In the past decade, numerical modelling has been increasingly used for simulating the mechanical behaviour of naturally fractured rock masses. In this paper, we introduce new algorithms for spatial and temporal analyses of newly generated fractures and blocks using an integrated discrete fracture network (DFN)-finite-discrete element method (FDEM) (DFN-FDEM) modelling approach. A fracture line calculator and analysis technique (i.e. discrete element method (DEM) fracture analysis, DEMFA) calculates the geometrical aspects of induced fractures using a dilation criterion. The resultant two-dimensional (2D) blocks are then identified and characterised using a graph structure. Block tracking trees allow track of newly generated blocks across timesteps and to analyse progressive breakage of these blocks into smaller blocks. Fracture statistics (number and total length of initial and induced fractures) are then related to the block forming processes to investigate damage evolution. The combination of various proposed methodologies together across various stages of modelling processes provides new insights to investigate the dependency of structure's resistance on the initial fracture configuration.

A massive rock and ice avalanche occurred on the western slope of the Ronti Gad valley in the northern part of Chamoli, Indian Himalaya, on 7 February 7, 2021. The avalanche on the high mountain slope at an elevation of 5600 m above sea level triggered a long runout disaster chain, including rock mass avalanche, debris avalanche, and flood. The disaster chain had a horizontal travel distance of larger than 17,600 m and an elevation difference of 4300 m. In this study, the disaster characteristics and dynamic process were analyzed by multitemporal satellite imagery. The results show that the massive rock and ice avalanche was caused by four large expanding discontinuity planes. The disaster chain was divided into five zones by satellite images and field observation, including source zone, transition zone, dynamic entrainment zone, flow deposition zone, and flood zone. The entrainment effect and melting water were recognized as the main causes of the long-runout distance. Based on the seismic wave records and field videos, the time progress of the disaster was analyzed and the velocity of frontal debris at different stages was calculated. The total analyzed disaster duration was 1247 s, and the frontal debris velocity colliding with the second hydropower station was approximately 23 m/s. This study also carried out the numerical simulation of the disaster by rapid mass movement simulation (RAMMS). The numerical results reproduced the dynamic process of the debris avalanche, and the mechanism of long-runout avalanche was further verified by parametric study. Furthermore, this study discussed the potential causes of disaster and flood and the roles of satellite images and seismic networks in the monitoring and early-warning.

This study experimentally analyzes the nonlinear flow characteristics and channelization of fluid through rough-walled fractures during the shear process using a shear-flow-visualization apparatus. A series of fluid flow and visualization tests is performed on four transparent fracture specimens with various shear displacements of 1 mm, 3 mm, 5 mm, 7 mm and 10 mm under a normal stress of 0.5 MPa. Four granite fractures with different roughnesses are selected and quantified using variogram fractal dimensions. The obtained results show that the critical Reynolds number tends to increase with increasing shear displacement but decrease with increasing roughness of fracture surface. The flow paths are more tortuous at the beginning of shear because of the wide distribution of small contact spots. As the shear displacement continues to increase, preferential flow paths are more distinctly observed due to the decrease in the number of contact spots caused by shear dilation; yet the area of single contacts increases. Based on the experimental results, an empirical mathematical equation is proposed to quantify the critical Reynolds number using the contact area ratio and fractal dimension.

In numerical modelling, selection of the constitutive model is a critical factor in predicting the actual response of a geomaterial. The use of oversimplified or inadequate models may not be sufficient to reproduce the actual geomaterial behaviour. That selection is especially relevant in the case of anisotropic rocks, and particularly for shales and slates, whose behaviour may be affected, e.g. well stability in geothermal or oil and gas production operations. In this paper, an alternative anisotropic constitutive model has been implemented in the finite element method software CODE_BRIGHT, which is able to account for the anisotropy of shales and slates in terms of both deformability and strength. For this purpose, a transversely isotropic version of the generalised Hooke's law is adopted to represent the stiffness anisotropy, while a nonuniform scaling of the stress tensor is introduced in the plastic model to represent the strength anisotropy. Furthermore, a detailed approach has been proposed to determine the model parameters based on the stress–strain results of laboratory tests. Moreover, numerical analyses are performed to model uniaxial and triaxial tests on Vaca Muerta shale, Bossier shale and slate from the northwest of Spain (NW Spain slate). The experimental data have been recovered from the literature in the case of the shale and, in the case of the slate, performed by the authors in terms of stress-strain curves and strengths. A good agreement can be generally observed between numerical and experimental results, hence showing the potential applicability of the approach to actual case studies. Therefore, the presented constitutive model may be a promising approach for analysing the anisotropic behaviour of rocks and its impact on well stability or other relevant geomechanical problems in anisotropic rocks.

The shear behavior is regarded as the dominant property of rock joints and is dramatically affected by the joint surface roughness. To date, the effect of surface roughness on the shear behavior of rock joints under static or cyclic loading conditions has been extensively studied, but such effect under impact loading conditions keeps unclear. To address this issue, a series of impact shear tests was performed using a novel-designed dynamic experimental system combined with the digital image correlation (DIC) technique. The dynamic shear strength, deformability and failure mode of the jointed specimens with various joint roughness coefficients (JRC) are comprehensively analyzed. Results show that the shear strength and shear displacement characteristics of the rock joint under the impact loading keep consistent with those under static loading conditions. However, the temporal variations of shear stress, slip displacement and normal displacement under the impact loading conditions show obviously different behaviors. An elastic rebound of the slip displacement occurs during the impact shearing and its value increases with increasing joint roughness. Two identifiable stages (i.e. compression and dilation) are observed in the normal displacement curves for the rougher rock joints, whereas the joints with small roughness only manifest normal compression displacement. Besides, as the roughness increases, the maximum compression tends to decrease, while the maximum dilation gradually increases. Moreover, the microstructural analysis based on scanning electron microscope (SEM) suggests that the roughness significantly affects the characteristics of the shear fractured zone enclosing the joint surface.

Quasi-NPR (negative Poisson’s ratio) steel is a new type of super bolt material with high strength, high ductility, and a micro-negative Poisson’s effect. This material overcomes the contrasting characteristics of the high strength and high ductility of steel and it has significant energy-absorbing characteristics, which is of high value in deep rock and soil support engineering. However, research on the shear resistance of quasi-NPR steel has not been carried out. To study the shear performance of quasi-NPR steel bolted rock joints, indoor shear tests of bolted rock joints under different normal stress conditions were carried out. Q235 steel and #45 steel, two representative ordinary bolt steels, were set up as a control group for comparative tests to compare and analyze the shear strength, deformation and instability mode, shear energy absorption characteristics, and bolting contribution of different types of bolts. The results show that the jointed rock masses without bolt reinforcement undergo brittle failure under shear load, while the bolted jointed rock masses show obvious ductile failure characteristics. The shear deformation capacity of quasi-NPR steel is more than 3.5 times that of Q235 steel and #45 steel. No fracture occurs in the quasi-NPR steel during large shear deformation and it can provide stable shear resistance. However, the other two types of control bolts become fractured under the same conditions. Quasi-NPR steel has significant energy-absorbing characteristics under shear load and has obvious advantages in terms of absorbing the energy released by shear deformation of jointed rock masses as compared with ordinary steel. In particular, the shear force plays a major role in resisting the shear deformation of Q235 steel and #45 steel, therefore, fracture failure occurs under small bolt deformation. However, the axial force of quasi-NPR steel can be fully exerted when resisting joint shear deformation; the steel itself does not break when large shear deformation occurs, and the supporting effect of the jointed rock mass is effectively guaranteed.

The initiating condition for the accelerated creep of rocks has caused difficulty in analyzing the whole creep process. Moreover, the existing Nishihara model has evident shortcomings in describing the accelerated creep characteristics of the viscoplastic stage from the perspective of internal energy to analyze the mechanism of rock creep failure and determine the threshold of accelerated creep initiation. Based on the kinetic energy theorem, Perzyna viscoplastic theory, and the Nishihara model, a unified creep constitutive model that can describe the whole process of decaying creep, stable creep, and accelerated creep is established. Results reveal that the energy consumption and creep damage in the process of creep loading mainly come from the internal energy changes of geotechnical materials. The established creep model can not only describe the viscoelastic–plastic creep characteristics of rock, but also reflect the relationship between rock energy and creep deformation change. In addition, the research results provide a new method for determining the critical point of creep deformation and a new idea for studying the creep model and creep mechanical properties.

A method for packing irregular particles with a prescribed volume fraction is proposed. Furthermore, the generated granular material adheres to the prescribed statistical distribution and satisfies the desired complex spatial arrangement. First, the irregular geometries of the realistic particles were obtained from the original particle images. Second, the Minkowski sum was used to check the overlap between irregular particles and place an irregular particle in contact with other particles. Third, the optimised advance front method (OAFM) generated irregular particle packing with the prescribed statistical distribution and volume fraction based on the Minkowski sum. Moreover, the signed distance function was introduced to pack the particles in accordance with the desired spatial arrangement. Finally, seven biaxial tests were performed using the UDEC software, which demonstrated the accuracy and potential usefulness of the proposed method. It can model granular material efficiently and reflect the mesostructural characteristics of complex granular materials. This method has a wide range of applications where discrete modelling of granular media is necessary.

Evaluating the fracture resistance of rocks is essential for predicting and preventing catastrophic failure of cracked structures in rock engineering. This investigation developed a brittle fracture model to predict tensile mode (mode I) failure loads of cracked rocks. The basic principle of the model is to estimate the reference crack corresponding to the fracture process zone (FPZ) based on the maximum normal strain (MNSN) ahead of the crack tip, and then use the effective crack to calculate the fracture toughness. We emphasize that the non-singular stress/strain terms should be considered in the description of the MNSN. In this way, the FPZ, non-singular terms and the biaxial stress state at the crack tip are simultaneously considered. The principle of the model is explicit and easy to apply. To verify the proposed model, laboratory experiments were performed on a rock material using six groups of specimens. The model predicted the specimen geometry dependence of the measured fracture toughness well. Moreover, the potential of the model in analyzing the size effect of apparent fracture toughness was discussed and validated through experimental data reported in the literature. The model was demonstrated superior to some commonly used fracture models and is an excellent tool for the safety assessment of cracked rock structures.

The mechanical behavior of coal is the key factor affecting underground coal mining and coalbed methane extraction. In this study, triaxial compression and seepage tests were carried out on coal at different gas pressures. The mechanical properties and failure process of coal were studied, as well as the acoustic emission (AE) and strain energy. The influence of gas pressure on the mechanical parameters of this coal was analyzed. Based on the conventional energy calculation formula, the pore pressure was introduced through the effective stress formula, and each energy component of coal containing gas was refined innovatively. The contribution of gas pressure to the total energy input and dissipation during loading was quantitatively described. Finally, the influence of gas pressure on coal strength was theoretically analyzed from the perspectives of Mohr–Coulomb criterion and fracture mechanics. The results show that the total absorbed energy comprises the absorbed energy in the axial pressure direction (positive) and in the confining pressure direction (negative), as well as that induced by the pore pressure (initially negative and then positive). The absorbed energy in the axial pressure direction accounts for the main proportion of the total energy absorbed by coal. The quiet period of AE in the initial stage shortens, and AE activity increases during the pre-peak stage under high gas pressure. The fractal characteristics of AE in three stages are studied using the correlation dimension. The AE process has different forms of self-similarity in various deformation stages.

The irregularity of jointed network poses a challenge to the determination of field mechanical parameters of columnar jointed rock mass (CJRM), and a reasonable prediction of deformation and strength characteristics of CJRM is important for engineering construction. The Voronoi diagram and three-dimensional printing technology were used to make an irregular columnar jointed mold, and the irregular CJRM (ICJRM) specimens with different dip directions and dip angles were prepared. Uniaxial compression tests were performed, and the anisotropic strength and deformation characteristics of ICJRM were described. The failure modes and mechanisms were revealed in accordance with the final appearances of the ICJRM specimens. Based on the model test results, the empirical correlations for determining the field deformation and strength parameters of CJRM were derived using the dip angle and modified joint factor. The proposed empirical equations were used in the Baihetan Project, and the calculated mechanical parameters were compared with the field test results and those obtained from the tunneling quality index method. Results showed that the deformation parameters determined by the two proposed methods are all consistent with the field test results, and these two methods can also estimate the strength parameters effectively.

Water-induced landslides in hydropower reservoirs pose a great threat to both project operation and human life. This paper examines three large reservoirs in Sichuan Province, China. Field surveys, site monitoring data analyses and numerical simulations are used to analyze the spatial distribution and failure mechanisms of water-induced landslides in reservoir areas. First, the general rules of landslide development in the reservoir area are summarized. The first rule is that most of the landslides have rear edge elevations of 100–500 m above the normal water level of the reservoir, with volumes in the range of 106–107 m3. When the volume exceeds a certain amount, the number of sites at which the landscape can withstand landslides is greatly reduced. Landslide hazards mainly occur in the middle section of the reservoir and less in the annex of the dam site and the latter half of the reservoir area. The second rule is that sedimentary rocks such as sandstone are more prone to landslide hazards than other lithologies. Then, the failure mechanism of changes in the water level that reduces the stability of the slope composed of different geomaterials is analyzed by a proposed slope stability framework that considers displacement and is discussed with the monitoring results. Permeability is an essential parameter for understanding the diametrically opposed deformation behavior of landslides experiencing filling-drawdown cycles during operation. This study seeks to provide inspiration to subsequent researchers, as well as guidance to technicians, on landslide prevention and control in reservoir areas.

Rock fracturing is often accompanied by electromagnetic phenomenon. As a vector field, in addition to the intensity that is widely concerned, the generated electromagnetic field also has obvious directionality. To this end, a set of electromagnetic antennas capable of simultaneous three-axis measurement is used to monitor the electromagnetic vector field generated from rock fracturing based on Brazilian tests. The signal amplitude on each axis can represent the magnitude of actual magnetic flux density component on the three axes. The intensity and directional characteristics of electromagnetic signals received at different positions are studied using vector synthesis. The directionality of electromagnetic radiation measured using a three-axis electromagnetic antenna shows that the direction of the magnetic flux intensity generated by rock fracturing tends to be parallel to the crack surface, and the measured signal intensity is greater in a direction closer to the crack surface.

A multi-purpose prototype test system is developed to study the mechanical behavior of tunnel supporting structure, including a modular counterforce device, a powerful loading equipment, an advanced intelligent management system and an efficient noncontact deformation measurement system. The functions of the prototype test system are adjustable size and shape of the modular counterforce structure, sufficient load reserve and accurate loading, multi-connection linkage intelligent management, and high-precision and continuously positioned noncontact deformation measurement. The modular counterforce structure is currently the largest in the world, with an outer diameter of 20.5 m, an inner diameter of 16.5 m and a height of 6 m. The case application proves that the prototype test system can reproduce the mechanical behavior of the tunnel lining during load-bearing, deformation and failure processes in detail.

Shield tunneling is easily obstructed by clogging in clayey strata with small soil particles. However, soil clogging rarely occurs in strata with coarse-grained soils. Theoretically, a critical particle size of soils should exist, below which there is a high risk of soil clogging in shield tunneling. To determine the critical particle size, a series of laboratory tests was carried out with a large-scale rotary shear apparatus to measure the tangential adhesion strength of soils with different particle sizes and water contents. It was found that the tangential adhesion strength at the soil–steel interface gradually increased linearly with applied normal pressure. When the particle size of the soil specimen was less than 0.15 mm, the interfacial adhesion force first increased and then decreased as the water content gradually increased; otherwise, the soil specimens did not manifest any interfacial adhesion force. The amount of soil mass adhering to the steel disc was positively correlated with the interfacial adhesion force, thus the interfacial adhesion force was adopted to characterize the soil clogging risk in shield tunneling. The critical particle size of soils causing clogging was determined to be 0.15 mm. Finally, the generation mechanism of interfacial adhesion force was explored for soils with different particle sizes to explain the critical particle size of soil with clogging risk in shield tunneling.

The wide engineered application of compacted expansive soils necessitates understanding their behavior under field conditions. The results of this study demonstrate how seasonal climatic variation and stress and boundary conditions individually or collectively influence the hydraulic and volume change behavior of compacted highly expansive soils. The cyclic wetting and drying (CWD) process was applied for two boundary conditions, i.e. constant stress (CS) and constant volume (CV), and for a wide range of axial stress states. The adopted CWD process affected the hydraulic and volume change behaviors of expansive soils, with the first cycle of wetting and drying being the most effective. The CWD process under CS conditions resulted in shrinkage accumulation and reduction in saturated hydraulic conductivity (ksat). On the other hand, CWD under CV conditions caused a reduction of swell pressure while has almost no impact on ksat. An elastic response to CWD was achieved after the third cycle for saturated hydraulic conductivity (ksat), the third to fourth cycle for the volume change potential under the CV conditions, and the fourth to fifth cycle for the volume change potential under the CS conditions. Finally, both swell pressure (σs) and saturated hydraulic conductivity (ksat) are not fundamental parameters of the expansive soil but rather depend on stress, boundary and wetting conditions.

Soil stabilization using nanomaterials is an emerging research area although, to date, its investigation has mostly been laboratory-based and therefore requires extensive study for transfer to practical field applications. The present study advocates nano-calcium carbonate (NCC) material, a relatively unexplored nanomaterial additive, for stabilization of low-plasticity fine-grained soil having moderate organic content. The plasticity index, compaction, unconfined compressive strength (UCS), compressibility and permeability characteristics of the 0.2%, 0.4%, 0.6% and 0.8% NCC-treated soil, and untreated soil (as control), were determined, including investigations of the effect of up to 90-d curing on the UCS and permeability properties. In terms of UCS improvement, 0.4% NCC addition was identified as the optimum dosage, mobilizing a UCS at 90-d curing of almost twice that for the untreated soil. For treated soil, particle aggregation arising from NCC addition initially produced an increase in the permeability coefficient, but its magnitude decreased for increased curing owing to calcium silicate hydrate (CSH) gel formation, although still remaining higher compared to the untreated soil for all dosages and curing periods investigated. Compression index decreased for all NCC-treated soil investigated. SEM micrographs indicated the presence of gel patches along with particle aggregation. X-ray diffraction (XRD) results showed the presence of hydration products, such as CSH. Significant increases in UCS are initially attributed to void filling and then because of CSH gel formation with increased curing.

Calcareous sand is widely present in coastal areas around the world and is usually considered as a weak and unstable material due to its high compressibility and low strength. Microbial-induced calcium carbonate precipitation (MICP) is a promising technique for soil improvement. However, the commonly adopted bio-augmented MICP approach is in general less compatible with the natural soil environment. Thus, this study focuses on the bio-stimulated MICP approach, which is likely to enhance the dominance of ureolytic bacteria for longer period and thus is deemed more efficient. The main objective of this paper is to investigate the compressibility of calcareous sand treated by bio-stimulated MICP approach. In the current study, a series of one-dimension compression tests was conducted on bio-cemented sand prepared via bio-stimulation with different initial relative densities (Dr). Based on the obtained compression curves and particle size distribution (PSD) curves, the parameters including cementation content, the coefficient of compressibility (av), PSD, relative breakage (Br), and relative agglomeration (Ar) were discussed. The results showed that av decreased with the increasing cementation content. The bio-cemented sand prepared with higher initial Dr had smaller (approximately 20%–70%) av values than that with lower initial Dr. The specimen with higher initial Dr and higher cementation content resulted in smaller Br but larger Ar. Finally, a conceptual framework featuring multiple contact and damage modes was proposed.