The ongoing advancements of large-scale water conservancy projects, deep ultralong tunnels, and deep geological storage facilities in geotechnical engineering present severe challenges to the safety of rock mass engineering. The shear strength of rock masses is crucial for engineering construction, as numerous instability incidents stem from shear-failure mechanisms. Therefore, the experimental studies of the shear mechanical properties of rock masses are highly necessary. However, traditional rock shear testing machines are only suitable for square specimens, suffering from uneven stress distribution caused by a rigid contact between the loading indenter and specimen pads. To address this problem, this study optimizes the performance of an existing laboratory rock shear testing machine. The modification simultaneously enables direct shear testing on cylindrical specimens and incorporates acoustic emission monitoring. It also improves the uneven distribution of normal stress on the rock specimens. This optimization is crucial for expanding specimen specifications, enhancing rock shear testing accuracy and scalability, and maintaining the long-term stability of rock engineering structures.

This study focuses on optimizing the TEST60 rock shear testing machine. The main body of the device comprises an operating box equipped with horizontal load bars on both sides to apply shear loads and a vertical load bar at the top to apply vertical loads. The front end of the horizontal load bar connects to upper and lower pads featuring arc-shaped grooves. These pads are secured via mounting holes and incorporate dedicated holes for acoustic emission sensors. The indenter of the vertical load bar is modified into a spherical shape, and springs are uniformly distributed along its lower edge, connecting to the outer wall of the pressure base to ensure uniform pressure distribution. Using this optimized shear testing machine, shear tests are conducted on cylindrical rock specimens with real-time acoustic emission monitoring. During shear testing, the vertical load rod and spherical indenter are manually lowered. Fine adjustments to the spherical indenter are possible throughout this process to ensure uniform vertical loading.

The shear stress displacement curve exhibits typical prepeak and postpeak mechanical characteristics, effectively illustrating the shear failure mechanism of the specimen. Acoustic emission monitoring reveals minimal activity during the initial loading, followed by a highly active phase after the peak. Combined with the spatial localization of acoustic emissions based on phase segmentation, this clearly reveals the damage evolution mechanism in the specimen, from microcrack initiation to macroscopic fracture. The peak values of the shear stress displacement curve and acoustic emission precisely correlate with the test results, consistent with actual rock mechanical properties. This validates the effectiveness and applicability of the optimized rock shear testing machine. The spherical indenter ensures the uniform distribution of normal stress across the rock specimen, and the concealed acoustic emission sensors enable multimethod collaborative monitoring.

The optimized rock shear testing machine overcomes the limitations of traditional machines, notably their applicability to only square specimens, by incorporating cylindrical pads with holes for acoustic emission sensors, thereby expanding the range of suitable specimen dimensions. The vertical loading head is upgraded to a spherical design, resolving the issues of uneven normal stress distribution and enhancing the accuracy of rock mechanical parameter testing. The optimized direct shear testing apparatus, when combined with techniques such as digital image correlation, enables further investigation into shear failure mechanisms, enabling the execution of a broader range of rock mechanical tests.

As underground space development progresses into deeper parts of the Earth, the geological conditions of rock formations become more complex. The change in the mechanical properties of rocks under pore water pressure has become a key focus in rock mechanics, particularly in deep underground projects such as resource extraction and tunnel construction. Pore water pressure testing is an important method for analyzing the effects of pore water pressure on rocks. However, traditional pore water pressure loading devices encounter challenges, especially in maintaining stability during long-term loading, leading to pressure fluctuations and decreases. Power outages can also result in pressure drops, which affects the accuracy and reliability of the experimental results. These issues have prevented indoor simulations from accurately replicating real deep rock water environments. Therefore, this study designed and developed a pore water pressure loading device that can simulate various water environments and ensure long-term rock hydration reactions. This development is crucial for understanding the mechanical changes in deep rocks under pore water pressure and ensuring the long-term safety and stability of underground projects.

The proposed pore water pressure loading device is designed to meet the requirements of long-term hydration reactions. The device consists of a symmetric structure, including the main body, inlet and outlet pipelines, and control valves. The structure is simple, reliable, and easy to operate. The pore water pressure loading process can be completed by rotating the control rods. Once the water pressure reaches the required level, the shut-off valves are adjusted to maintain the desired pressure for the test. Minor pressure deviations during long-term loading can be corrected by manually adjusting the control rods in real time, thus ensuring stable pore water pressure and accurately simulating rock water conditions. The device was used in conjunction with Sichuan University’s Deep High-Stress High-Permeability Environment Simulation System to treat rock samples for 60 days, during which pore water pressure changes were recorded. After treatment, Brazilian splitting tests were conducted using an MTS815 testing machine to determine the Mode I fracture toughness of the different samples.

During the 60-day loading process, the pore water pressure slightly decreased on Days 16, 42, and 58 with a reduction of approximately 0.5 MPa, indicating that the device maintained stable pore water pressure with minimal fluctuations, meeting the experimental requirements. The results also showed differences in fracture toughness among the three marble samples. Specifically, samples simulating deep rock water conditions (Sample A) and those soaked in a NaOH solution (pH = 9, Sample B) showed a reduction of 10.1% and 7.6%, respectively, compared with those under natural conditions (Sample C). This finding indicates that rock fracture toughness decreases, to some extent, under water saturation, particularly in deep water environments where pore water pressure promotes crack propagation, further reducing fracture toughness.

The pore water pressure loading device developed in this study achieves long-term, stable, and high-precision pore water pressure loading, meeting the experimental needs for sustained pressure applications. The device can simulate different water environments by adjusting the water solution in the internal tank, creating ideal conditions for analyzing rock hydration reactions in various underground environments. The mechanical adjustment of the device, without the need for power, makes it simple to operate and compatible with conventional rock mechanics testing equipment, such as triaxial rock testing machines. The development and use of this device will facilitate further experimental studies on the effects of pore water pressure on rock mechanical properties under more complex conditions.