The natural mechanic condition of the rock mass is discussed. According to the unloading test of the rock mass, a study IS done on the relationship between stress and strain and its characteristics, the mechanical parameters are offered and their difference from those or the general rock mass is pointed out.


During the endless geological action, many structural planes with different volumes and directions have developed in the rock mass. Therefore the rock mass today is a damaged one by different geological actions and the stress in the rock mass is residual. If the rock mass is excavated, the residual stress would be further released. For example. the rock mass of the Three Gorges Project permanent ship-lock slopes had ever undergone six great structural actions: Before Jinning (Dynasty) action, Jinning (Dynasty) action, Chengjiang action, Yinzhi action. Yanshan action and Xishan action. Affected by many structural actions, the rock mass reached its present residual stress which is shown as a o-

Experiment models and materials

In order to study the mechanical characteristics of the unloading rock mass, the rock mass of the Three Gorges Project permanent ship-lock slopes is taken as the object and the analogue experiment theory and method are used to search the Stress-Strain Relationship (SSR) or the unloading rock mass. In the analogue test geo-mechanical model materials are also made use of. The local analogue technique is used in the test, with which different geometrically similar ratios about the sizes of the models are chosen and different structural conditions of the site rock mass are simulated in room to study the effect of sizes, anisotrophy, unloading mechanical characteristics and their parameters. The size of the experiment models is 25cm X 25cm X 25cm and the geometric ratios are CL=3,9.27, 81 respectively. Because of the multitude of the geologic structure planes in the area, it is difficult for the local analogue to reveal all the real conditions. So after studying the effect of all kinds or structural planes on the strength and deformation of the rock mass,we decide to research mainly the high-dip-angle-superior -engineering planes referred in the geologic research report, which have great influence in anisotropy and unloading mechanical characteristics on the slope rock mass.

Experiment methods of loading and unloading

The tri-axial test apparatus specially designed for the experiment are adopted. X represents the strike of the axle of permanent ship-lock. Y is vertical to The axle in the horizontal plane (the main unloading direction). Z is vertical to the horizontal plane. After loading, the unloading test by simulating the rock mass excavation is done. In the test, a xis unchanged, a y and 0 L are unloaded to zero in 6 stages. (sec Figure 2.point C). It is mainly to simulate the unloading process in Direction Y. After 0 y reaches zero, tensile stress is added continually until the break of the rock mass.

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