Rock specimen has properties of material strength and supporting capacity in pseudo-triaxial stress slate. The material Strength is a decreasing function with plastic deformation, but the variation of supporting capacity is very complex with the loading path, which influence specimen's deformation greatly. In this paper, experiments of loading and unloading path were carried in MTS servo-controlled testing machine, and mechanical property of marble, siltstone, sand and coal were studied. To compare the relations between confining pressure and residual strength or peak strength, the internal friction coefficient of specimen beyond yield is studied. The relation between supporting capacity and deformation property of specimens was discussed.
Until now the compression. Strength of rock specimen in laboratory was obtained by the increasing of axial stress while the Confining pressure keeping constantly. And the shear strength was obtained by the increasing of shear stress while the normal stress keeping constantly But the stress path is very complicated for rock failure in geo-engineering. But the effect of loading path on the experimenlal results has not been understood clearly. In fact rock specimen under pseudo-triaxial loading has two properties of material strength and supporting capacity, which has not been distinguished clearly. For example, a broken specimen, cracked into two parts completely, may supports axial stress by fiction as well. Rock is heterogeneous. Material in the specimen has different strength, and reaches its supporting capacity to produce Plastic deformation. The strength of a specimen is only a parameter of manifestation of the whole material in the specimen.In other words, Some material yields and produces plastic deformation before specimen's peak stress. Usually, the yield Stress increases slower than the strength of specimen with confining pressure.
In this paper. it is said as effective loading path when the difference between supporting capacity of specimen and the axial stress, or /(0)) -01, is decreasing, and said as effective unloading path when /(0)) -01 is increasing. That means, the increase of axial stress or decrease of confining pressure is effective loading, and the decrease of axial stress or increase of confining pressure is effective unloading. It is easy to understand that the variance of stresses in elastic state does not influence on specimen strength,
Axial compression. confining pressure reduction and ratio loading arc all monotonous effective loading paths. or monotonously close to the strength curve, therefore they make the material in specimen yield gradually according to its strength from low to high. The experimental results of granite and tonalite showed clearly that the strength of specimen was decided by confining pressure, and the fracture was brittle, no relation to loading path. (Swanson, 1971).