A Novel Mogi Type True Triaxial Testing Apparatus and Its Use to Obtain Complete Stress–Strain Curves of Hard Rocks

A true triaxial apparatus (TTA) was designed and fabricated at Northeastern University, Shenyang, China, by modifying the original Mogi type testing apparatus to emulate three-dimensional stress paths in deep mining and tunneling excavations. Such an apparatus can be used to investigate deformation and brittle failure behaviors of hard rocks as well as the cause of rockbursts. The novel TTA can capture the post-peak behavior of a 50 × 50 × 100 mm³ specimen. Technical improvements such as a considerable increase of the stiffness of the loading frames were implemented to deal with difficulties in TTA testing. The accuracy of the volume change measurement was improved and a combined pneumatic and hydraulic technique was applied to create a “floating” vertical loading frame. The end friction effect and the loading gap effect were evaluated using a series of tests. Repeatability tests, brittle failure tests in a loading stress path and an unloading stress path (unloading of σ ₃) were carried out on granite specimens to verify the performance of the TTA. The test results show that the apparatus achieves its original design goal.