Given that the drilled core diameter decreases with increasing depth and there is no true triaxial apparatus for testing the complete stress–strain behavior and long-term deformation behavior of small-sized cores, a geological environment true triaxial apparatus was developed. The apparatus was mainly designed to obtain the complete stress–strain curves of hard rock specimens measuring 25 mm×25 mm×50 mm, while also considering the measurement of long-term deformation. This device focuses on solving 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 deformation behavior of Xiling ultra-deep drilling cores was measured using this apparatus.
Ultra-deep drilling investigation is an important method to evaluate the stability of deep geological structures. The environment of ultra-deep core drilling is characterized by high in-situ stress, many large fault structures, and abundant local rock mass fissure water. In addition, the in-situ stress under the action of structural stress presents a true triaxial stress state (σ1 > σ2 > σ3). Therefore, studying the mechanical behavior of ultra-deep drill cores under true triaxial stress can help the analysis of the mechanism of deep and ultra-deep geohazards.
Mogi (1971) developed the first rock true triaxial apparatus in 1971, which proved that σ2 affects the strength and deformation of rock. In 1989, Xu et al. (1990) designed China's first rock true triaxial apparatus. The above two true triaxial apparatus can be mainly used to obtain the pre-peak failure behavior of rocks. To obtain post-peak curves of hard rock, Haimson & Chang (2000) developed a true triaxial apparatus with servo control, but only partial post-peak curves were acquired due to the limitations of the testing technique. Since 2000, some true triaxial apparatus with high stress loading capacity have been successfully developed, and the monitoring of the complete stress–strain curve of hard rock has also been achieved (Zhang et al. 2017). In addition, for the long-term failure behavior, fluid coupling failure behavior, and temperature coupling failure behavior of hard rock, some true triaxial apparatus with corresponding functions have also been successfully developed (Nasseri et al. 2014 and Feng et al. 2018).
