This research experimentally analyzed the impacts of various water cement (W/C) ratios of ultrafine cement grout material and normal loads F_N applied to fractures on grout nonlinear flow behavior through a rough plexiglass fractured sample. An effective self-made apparatus was designed and manufactured to conduct the stress-dependent grout flow tests on the plexiglass sample containing rough fractures. At each W/C ratio, the grout pressure P increased from 0 to 0.9 MPa, and the normal loads F_N ranged from 666.3 to 1467.8 N. The results of the experiments indicate that (1) the Forchheimer’s law can be used to express the results of grout nonlinear flow through rough fractures. Moreover, both nonlinear coefficient a and linear coefficient b in Forchheimer’s law decreased with the increase of the W/C ratio, but increased with the increase of the F_N value. (2) For normalized transmissivity, with the increase of Re, the decline of the T/T₀–Re curves means that the grout flow behavior through the fracture mainly went through three stages: the viscosity effect, then the weak inertia effect, and finally the strong inertia effect. The three stages showed that with the increase of Re, the grout flow state changed from linear to nonlinear. Moreover, with the increase of the W/C ratio, the Forchheimer coefficient β decreased. (3) At a given F_N, the critical grout hydraulic gradient J_c decreased, but the critical Reynolds number Re_c increased as the W/C ratio increased; at a given W/C ratio, J_c increased, but Re_c decreased as F_N increased.

Short-term and creep tests of fractured sandstone with different degrees of damage prepared using pre-peak and post-peak unloading tests on intact sandstone were carried out using a servo-controlled rock mechanics system. Based on our experimental results, the influence of confining pressure and damage on short-term mechanical behavior of fractured sandstone with different degrees of damage was first analyzed. The results show that the peak strength, residual strength, elastic modulus, and secant modulus of fractured sandstone increase linearly with increasing confining pressure, but decrease with increasing damage. The short-term failure modes depend on the damage and change from typical shear failure modes to multiple shear failure modes with increasing damage. Then, the influence of the differential stress, confining pressure, and the degree of damage on the creep mechanical behavior of fractured specimens was further investigated. The axial instantaneous strain and creep strain increase linearly with increasing differential stress, and the specimens exhibit significant time-dependent behavior under high stress. The steady creep rate increases with increasing stress, but it decreases with increasing confining pressure and damage. However, the long-term strength and creep failure strength of fractured specimens increase linearly with increasing confining pressure, but they decrease linearly with increasing damage. The creep failure modes of fractured specimens are also the main shear failure modes, which are similar to the short-term failure modes.

Grouting reinforcement is a pivotal approach to enhancing the integrity and load-bearing capacity of fractures in surrounding rock. In this study, standard limestone specimens were fractured through uniaxial compression. Then, the specimens were reinforced with grouting, using ultrafine cement paste containing varying mass fractions of enhancers and a grouting apparatus developed by the authors. After the specimens were cured under standard conditions for 28 days, CT scanning technology was used to investigate the microstructure and grouting effect characteristics of grouted bodies containing different mass fractions of enhancers from a mesoscopic perspective. Then, uniaxial compression tests were conducted on those grouted specimens. The experimental results revealed that the content of the enhancer significantly affected the post-peak characteristics, mechanical parameters, and failure modes of the grouted specimens. When the content of the enhancer increased from 2.50 wt.% to 15.00 wt.%, the uniaxial compressive strength of the grouted specimens exhibited a positive correlation with the enhancer content, with the maximum improvement rate reaching 18.10% compared to the residual strength. However, when the enhancer content ranged from 15.00 wt.% to 20.00 wt.%, the uniaxial compressive strength was negatively correlated with the enhancer content. At an enhancer content of 15.00 wt.%, the overall stability of the grouted specimens was optimal, with all mechanical parameters reaching their maximum values. Utilizing three-dimensional CT scanning and reconstruction technology, it was observed that when the enhancer content was less than 15.00 wt.%, the cracks were concentrated in the limestone matrix rather than in the grouted solid in the edge regions of grouted specimens. However, in the whole specimens, the cracks in the grouted solid exceeded that in the limestone matrix. Conversely, when the enhancer content was greater than 17.50 wt.%, the grouted solid was predominantly distributed within the edge fissures of the specimens, while the internal regions exhibited a lower volume fraction of the grouted solid. In this scenario, the volume fraction of the grouted solid in the specimens was significantly lower than that of the fissures.

The long-term stability of deep underground excavations near aquifer-bearing strata is primarily controlled by the time-dependent deformation and permeability changes in the surrounding rock mass under the combined effects of mechanical loading and groundwater seepage. This study experimentally investigates the creep behavior and permeability evolution of tuff specimens subjected to stepwise reductions in confining pressure under coupled seepage and stress conditions. Conventional triaxial compression tests were conducted to determine the peak strength at confining pressures of 10, 15, and 20 MPa. Subsequently, triaxial creep tests were performed, maintaining axial stress at 70% of the previously established peak strength, with a constant seepage pressure of 4 MPa, while progressively decreasing the confining pressure. The results clearly reveal a three-stage creep process—with instantaneous, steady-state, and accelerated phases—with the radial strain exceeding axial strain and ultimately dominating at failure. This indicates that failure is characterized by significant volumetric expansion. At the specified initial confining pressures of 10 MPa, 15 MPa, and 20 MPa, the tuff specimens exhibited volumetric strains of −1.332, −1.119, and −0.836 at failure, respectively. Permeability evolution depends on the creep stage, showing a pronounced increase during the accelerated creep phase that often surpasses the cumulative permeability changes observed earlier. The specimen’s permeability at failure increased by factors of 3.97, 3.21, and 3.61 compared to the initial stage of the experiment, respectively. Additionally, permeability evolution exhibits a strong functional relationship with volumetric strain, which can be effectively modeled using an exponential function. The experimental findings further indicate that, as the confining pressure is gradually reduced, the permeability evolves following a clear exponential trend. Additionally, a higher initial confining pressure slows the rate at which permeability increases. These findings clarify the three-stage creep behavior and the associated evolution of the permeability index in tuff under coupled seepage–stress conditions. Additionally, they present a quantitative model linking permeability to volumetric strain, offering both a theoretical foundation and a new approach for assessing the long-term stability risks of deep underground engineering projects.