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Characterizing the changes in the meso-structure of granite against thermo-hydro-mechanical (THM) effect is of great significance to understand the variation mechanism of granite hydro-mechanical properties in disposal system. In this study, an improved meso-structure-based DEM model (I-MSBM) was first developed to adequately consider the weakening effects of the mineral grain bonds caused by THM loading, in which the aperture-dependent strength and elastic modulus weakening factors of pores were determined by experiments. Comparisons between the numerical and experimental results indicated that the developed model was suitable to distinguish the THM responses of varying minerals, mineral boundaries and pores in the granite. Based on the I-MSBM, the changes in the permeability and macro mechanical properties, i.e., the uniaxial compressive strength and elastic modulus of granite were investigated under the conditions of coupled temperatures (25–225 °C), pore pressures (0–12 MPa) and confining pressure (15 MPa), and the main mechanisms affecting the varied macro hydro-mechanical properties were further discussed. The results indicate that the permeability and macro mechanical properties of granite are more sensitive to the high pore pressure (P > 4 MPa) at moderate temperatures (T < 125 °C), which is mainly due to the variation in the pore aperture induced by high pore pressure. Furthermore, the macro hydro-mechanical properties are more susceptible to temperature higher than 125 °C owing to the accelerated development of the thermal-induced micro cracks. The findings of this study are helpful for understanding the complex THM coupling damage to the granite in the nuclear waste disposal systems. Highlights: An improved meso-structure-based DEM model was developed to characterize the THM coupled effects on weakening the bonds among the mineral grains, in which the aperture-dependent weakening factors of pores were determined. The THM responses of varying minerals, mineral boundaries and pores in the granite were distinguished via the improved meso-structure-based DEM model The changes in the macro hydro-mechanical properties of granite subjected to THM coupling in disposal system were clarified, and the mechanisms that affect the varied macro hydro-mechanical properties were further revealed. [ABSTRACT FROM AUTHOR]

Yielding supports have shown significant advantages in overcoming the problems of large deformations caused by tunnelling in squeezing ground. The purpose of this study is to provide an analytical design method for yielding supports from the perspective of coordinated deformation. The analyses are carried out on the deformation behaviour of two yielding members in yielding supports, including the shotcrete lining with highly deformable elements and the steel arch with sliding joints. Their deformation process is divided into three stages, and the main reasons for their contraction in each stage are explained. The unified pressure–displacement expressions for the shotcrete lining with highly deformable elements and the steel arch with sliding joints are provided. Considering different yielding displacements of two yielding members, six deformation cases of yielding supports are discussed. Theoretical formulas are proposed to describe the deformation behaviour of yielding supports in each case. The reliability and effectiveness of the analytical model are validated by comparison with the previous numerical results, and a good agreement between them is achieved. Finally, a parametric study of the influence of steel arch spacing and yielding length is carried out. The results show that the support pressure increases as the steel arch spacing decreases. The bearing capacity of the yielding supports is insufficient and the yielding members would cause failure, if the steel arch spacing is too large. When the yielding length of one member is much greater than that of the other, it is easy to found that one member has failed severely while the other is still in the yielding stage. [ABSTRACT FROM AUTHOR]

In tunnel construction, excavation stoppages are often encountered due to many irresistible factors. This study reveals the influence mechanism of excavation stoppages on tunnel mechanical response by using mathematical analytical method. Firstly, the mathematical expressions for stress release coefficient considering multiple excavation stoppages are redefined and provided, based on the previous stress release coefficient in the negative exponential function form. Secondly, the mechanical model of a deep circular lined tunnel considering excavation stoppages is established, and the unified analytical solutions for tunnel displacement and liner pressure during the unlined and lined (excavation and excavation stoppage) stages are provided, respectively. Furthermore, the proposed solutions are able to capture the average tunnel deformation and liner pressure in Rongjiawan tunnel (an excavation stoppage took place) and agree well with the numerical results. Finally, a parametric investigation is performed, including the influences of starting time and duration time of excavation stoppage, excavation rate and liner installation time. Results show that both the final tunnel displacement and liner pressure are unaffected by starting time and duration time of excavation stoppage. However, their convergence rates are significantly affected by these two excavation stoppage factors. A higher excavation rate before stoppage causes a larger tunnel displacement and a smaller liner pressure. The influence of re-excavation rate is too low to be paid attention. A higher liner pressure can be induced by an earlier liner installation. The findings in this study are helpful for the evaluation of excavation stoppage-caused influence in tunnels. [ABSTRACT FROM AUTHOR]

The mechanical responses of tunnels reinforced by combined bolt–cable system are investigated in this paper. An analytical model of the convergence-confinement type is proposed that accounts for the sequential installation of finite-length fully grouted bolts and cables, as well as the effects of intermediate principal stress. The rock mass is assumed to be strain-softening material, obeying the unified strength theory or the Drucker–Prager criterion. The analytical model is divided into three stages including before bolt installation, bolt–rock interaction and after cable installation, of which the latter two stages are the focus of this study. According to the distribution and extent of the plastic zone and the relative bolt–cable lengths, six forms during the bolt–rock interaction and ten forms after cable installation are categorized and solved. Using the critical support pressures for the transitions of different forms, the modified whole-process ground reaction curve is obtained and validated by 2D numerical simulations and an existing model. Furthermore, the fictitious pressure is introduced to quantify the three-dimensional space effect of the tunnel face, whose solution is given by combining the proposed ground reaction curve and the longitudinal displacement profile proposed by Hoek. By using the strain compatibility conditions of bolts, cables and rock, the longitudinal tunnel displacement, stresses and plastic radii considering the sequential installation of bolts and cables are obtained. The results of the proposed model agree well with the 3D numerical simulations. Parametric analyses are conducted to investigate the effect of intermediate principal stress on the tunnel responses by different criteria. The proposed model provides a convenient alternative tool for the combined bolt–cable design of tunnels at the preliminary stage. [ABSTRACT FROM AUTHOR]

In situ rock mass usually contains discontinuities in various forms, including fissures, joints and fractures, etc., and it becomes more heterogeneous when the discontinuities are backfilled with artificial grouting. It is incredibly challenging to obtain an accurate wave velocity model for those rocks with random inclusions, making the AE sources more difficult to locate. This study first proposed a projective reconstruction method (PRM) to obtain the wave velocity model for heterogeneous rock. An improved AE source localization algorithm was proposed using the arrival time-based governing equations. Flawed sandstone samples with various inclination angles were then prepared and subjected to unconfined compression until failure. The fragments were restored to generate post-grouted samples with random inclusions. The feasibility and validation of the PRM-based wave velocity model and the proposed AE source localization algorithm were verified by pencil lead break tests. The AE source localization results were compared using three wave velocity models, i.e., averaged wave velocity model, 1D approximated wave velocity model and PRM-based wave velocity model. The results indicate that the PRM-based AE source localization for the post-grouted samples is much more accurate than the other two models. Finally, the fracturing mechanisms of the post-grouted samples with random inclusions under uniaxial compression conditions were illuminated based on the results of the AE source localization and moment tensor inversion. It is confirmed that the PRM-based wave velocity model was promising for the AE source localization in heterogeneous rocks with random inclusions. Highlights: A projective reconstruction method for obtaining the wave velocity field of heterogeneous rocks. More accurate AE source localization was achieved for post-grouted sandstones with random inclusions. The proposed PRM-based wave velocity model are promising for microseismic localization application in the practical field. [ABSTRACT FROM AUTHOR]

The safety of circumferential yielding lining is influenced significantly by the shotcrete hardening property. However, presently, negligible research attention has been paid to this topic. This study aims to establish an analytical model for determining the support stiffness of circumferential yielding lining. Here, the shotcrete hardening property is considered. The entire deformation process of circumferential yielding lining is divided into three stages according to the mechanical characteristic of the yielding elements. A comparison of the element yield time and shotcrete age yields two cases that need to be discussed. Furthermore, analytical solutions for the time-dependent lining stiffness in different cases are proposed based on the previous shotcrete hardening equations. The corresponding equation for the lining pressure is also provided to evaluate the safety of shotcrete lining. The proposed analytical model is applied effectively in the Saint Martin La Porte access adit and predicts the average response of the monitoring tunnel convergences. Finally, a comprehensive parametric investigation is carried out, which includes the effects of yield stress, elastic modulus and length of yielding elements and ground stress on the shotcrete lining safety. Certain inspiring findings and effective recommendations for the design of circumferential yielding lining are provided. Highlights: Equations for time-dependent stiffness of circumferential yielding lining are proposed. The real-time evaluation of shotcrete lining safety is performed. The influences of yielding element parameters and ground stress are discussed. [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]

This paper presents the analytical solutions for the responses of tunnels excavated in rock masses exhibiting strain-softening behavior. Since previous analyses give little consideration to the effect of the intermediate principal stress on the strain-softening rock behavior, the unified strength theory was introduced to analyze the tunnel response. Four cases of different definitions of the elastic strain in the softening and residual regions, used in the existing solutions, were considered. The tunnel displacements, stresses, radii of the softening and residual zones and critical stresses were deduced. The proposed solutions were verified by comparing with numerical simulations, model tests and existing solutions. Furthermore, the solutions of the four cases were compared with each other to investigate the influence of the elastic strain expressions on the tunnel responses. The results showed that the intermediate principal stress coefficient b has a significant effect on the tunnel displacements, stress fields, and plastic radii. Parametric studies were performed to analyze the influences of the softening and residual dilatancy coefficients, softening modulus and residual strength on the tunnel responses. The parametric analysis indicated that the existing models should be carefully evaluated in the analysis of tunnels constructed through average-quality rocks; the proposed solutions outperformed the existing models in solving the mentioned problem. [ABSTRACT FROM AUTHOR]

Shale is one of the most widely distributed and common types of surrounding rock in underground structures such as tunnels. However, the mechanical strength of shale decreases significantly under the combined action of stress and osmotic pressure. In this study, we conducted triaxial compression tests under variable osmotic and confining pressures to evaluate the mechanical behavior of shale under a combination of stress and seepage. The peak mechanical strength of shale decreased as the osmotic pressure increased, which resulted in five different macro failure modes corresponding to different levels of osmotic pressures. Furthermore, high confining pressures induced failure of the compression band which considerably inhibited permeability, with stages of shale fracture developing in correspondence with changes in the permeability. The minimum permeability of shale occurred before the expansion point. While the confining pressure played an important role in the growth of cracks, the osmotic pressure had a relatively weak effect. The quantitative relationship between permeability, axial strain, and volumetric strain found in this study sufficiently reflects the evolution law of shale permeability during the deformation and failure processes. Based on these results, we developed a new damage constitutive model for rock under osmotic pressure by introducing statistical damage theory. The model can reflect the stress–strain relationship better under different confining and osmotic pressures, can accurately capture the key points of the mechanical response, and depends on parameters that are simple to calculate, which is important for engineering applications. [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]