How to accurately and nondestructively obtain inner stress evolutional data of rock and soil material is helpful to study physical mechanism of material failure affected by confining pressures. The triaxial compression tests of sandstone under confining pressures of 0.0

0.5 and 1.0 MPa are designed based on the particle flow code (PFC) software. According to failure shape

macro-stress- strain curve and stress data in x

y

xy directions at nine monitoring points on left and right sides or middle of specimen

mechanical mechanism and influence by confining pressure of material failure process are analysed. It is found that the shear band located from upper left corner to lower right one develops enough than shear band situated from upper right corner to lower left corner

which is limited with increase of confining pressure; that is to say shear band tends to expand with same velocity because of confining pressure. It can also be indirectly proved by failure shape explained by microcracks; and that confining pressure makes strain more obvious.