In laboratory experimental techniques, we have different experimental approaches to realize in situ stress state around the underground, true triaxial test, confined triaxial compression test and confined triaxial extension test. True triaxial test can generate a general stress state and adopted rectangular specimen. On the other hand, later two testing methods can only restricted stress state of σ2=σ3?or σ1=σ2 and used cylindrical specimen of f50x100mm. The true triaxial test is superior to obtain continuously deformation behaviors from ductile under compression regime to brittle under extension regime. We had rather few data related to confined triaxial extension test and also true triaxial compression test.

To investigate the deformation behavior with different pressure medium for σ1 and σ2 under extensional stress state, we conducted true triaxial compression and extension test using rectangular specimens of 35x35x70mm, and confined triaxial compression and extension test using cylindrical specimens of φ50x100mm. During extensional stress loading, σ1 and σ2 in different shape specimen were applied respectively by rigid piston and confining fluid medium. Main object in this research is to investigate the differences on deformation behaviors and fracture pattern of Kimachi sandstone with different loading system, a confining medium and regid piston.

The maximum principal stress increased by some amount of stress in changing from compressional stress state to extensional stress state. The brittleness defined as reduction in the maximum principal strain is observed under increasing of the intermediate principal stress, transition characteristics are also observed from compression state to extension state. The final maximum principal stress (σ1 and σ2) with rigid piston showed higher value than those with confining fluid medium. These difference are attributed to the existence of friction resistance by steel end pieces for σ1 and σ2.

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