Abstract: To study the deformation and fracture behavior of ultra-deep drilling core, an ultra-deep drilling core geological environment true triaxial apparatus was developed. The apparatus was mainly designed to obtain the complete stress-strain behavior, cyclic loading and unloading fracture behavior, and time-dependent deformation behavior of specimens (25 mm×25 mm×50 mm) processed by ultra-deep drilling cores under a true triaxial stress, mainly solving the technical problems such as inaccurate measurement of specimen deformation, low stiffness of the apparatus, mutual interference between pressure chamber oil pressure and actuator pressure. The device was used to test the mechanical behavior of the cores from the Xiling engineering exploration hole at the Sanshandao Gold Mine of Shandong Gold Group under the true triaxial stress. It was found that the true triaxial cyclic loading and unloading weakened the brittle fracture characteristics of the specimen, and induced deformation in three directions to show anisotropy. In the multi-stage true triaxial creep test, the deformation of specimen in three directions showed primary creep, steady creep, and accelerated creep. The creep strain rates in the three directions increased approximately linearly with deviatoric stress.

Key words: ultra-deep drilling core, true triaxial test, complete stress-strain, cyclic loading and unloading, time-dependent failure