Development and monitoring application of a high-pressure splitting grouting test system for deep rock masses

High-pressure laboratory splitting grouting tests are essential for validating grouting theories and assessing the reliability of numerical simulation results, thereby providing a fundamental basis for practical engineering applications. However, existing experimental facilities are limited in their ability to simulate the coupled effects of high in-situ stress and high injection pressure, accommodate large-scale specimens, adapt to various grouting materials, and perform multi-dimensional synchronous monitoring. To investigate the mechanisms of splitting grouting in deep rock masses under the coupled effects of high confining stress and high splitting pressure, a high-pressure splitting grouting testing apparatus for deep rock masses was independently developed. The system consists of three core subsystems: a high-precision grouting control subsystem, a true triaxial stress loading subsystem, and a multi-dimensional synchronous monitoring subsystem. The true triaxial stress loading subsystem employs a KDSH1000 high-pressure, high-speed servo pump to achieve servo-controlled loading in the three principal stress directions. Grouting pressure is provided by a KDHB100S dual-cylinder high-precision injection pump, which is connected to the specimen through a high-pressure grouting mixing tank. The apparatus can generate a maximum injection pressure of 70 MPa and a maximum loading capacity of 4,500 kN along each of the X-, Y-, and Z-axes, while accommodating cubic specimens with edge lengths of up to 300 mm. It also enables the synchronous acquisition of multi-dimensional data, including conventional mechanical parameters, fluid pressure and flow characteristics, acoustic emission (AE) signals, and ultrasonic wave responses. To verify the reliability and stability of the testing apparatus, splitting grouting experiments were conducted on specimens subjected to three cyclic loading paths: constant-amplitude cyclic loading, stepwise cyclic loading, and variable upper-limit stepwise cyclic loading. Multi-dimensional data, including injection pressure histories, failure morphologies, AE signals, and ultrasonic wave data, were synchronously collected and analyzed. The results demonstrate that the apparatus can accurately reproduce complex stress paths representative of deep rock formations while providing stable and reliable data acquisition. Under all three cyclic loading paths, the cumulative AE ring-down counts exhibited a pronounced stepwise increase throughout the splitting grouting process. During the stable seepage stage, AE activity remained at a high level because of the combined effects of compressive damage along the fracture surfaces and shear deformation on the rough fracture interfaces. During the initial stress loading and cyclic loading–unloading stages, only low-energy, sparsely distributed AE events were detected, whereas the splitting grouting stage generated high-energy, densely distributed AE events. The spatial distribution of AE events corresponded well with the propagation characteristics of splitting fractures. Moreover, different cyclic loading modes produced significantly different levels of internal damage in the specimens, following the order: variable upper-limit stepwise cyclic loading > constant-amplitude cyclic loading > stepwise cyclic loading. Furthermore, splitting grouting increased the ultrasonic frequency-domain energy of specimens subjected to variable upper-limit stepwise cyclic loading and constant-amplitude cyclic loading, whereas it decreased the ultrasonic frequency-domain energy of specimens subjected to stepwise cyclic loading.