Abstract: The cavity effect in granular matter is studied experimentally. When pulling a stick out the granular matter, it create a sustained cavity under it, and all the system＇s construction will be breakdown because of the cavity, the breakdown situation can be explored by the friction force of the stick. The relationship between the breakdown time t and the cavity＇s depth h is exponential, t is proportion to exp（h/λ） that we obtained by experiment, and it is unaffected by changing the granular size. We do an two-dimension experiment at the same time, to understand the inside situation of the granular matter clearly.

Key words: granular matter, cavity, force chain, breakdown time