Search

To explore the influence of the working face excavation rate on the rock deformation mechanism and seepage characteristics, deformation and seepage tests of sandstone under different loading and unloading stress paths, such as constant axial pressure unloading confining pressure and loading axial pressure unloading confining pressure, were carried out. Particle Flow Code in 3 Dimensions (PFC3D) and Python were used to realize fluid-solid coupling, and numerical simulation calculations were performed along the test path to analyze the influence of the unloading rate on the fine-scale deformation mechanism and permeability characteristics of sandstone, and the relationship between crack type and permeability was obtained. A sandstone fracture mechanics model is established to analyze the stress concentration degree at the end of the branch crack of the test path. The results show that the rate of confining pressure unloading is inversely proportional to the strain. Additionally, permeability correlates with the principal stress difference in an exponential manner. Interestingly, the sensitivity of permeability to stress shows an inverse trend with the unloading rate of confining pressure. Furthermore, there exists a linear relationship between permeability and the number of cracks. During the unloading process, tensile cracks predominate, and the propagation of shear cracks lags behind that of tensile cracks. The proportion of tensile cracks decreases with the increase of the unloading rate when the axial pressure is unchanged but increases when axial pressure is added, resulting in axial compression deformation and expansion deformation along the unloading direction. These research outcomes offer theoretical insights for the prudent selection of mining rates, and they hold significant implications for mitigating water inrush disasters in deep mining operations. [ABSTRACT FROM AUTHOR]

Coral reef limestone is a biogenic limestone with a hyperdeveloped pore structure. The complex structural features of coral reef limestone can directly affect its mechanical and seepage characteristics, which are not yet well-understood. The pore structure information of two coral reef limestone specimens, including one with coarse pores (CR-C) and one with fine pores (CR-F), was obtained by combining CT scanning technology, 3D visualization reconstruction, and pore identification algorithms. The pore structure characteristics were interpreted considering the porosity's spatial variability, fractal dimension, and tensorial anisotropy. A seepage tensor derived from the geometry component of Hagen-Poiseuille's law was proposed to assess the seepage anisotropy, which was then verified through seepage simulations along different directions. It was found that the spatial distribution of the porosity of CR-C was less uniform than that of CR-F. Both testing specimens had distinct pore structures with obvious directionality that varied with sampling size. Seepage simulations on a representative elementary volume (REV) of CR-C revealed that the permeability along the identified dominant seepage direction was indeed the largest. Some empirical correlations between the permeability and the deviation angle from the dominant seepage direction were established. This study highlights the importance of considering structural anisotropy when interpreting the properties of coral reef limestone. [ABSTRACT FROM AUTHOR]

In this study, the bond-slip behavior between textile reinforced concrete (TRC)-confined concrete and rebars both at ambient temperature and after ISO 834 standard fire was investigated experimentally through pull-out tests. The failure modes, ultimate load, average ultimate bonding stress and energy consumption were obtained and discussed. Effects of rebar diameters (12 mm, 14 mm and 16 mm), bonding lengths (2.5d, 5d and 7.5d) and high-temperature exposure durations (30 min and 60 min) on the bonding behavior between TRC-confined concrete and rebars were examined emphatically. It was observed that the damage modes of specimens changed from pull-out failure at ambient temperature into splitting failure for both TRC-confined specimens and companion unconfined specimens exposed to high temperatures. The results obtained demonstrated that both the ultimate load and the average ultimate bonding stress can be significantly improved by the utilization of TRC jackets after 30 min exposure, and the increment of the average ultimate bonding stress can reach up to 90% as compared to those unconfined specimens. However, the enhancement of TRC jackets became unobvious after 60 min. Finally, based on test results and Prigogine's dissipative structure theory, the energy analysis of bond-slip curves was evaluated to verify the enhancement of TRC jackets quantitatively. [ABSTRACT FROM AUTHOR]

The arrival behavior of elastic waves in a naturally fractured rock is studied based on numerical simulations. We use the discrete fracture network method to represent the distribution of a natural fracture system and employ the displacement discontinuity method to compute the propagation of elastic waves across individual fractures. We analyze macroscopic wavefield arrival properties collectively arising from the interaction between elastic waves and numerous fractures in the system. We show that the dimensionless angular frequency ῶ = ωZ/κ exerts a fundamental control on the arrival behavior of a plane wave traveling through the fractured rock, where ω, Z, and κ are the angular frequency, seismic impedance, and fracture stiffness, respectively. An asynchronous arrival phenomenon of the wave energy occurs and becomes more significant with an increased ῶ. Two regimes are identified according to the two-branch dependency of the fractal dimension D of the FFAW on ῶ, where the wave arrival behavior is within a non-fractal regime for ῶ smaller than the critical frequency ῶc ≈ 1.0, and enters the fractal regime for ῶ ≥ ῶc. The self-affine properties of the FFAW, i.e., the roughness exponent α and the correlation length lc, both linearly decrease as a function of the exponent ξ (with ῶ = 10ξ) in the fractal regime. Early breakthrough of wave transport occurs in regions with relatively low fracture density, while late-time arrival happens in regions of high fracture density. Highlights: Elastic wave transport in a naturally fractured rock is studied based on numerical simulations. Asynchronous arrival phenomenon of the wave energy becomes more significant with an increased dimensionless angular frequency ῶ. The front of first arrival waves is characterized by properties correlated with ῶ in the fractal regime. Early breakthrough of wave energy occurs in regions of low fracture density, while late-time arrival happens in regions of high fracture density. [ABSTRACT FROM AUTHOR]

Due to the complexity of the talus-like rock mass with different values of volumetric block proportion (VPB), it is thus crucial to explore the VBP effect on the excavation-induced ground responses. We conduct a series of 2D DEM (discrete element method) simulations on a common circular tunnel excavation in the talus-like rock mass with different VBPs (0%, 15%, 50%, 85% and 100%). For each VBP, two support scenarios, i.e., unsupported and supported by a rigid lining, are considered. The micro characteristics of the excavation-induced ground responses, including the contact force, force chain, coordination number and shear-slip contact, and the stress distribution and ground settlement are elaborated in detail. Accordingly, three types of talus-like rock masses are identified as soil-, hybrid- and rock-types, corresponding to VBP = 0–15%, 50%, and 85–100%, respectively. It is found that the lining support is essential for maintaining the ground stability of a tunnel excavation in the soil- and hybrid-type talus-like rock masses while the backbones formed by rock blocks in the rock-type talus-like rock mass can provide a certain support for the surrounding ground. Our findings have important implications for optimizing the construction scheme of tunnel excavation in different types of talus-like rock masses. [ABSTRACT FROM AUTHOR]

Due to the sensitivity of sandy soil's mechanical behavior to the particle shape, it is thus of importance for interpreting the effect of particle shape on the ground response induced by tunnel excavation in sandy formations. We conducted a series of 2D DEM (discrete element method) simulations on a common circular tunnel excavation in sandy soil with variable-shaped particles, which are characterized as two descriptors, i.e., aspect ratio (AR) and convexity (C). The macroscopic responses and the microscopic characteristics of the sandy ground are elaborated in detail. The simulation results show obvious asymmetrical features of the excavated ground, which results from the ground heterogeneity caused by the irregular particle shape. In addition, we investigate the roles of AR and C on the ground response and find that reducing AR or increasing C will enlarge the ground settlement, i.e., the sandy ground deformation is more sensitive to the particles with more irregular shapes. However, elongated particles are beneficial for the generation of soil arching with stronger bearing capacity and thus reduce the soil pressure on the tunnel lining. Our findings have important implications for the safety assessment of the tunnel excavation, as well as other underground structure construction in sandy formations. [ABSTRACT FROM AUTHOR]

The talus-like rock mass is a special kind of geomaterial widely distributed in southwestern China, which has induced serious engineering disasters for tunneling engineering. However, the mechanical behavior of the talus-like rock mass remains unclear as the previous studies mainly focused on similar geomaterials such as the soil–rock mixtures. In this paper, we have carried out both experimental and discrete element method (DEM)-based numerical analyses to investigate the shearing characteristics of the talus-like rock mass collected from a real project site. Large-scale direct shear tests reveal that the strength parameters increase with the block content, which is different from the traditional soil–rock mixture. A dependence has been discovered in that the specimen dilation becomes more obvious under lower normal stress and larger block content. It is also observed that higher normal stress is beneficial for crushing blocks. The force chains obtained in the DEM simulations show that distinct internal structures are generated in the rock samples with different block contents. The distribution of coordination number establishes the dependence of fabric stability on block content during shearing. Bond-break evolution reveals the tendencies of crushed particles were consistent with those of experimental tests. The findings provide a more in-depth understanding about the mechanical behavior of the talus-like rock mass, which helps to uncover the cause of the collapse of the real tunnel project. [ABSTRACT FROM AUTHOR]

The rock breaking mechanism plays a significant role in TBM (tunnel boring machine) performance prediction, which has not yet been understood satisfactorily. In this work, a series of rotary cutting simulations via a particle-based DEM (discrete element method) for mudstone were conducted to explore the rock breaking mechanism and cutter-rock interaction during TBM tunnelling. Different cutting scenarios were considered. The rock breaking events were inferred from the breakage of bonds between particles constituting the mudstone. The spatial distribution of microcracks was obtained in the form of bond breaks between particles. The cracking features of mudstone are different from those reported for hard rocks. The characteristics of the contact stress distribution were also analysed and fitted by a combined Hertz-Weibull model. The distribution ranges of bond breaks and cutter-rock contact stress are both positively related to the penetration depth but not largely dependent on the cutting radius. However, the side force has a negative relationship with the cutting radius, and this dependency is more significant with increasing penetration depth. The bond-break distribution is skewed to the outer side of the cutter due to different cutting trajectories and centrifugal effects. Moreover, the bonds tend to break towards the existing cutting groove due to the release of radial confinement. The variation in cutting forces interrelates with the bond breakage events, which reduces the local stiffness and strength of the rock. The existence of the former cutting groove yields eases rock breakage and associated smaller cutting forces. [ABSTRACT FROM AUTHOR]

The stability analysis of rock blocks on man-made excavation faces (e.g. tunnel, cavern, and slope) subject to seismic loads is an important issue in the field of rock engineering. This paper proposes a generalized block theory (GBT) by combining a pseudo-static method and the traditional block theory to evaluate the stability of blocky rock masses during earthquake activities. In our analysis, the basic safety factors are derived considering time-varying seismic loads to determine the stability of a rock block at each time step. Afterwards, two new parameters, Pu and Vu, are used to evaluate the seismic stability of a rock block, where Pu is the instability probability defined as the ratio of the time for the block becoming unstable to the total seismic loading time, and Vu is the probabilistic instability volume defined as Pu times the block volume. As for a blocky rock mass system, its probabilistic instability volume is the sum of Vu of all seismically unstable blocks and the instability probability is the ratio of its probabilistic instability volume and total volume of seismically unstable blocks. Through the simulation of a generic slope excavation, we observe that seismic loads significantly affect the stability and kinematics of a rock block during an earthquake. For a blocky rock mass, both Pu and Vu decay with the epicentral distance, in general following an inverse power law trend. Furthermore, it is found that the local site effect also has a strong influence on the slope stability under seismic loads. [ABSTRACT FROM AUTHOR]

Tayi tunnel is one of the component tunnels in the Jian-Ge-Yuan Highway Project located in Yunnan Province, southeast of China. It mainly passes through talus-like formations comprised of rock blocks of diverse sizes and weak interlayers with clayey soils with different fractions. Such a special composition leads to the loose and fractured structure of talus-like formations, which is highly sensitive to the excavation perturbation. Therefore, Tayi tunnel has become the controlled pot of the whole highway project as the construction speed has to be slowed down to reduce the deformation of surrounding talus-like rock mass. To better understand the tunnel-induced ground response and the interaction between the surrounding rock mass and tunnel lining, a comprehensive in situ monitoring program was set up. The in situ monitoring contents included the surrounding rock pressure on the primary lining, the primary lining deformation, and the stress of steel arches. Based on the monitoring data, the temporal and the long-term spatial characteristics of mechanical behavior of surrounding rock mass and lining structure due to the excavation process were analyzed and discussed. It is found that the excavation of lower benches released the surrounding rock pressure around upper benches, resulting in the decrease of the surrounding rock pressure on the primary lining and the stress of steel arches. In addition, the monitoring data revealed that the primary lining sustained bias pressure from the surrounding rock mass, which thereby caused unsymmetrical deformation of the primary lining, in accordance with the monitored displacement data. A dynamically adaptive support system was implemented to strengthen the bearing capacity of the lining system especially in the region of an extremely weak rock mass. After such treatment, the deformation of the primary lining has been well controlled and the construction speed has been considerably enhanced. [ABSTRACT FROM AUTHOR]

It is important to consider machine-block interaction when assessing the stability of tunnels excavated by tunnel boring machine (TBM) in blocky rock mass as the behavior of rock blocks changes with TBM progression. The cutterhead motion (advancement and rotation) alters the geometry of rock blocks and the cutters acting on the block. As a consequence, the contact forces between the cutterhead and rock blocks change, which leads to the variation of block kinematics and stability. This paper extends the traditional block theory to analyze the life-cycle behavioristics (morphology, mechanics, kinematics and stability) of rock blocks during TBM tunneling considering TBM-block interaction. The morphology analysis follows the traditional block theory but considers TBM-block interaction. The cutterhead-block interaction is decomposed into a normal component and a tangential component, both of which depend on cutterhead motion. The removability conditions and mechanical equations for three traditional translational modes and two rotational modes are derived, based on which the kinematics and stability of rock blocks during TBM tunneling can be determined. Implementation of the proposed methodologies in reflecting the dynamic evolution of block behavioristics in the process of TBM tunneling is demonstrated through a generic example. [ABSTRACT FROM AUTHOR]

Equations 3, 4, 5, 11, 13, 15, 17, 18, 19, 20, 22, 23, 24, 25, 26, 27 and 31 are reported incorrectly. The correct equations are given below. [ABSTRACT FROM AUTHOR]