Abstract: Many field observational evidences are presented in the present paper, which indicate that the effective pressure within the rock mass can become zero or even negtive. The main changes in physical properties of the rock mass under such stress state as compared with that under normal stress state are as follows: significant increase in permeability, great increase in the rate of vertical displacement of ground surface and poor stability of the rock mass. The possible reasons for the above changes are also discussed in this paper.