The authors found a type of sidewall failure during the construction of a tunnel with high geostress. And relative strong rockburst usually occurred in the areas nearby during the excavation. This paper shows this type of failure called Crisp Fissure Failure and some characteristics of the failure. Reasons of the failure are also discussed. Further more, the authors studied the fracture distribution under this type of failure by Ground Penetrating Radar. Suggestions of support in a high risk tunnel are put forward based on the research of the Crisp Fissure Failure.