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Thin Spray-On Liner (TSL) has been widely used in underground engineering support such as mining due to its convenient operation and good support effect. The TSL applied to the rock is inevitably affected by dynamic loads from excavation. In order to study the support effect of TSL on a rock under dynamic load, a series of uniaxial compression experiments under static and dynamic loads are carried out on the TSL-coated specimen, and the costing thickness is 1 mm, 3 mm and 5 mm, respectively. The results showed that the TSL can improve the strength and peak strain of sandstone, and the degree of improvement is more obvious with the increase of costing thickness under static and dynamic load, and the loading rate of the dynamic load is about 70 ∼ 110 s−1. It was also found that the dynamic strength increases exponentially with strain rate, with the costing thickness increases, the strain rate index of sandstone increases. The good bonding ability and tensile properties of TSL ensure support to rock. Application of TSL can delay the time for cracks to appear in the specimen and increase the energy required for the failure of the specimen. The phenomenon was also simulated using ABAQUS, and the simulations show that TSL provides passive support that the pressure of TSL gradually increases with the deformation of the sandstone during the impact, and can delay the failure time of the specimen and reduce the number of cracks. That is consistent with the experiment. [ABSTRACT FROM AUTHOR]

In this study, several chemical grouting tests of fractured porous sandstone were performed based on the low-field nuclear magnetic resonance (NMR) technology, and the nuclear magnetic signal characteristics (NMR T2) of slurry were tracked and tested in real-time. The variations in the injected slurry volume, effective grouting time and slurry permeability with different confining pressures, injection pressures and fracture inclination angles were quantitatively analysed. Besides, the relative filling degree (RFD) was proposed to comprehensively investigate the diffusion processes under different grouting conditions. The results indicate that both the high confining pressure (e.g., 20 MPa) and large fracture inclination angle (e.g., 27°) induce the decrease in injected slurry volume by the rate of 16.7 and 44%, while the effects induced by injection pressure is not obvious. The permeability of slurry in fractured porous sandstone linearly decrease as the confining pressure and fracture inclination angle increase, coinciding with the decrease amplitude of 67.1 and 44%, respectively. The slurry diffusion pattern in fractured porous sandstone is strongly dependent on the confining pressure, injection pressure and fracture inclination angle. Both the relative filling degrees of rock pores and fractures decrease with increasing confining pressure and fracture inclination angle. Highlights: The diffusion processes of chemical slurry under several external conditions and grouting parameters have been monitored in real-time by the low-field NMR technique. The variations in the injected slurry volume and diffusion patterns with different grouting conditions were comprehensively studied. The expression of the relative filling degree of slurry in fractured porous sandstone was proposed. And the effects of various grouting conditions on the relative filling degree were analyzed based on the evolution of NMR T2 curves. [ABSTRACT FROM AUTHOR]

Due to the invisibility and complexity of underground engineering rock masses, it is extremely difficult to observe the diffusion process of dynamic water grouting. Therefore, studying the diffusion mechanism of grouting in underground fractured rock masses with flowing water is crucial in preventing water inrush and improving the rock mass tightness. In this study, a novel dynamic water grouting test system based on low-field nuclear magnetic resonance (LF-NMR) was developed to investigate the dynamic grouting diffusion process in fractured sandstone with flowing water. Then, the NMR characteristics of superfine cement-sodium silicate slurry and free water were compared, and the influences of water flow velocity, grout flow velocity and fracture apertures on the grouting diffusion characteristics of fractured sandstone with flowing water were systematically investigated. The test results show that the increase in the grout flow velocity and fracture aperture can effectively improve the dynamic water grouting efficiency, while the increase in the water flow velocity will reduce the dynamic water grouting efficiency. When the grout flow velocity is lower than the water flow velocity, the dynamic water grouting test cannot be successfully performed. Additionally, the slurry filling ratio of the pores peaks when the flow velocities of grout and water are identical. The findings in this paper can provide meaningful reference and guidance for grouting parameter selection in grouting engineering practice. Highlights: A novel dynamic water grouting test system which can monitor the variations of slurry and water in pores and fractures of real rock is developed. The grout diffusion process in the rock pores and fractures under flowing water condition shows an obvious three stages evolution. The influences of dynamic water grouting parameters such as grout flow velocity, water flow velocity and fracture aperture on grouting efficiency are investigated. [ABSTRACT FROM AUTHOR]

Weak inclusions play a significant role in regulating the deformation and fracturing behavior of rock masses. In this study, sandstone specimens filled with circular inclusions were subjected to diametrical compression until failure. The local strain concentration intensity and cracking process at the rock-inclusion interface were specifically investigated using the integrated strain gauge (SG) and digital image correlation (DIC) techniques. The experimental results reveal that the presence of inclusions introduces more complex deformation and fracturing behaviors in the infilled sandstone specimens. The stiffness of the infilled sandstones, with either low-strength or high-strength inclusions, initially increases and then decreases as the inclusion diameter increases. The effective load-carrying capacity of the infilled sandstone is influenced by the competing effects of inclusion enhancement and hole weakening. Moreover, a more pronounced strain concentration occurs at the inclusion boundary when the inclusion diameter is larger and the inclusion strength is lower. The crack mode in the infilled red sandstone is independent of the strength of the inclusion. For larger hole diameters, fracture planes form along the compression line, whereas interfacial debonding occurs in red sandstone with smaller hole diameters. Finally, the stress distribution characteristics along the loaded diameter and transverse diameter were analyzed using elastic finite element analyses, which can well explain the cracking mechanism of the infilled disc specimens. [ABSTRACT FROM AUTHOR]

The frost damage behaviors of rock mass largely depend on its water content. Therefore, it is of great practical significance to investigate the effects of the saturation degree on the evolution of unfrozen water content and the critical degree of saturation of rock mass. In this paper, sandstone samples were diamond-cored and prepared to hold different degrees of saturation. The samples were frozen to a minimum temperature of – 25 °C, during which the nuclear magnetic resonance (NMR) measurements were repeatedly performed to explore the changes in the transverse relaxation time (T2) distribution curves and the unfrozen water contents. The results indicate that most of the pore water at room temperature exists in the relatively smaller pores when the degree of saturation is lower than 60%. Upon freezing, the sandstone holding a lower degree of saturation exhibits a lower freezing speed and a higher residual unfrozen water content. Furthermore, the amounts of the unfrozen bound water of the 100% and 60% saturation samples slightly increase as the temperature drops to − 1 °C, whereas the unfrozen bound water content for the 30% saturation sample monotonically decreases during the freezing test. Based on the mechanical tests and the microscopic observation of the rock surfaces after cyclic freeze–thaw weathering, the critical degree of saturation for the studied sandstone was derived as 50%. The findings in this study provide meaningful guidance on the freeze–thaw damage prevention of the rock mass in cold regions. Highlights: The pore water distribution of the partially-saturated sandstones under freezing was quantitatively studied. An unfrozen water content prediction equation was proposed for the partially-saturated sandstones. The critical degree of saturation for the studied sandstone was determined. [ABSTRACT FROM AUTHOR]

In practical engineering, the mechanical properties of the surrounding rock often reflect the bearing capacity of the support. To investigate the relations between the surrounding rock and the support, solid specimens, hollowed cylinders, and hollowed cylinders filled with two kinds of cement mortars are tested under unconfined and conventional triaxial compressions. The effects of the infilling on the stress-strain curves, deformation features, mechanical properties, and failure patterns are schematically investigated. The results show that under the triaxial compression condition, each infilled specimen exhibits obvious residual carrying capacity though a slight stress drop occurs after the peak stress. The cement mortar exerts a positive effect on the carrying capacity of the rock, and the infilling having a higher strength and stiffness contributes to a more pronounced enhancement of the overall strength of the specimens. Under the triaxial compression condition, merely a dominated shear fracture can be seen on the surfaces, and with relatively high confining pressure (σ3 = 20 and 30 MPa), both the rock and cement mortar were cut into two parts by the dominated shear fracture. The laboratory tests in this study provide a simple and feasible way of investigating the interaction of the support system with the surrounding rock. [ABSTRACT FROM AUTHOR]

Understanding the micromechanical mechanism of the rock creep process is of great importance for studying the macroscopic time-dependent behavior of rocks. In this study, the evolution characteristics of the microstructure (cracks and pores) of saturated sandstones under short term and creep uniaxial compression conditions were investigated with the nuclear magnetic resonance (NMR) technique. The samples were first loaded to different stress levels and creep stages and then completely unloaded for NMR testing. Based on the testing results, the macroscopic deformation behavior, moisture migration law, pore size distribution, porosity, and microstructure change of the each sample under the short-term loading or different stages of creep were quantitatively analyzed. After that, by introducing a nonlinear elasto-viscoplastic damage creep model (EVP) by Zhao et al. (18:04017129, 2018), the relationships between the macroscopic irreversible strains and microscopic porosity increments were established. Overall, it was observed that: (1) regardless of the stress level, the magnitudes of the axial and lateral critical strains of samples at the onset of the accelerating creep stage are both relatively constant, and the axial strain is almost comparable to that at the peak stress in the short-term test, while the lateral strain is larger than that of the short-term test. (2) During the mechanical tests, the moisture in the samples migrates from large pores into small pores, and after mechanical tests, the porosities of the samples increase, in which the small pores always account for a larger proportion. (3) Corresponding to the three creep stages, the porosity of the sample increases slightly after the transient stage, increases to a constant value that is largely independent of stress after the steady stage, and increases significantly after the creep failure. In particular, compared to the initial porosity of 6.7%, the average porosities of samples taken to the onset of the tertiary stage and creep failure is 7.49% and 8.71%, increasing by 16.7% and 29.8%, respectively. (4) The porosity growth of sandstone during the brittle creep is mainly driven by the microscopic subcritical crack growth along the grain boundaries. [ABSTRACT FROM AUTHOR]

Investigations of the flow and diffusion mechanism of grout slurry in underground fractured rock mass are of critical significance to guarantee the grouting reinforcement of deep soft rock tunnel and to ensure the construction safety. This paper reports a series of permeation grouting tests in fractured sandstone samples based on the low-field nuclear magnetic resonance (LF-NMR) technique. During the grouting process, the NMR signal characteristics of the grout were monitored in real-time, and the parameters such as the slurry injection volume, effective grouting time and grout filling speed were comprehensively analyzed under various temperatures, confining pressures, flow velocities and numbers of fractures. The results show that the final injection amount of the grout decreases with increasing temperature and confining pressure. The effective grouting time is inversely proportional to the flow velocity, while it is positively related to the temperature. The filling speed decreases with increasing the temperature and confining pressure, while it increases with increasing the grout flow velocity. Compared to the single fractured sample, the sample with more fractures is injected with more grout slurry in a shorter effective grouting time and a higher filling speed. Under the conditions of high confining pressure (e.g., 15 MPa) or high flow velocity (e.g., ≥5 ml/min), the grout slurry preferentially transports into the micropores, and then the mesopores and macropores. However, the temperature and number of fractures have equal effects on the grout flow and diffusion in the three types of pores. The grouting filling performance becomes worse when the temperature and confining pressure are higher. The findings in this study provide meaningful guidance for the grouting reinforcement of deep soft rock tunnel. [ABSTRACT FROM AUTHOR]

The precise determination of unfrozen water content is of great significance to understand the freezing process of pore water and reveal the freeze-induced damage mechanism for frozen rock mass. In this study, low field nuclear magnetic resonance (LF-NMR) tests were conducted on five types of frozen sandstone samples. The changes in the T 2 distribution and unfrozen water content (including unfrozen bound water content and unfrozen movable water content) were obtained and then analyzed. The experimental results indicate that there is little unfrozen movable water exists in the sandstone sample at the temperature of -5 °C and lower, whereas the amount of the unfrozen bound water is slowly decreased during the freezing process. The pore size distribution largely dominates the speed of water-ice phase transition and the total ice volume in the sandstone samples. A larger equivalent average pore radius (r m) leads to a higher speed of water-ice phase transition. The freeze-induced strains of the frozen sandstones were theoretically calculated based on a frost heave model. It is indicated that the freezing strain increases rapidly in the temperature range of 0 °C to -10 °C, while tends to be stable with further decreasing the temperature. Owing to the different pore size distributions, the movable water induces more substantial freezing strain than the bound water in terms of the Wuding sandstone and Linyi sandstone having an r m larger than 1.6 μs. However, it is exactly inverted for the other three sandstones including the Tengchong sandstone, Luzhou sandstone and Zigong sandstone with much smaller r m. The study provides a reasonable method for quantifying the unfrozen water content and freeze-induced strain in frozen rock, as well as important guidance for the design and construction of rock engineering in cold regions. [ABSTRACT FROM AUTHOR]

In this study, the low-field nuclear magnetic resonance (NMR) testing technique was adopted to experimentally investigate the seepage processes of chemical grout flow in fractured rock under various temperatures and confining pressures. The variations in the NMR parameters, including the tested porosity, transverse relaxation time (T 2) distribution and T 2 peak area, were quantitatively analysed to study the seepage characteristics under different grouting conditions. The changes in the grout viscosity as a function of the temperature were studied. Then, the effects of injection pressures and temperatures on the seepage characteristics of porous sandstone with a single fracture and a constant confining pressure of 20 MPa were systematically investigated. The results indicated that the viscosity of the grout shows obvious time-dependent and temperature-dependent characteristics. The flow pattern of the grout is greatly dependent on the injection pressure, whereas it is less affected by the temperature. The permeability of the sandstone shows an increasing trend as the injection pressure increases and the temperature decreases. [ABSTRACT FROM AUTHOR]

Investigation of the seepage characteristics and the corresponding repairing effects of slurry on fractured rock mass is of great significance to grouting theory and operation parameters designing. Several chemical grouting tests under different temperatures and grouted pressures were conducted and the seepage processes were monitored by the LF-NMR (low field nuclear magnetic resonance) technique. The influence of temperature on the flow properties, grouted volume and permeability of slurry were comprehensively studied through the analysis of the NMR T 2 response. The effects of temperature on the mechanical performance of grouted specimens after curing were investigated through the directly shear tests. The results indicated that the temperature has an obvious impact on the seepage characteristics, the higher the temperature, the lower the permeability. The mechanical performance of grouted specimen is closely dependent on the grouting temperature, the inter friction angle φ and the cohesion c exponentially decrease as the temperature increases. A temperature dependent shear strength mechanical model of slurry bonding layer was proposed and verified. The findings of this study are useful for the parameters designing of grouting operation in deep-buried tunnels. [ABSTRACT FROM AUTHOR]

In this paper, to study the influence of grouting on meso-failure mechanism, acoustic emission (AE) localization technique was used to monitor the micro-cracking during the uniaxial compression tests on single flawed sandstone specimens with or without grouting. Due to the inhomogeneous geometrical structure of the flawed specimens, the mesoscopic damage evolution is also inhomogeneous during the loading process. As a result, the wave velocity of the specimens varied temporally and spatially, which greatly increased the difficulty in accurate AE localization. To solve this problem, the wave velocities versus the axial load for the studied sandstone were first measured, and the stress concentration effect near the flaw was also theoretically analyzed. Accordingly, the localization arithmetic was also improved to match the complex wave velocity field. Results show that compared to the specimens without grouting, more microcracks (particularly the shear microcracks) are induced in the grout-infilled specimens prior to the final failure of the specimens. Moreover, the effect of grouting on microcrack initiation also varies with the oblique angle of the flaw. For θ (flaw inclination) ≤30°, grouting significantly increases the shear microcrack at macrocrack propagation stage which states that the axial tensile cracking was limited by the supporting effect of the grout material and made the failure mode trend to shear mode. For θ = 45° and 60°, grouting improved the ratio of shear microcrack during the whole failure process, indicating that the direction of maximum shear stress is close to the flaw direction. Hence, failure develops in stable shear mode. For θ = 75° and 90°, the primary microcrack generated during the first macrocrack initiation changed to shear mode, but the grouting has little influence on the subsequent crack propagation. So that, the reinforcement effect is poor in this set of specimens. • The AE localization technique was improved by adopting a temporally and spatially varied wave velocity model. • Grouting decreases the stress concentration near the prefabricated flaw, which changes the mechanism of crack evolution. • The development of a macro crack in the flawed specimen follows a tensile-shear-alternating meso damage evolution mode. [ABSTRACT FROM AUTHOR]

Understanding the micro-mechanism of brittle creep in saturated rocks and studying their mechanical behaviors after creep damage are of great significance to the design of rock engineering and the prediction of the long-term evolution of the Earth's crust. In this study, we first performed a series of uniaxial creep tests with different creep stage cut-offs on saturated intact sandstones. The brittle creep mechanism of the specimens was explored microscopically by analyzing P-wave velocity, acoustic emission (AE), and nuclear magnetic resonance (NMR) measurements. Then, specimens with different degrees of creep damage were subjected to a secondary short-term loading test or creep test, and their short- and long-term mechanical behaviors, together with AE monitoring, were systematically investigated. The experimental results show that (1) the anisotropy of the specimens increases significantly during creep, with the axial and lateral P-wave velocities exhibiting a difference of 30% at the onset of tertiary creep. (2) The tensile cracks dominate the transient creep stage and the early part of the steady creep stage, while the shear cracks dominate thereafter. (3) The porosities of the specimens increase with the loading stages, and the porosity increments corresponding to small pores are greater than those corresponding to large pores, for all creep stages. (4) Specimens with different degrees of creep damage show greater deformability, and their short-term and creep mechanical parameters, except for uniaxial strength, change dramatically. (5) The AE characteristics of creep-damaged specimens differ significantly from those of intact specimens, and the Kaiser stress memory effect was observed during reloading, with the memory stress being the previous creep stress. [ABSTRACT FROM AUTHOR]