the diseases were summarized by analyzing the phenomena and data as well as the results of special tunnel inspection. Based on the on-site geostress test results during construction, it was assumed that the seriously cracked secondary lining was in the critical state of failure, thus the load borne by the structure was deduced. The formula for calculating safety coefficient of concrete structure with additional lining was deduced, and the relationship between the safety coefficient of tunnel with additional lining and the thickness of additional lining was obtained.