Study on collapse mechanism at tunnel export and its prevention measure

Collapse accident frequently occurs at tunnel export. In order to reveal the mechanism of this kind of tunnel collapse, three-dimensional elastic-plastic calculation model is established according to Maanshan mountain tunnel. Thick soft overburden, untimly support and rainwater infiltration are considered as the main influencing factors of the tunnel collapse. Its special geological condition is determinative factor and rainwater infiltration is inducing factor. The study shows that collapse at tunnel export is induced by tunnel face instability and loose rock and soil mass collapse at tunnel roof. This kind of tunnel collapse belongs to the failure model which is induced by the combined action of shearing and tension. The support behind excavating face can prevent plastic zone extending to ground surface. It is better that the distance from excavating face is shorter. So the primary support should be constructed timely and advanced support is necessary. The best length of advanced support is 5 m. At the end of this paper, the influence of rainfall to surrounding rock is analyzed through field monitoring data of surface settlement and surrounding rock pressure. It is presented that rainwater infiltration can accelerate the failure of surrounding rock and its influence effect mainly reflects on 1 to 2 days after rainfall. So waterproof and drainage measures should be done to prevent the collapse for rainy season construction.