The squeezing scenario in deep weak rock tunnels can hinder underground construction. However, due to the limitations of test technologies at hand, the real excavation stress path cannot be mimicked in the laboratory. Thus, the large deformation mechanism of deep weak rocks still remains unclear. For this, a true triaxial apparatus (TTA) to investigate the mechanical responses of deep weak rock under excavation stress paths in field and reveal the squeezing mechanism of deep tunnels is assembled and developed at Northeastern University, China. The apparatus can perform instantaneous unloading in σ3 direction based on electromagnetism technology. In addition, uniform loading and deformation measurements can be carried out based on the proposed linked interlocking clamp and antifriction device, even if the sample has a strong dilatation deformation performance. Next, a bore trepanning is designed to capture noiseless acoustic emission (AE) signals for deep weak rock at a low threshold. Finally, two tests were are conducted using this instrument to preliminarily understand the failure and deformation features of deep weak rock based on fractured marble. The results show that the complete stress–strain curves of fractured marble have the characteristics of low strengths and large deformations, and the larger deformation and the more serious failure occur when the fractured marble enters the post-peak state after excavation. The results show that the developed apparatus is likely to be applicable for deep weak rock engineering.

Resonance effects in parallel jointed rocks subject to stress waves are investigated using transfer functions, derived from signals generated through numerical modelling. Resonance is important for a range of engineering situations as it identifies the frequency of waves which will be favourably transmitted. Two different numerical methods are used for this study, adopting the finite difference method and the combined discrete element-finite difference method. The numerical models are validated by replicating results from previous studies. The two methods are found to behave similarly and show the same resonance effects; one operating at low frequency and the other operating at relatively high frequency. These resonance effects are interpreted in terms of simple physical systems and analytical equations are derived to predict the resonant frequencies of complex rock masses. Low frequency resonance is shown to be generated by a system synonymous with masses between springs, described as spring resonance, with an equal number of resonant frequencies as the number of blocks. High frequency resonance is generated through superposition of multiple reflected waves developing standing waves within intact blocks, described as superposition resonance. While resonance through superposition has previously been identified, resonance based on masses between springs has not been previously identified in jointed rocks. The findings of this study have implications for future analysis of multiple jointed rock masses, showing that a wave travelling through such materials can induce other modes of propagation of waves, i.e. spring resonance.

An open-source MATLAB application (app) named Discontinuity Intensity Calculator and Estimator (DICE) was developed in order to quantitatively characterize the fractures, or in more general, discontinuities within a rocky outcrop in three-dimensional (3D) digital data, such as digital outcrop model (DOM). The workflow proposed for the parametrization of the discontinuities consists of the following steps: (1) Analysis and mapping of the fractures detected within the 3D DOMs; (2) Calculation of the orientation, position and dimensions of discontinuities that are represented by best-fit circular planes; (3) Determining the discontinuity parameters (dimension, distribution, spacing and intensity) by the DICE algorithm using different 3D oriented sampling techniques (3D oriented scanline, 3D oriented circular scan window and spherical scan volume). Different sampling methods were bench tested with a synthetic, as well as a natural case study, and compared in order to understand the advantages and limitations of each technique. The 3D oriented circular scan window appears to be the most effective method for fracture intensity estimation with high accuracy (error 0.4%) and stability with variations in scan radius.

A variety of coal room and pillar mining methods have been efficiently practiced at depths of up to 500 m with least strata mechanics issues. However, for the first time, this method was trialled at depths of 850–900 m in CSM mine of Czech Republic. The rhomboid-shaped coal pillars with acute corners of 70°, surrounded with 5.2 m wide and 3.5–4.5 m high mine roadways, were used. Pillars were developed in a staggered manner with their size variation in the Panel II from 83 m × 25 m to 24 m × 20 m (corner to corner) and Panel V from 35 m × 30 m to 26 m × 16 m. Coal seam inclined at 12° was affected by the unusual slippery slickenside roof bands and sometimes in the floor levels with high vertical stress below strong and massive sandstone roof. In order to ensure safety, pillars in both the panels were continuously monitored using various geotechnical instruments measuring the induced stresses, side spalling and roof sagging. Both panels suffered high amounts of mining induced stress and pillar failure with side-spalling up to 5 m from all sides. Heavy fracturing of coal pillar sides was controlled by fully encapsulated steel bolts. Mining induced stress kept increasing with the progress of development of pillars and galleries. Instruments installed in the pillar failed to monitor actual induced stress due to fracturing of coal mass around it which created an apprehension of pillar failure up to its core due to high vertical mining induced stress. This risk was reduced by carrying out scientific studies including the three-dimensional numerical models calibrated with data from the instrumented pillar. An attempt has been made to study the behavior of coal pillars and their yielding characteristics at deeper cover based on field and simulation results.

To perform landslide susceptibility prediction (LSP), it is important to select appropriate mapping unit and landslide-related conditioning factors. The efficient and automatic multi-scale segmentation (MSS) method proposed by the authors promotes the application of slope units. However, LSP modeling based on these slope units has not been performed. Moreover, the heterogeneity of conditioning factors in slope units is neglected, leading to incomplete input variables of LSP modeling. In this study, the slope units extracted by the MSS method are used to construct LSP modeling, and the heterogeneity of conditioning factors is represented by the internal variations of conditioning factors within slope unit using the descriptive statistics features of mean, standard deviation and range. Thus, slope units-based machine learning models considering internal variations of conditioning factors (variant slope-machine learning) are proposed. The Chongyi County is selected as the case study and is divided into 53,055 slope units. Fifteen original slope unit-based conditioning factors are expanded to 38 slope unit-based conditioning factors through considering their internal variations. Random forest (RF) and multi-layer perceptron (MLP) machine learning models are used to construct variant Slope-RF and Slope-MLP models. Meanwhile, the Slope-RF and Slope-MLP models without considering the internal variations of conditioning factors, and conventional grid units-based machine learning (Grid-RF and MLP) models are built for comparisons through the LSP performance assessments. Results show that the variant Slope-machine learning models have higher LSP performances than Slope-machine learning models; LSP results of variant Slope-machine learning models have stronger directivity and practical application than Grid-machine learning models. It is concluded that slope units extracted by MSS method can be appropriate for LSP modeling, and the heterogeneity of conditioning factors within slope units can more comprehensively reflect the relationships between conditioning factors and landslides. The research results have important reference significance for land use and landslide prevention.

The mechanical characteristics of crystalline rocks are affected by the heterogeneity of the spatial distribution of minerals. In this paper, a novel three-dimensional (3D) grain-based model (GBM) based on particle flow code (PFC), i.e. PFC3D-GBM, is proposed. This model can accomplish the grouping of mineral grains at the 3D scale and then filling them. Then, the effect of the position distribution, geometric size, and volume composite of mineral grains on the cracking behaviour and macroscopic properties of granite are examined by conducting Brazilian splitting tests. The numerical results show that when an external load is applied to a sample, force chains will form around each contact, and the orientation distribution of the force chains is uniform, which is independent of the external load level. Furthermore, the number of high-strength force chains is proportional to the external load level, and the main orientation distribution is consistent with the external loading direction. The main orientation of the cracks is vertical to that of the high-strength force chains. The geometric size of the mineral grains controls the mechanical behaviours. As the average grain size increases, the number of transgranular contacts with higher bonding strength in the region connecting both loading points increases. The number of high-strength force chains increases, leading to an increase in the stress concentration value required for the macroscopic failure of the sample. Due to the highest bonding strength, the generation of transgranular cracks in quartz requires a higher concentrated stress value. With increasing volume composition of quartz, the number of transgranular cracks in quartz distributed in the region connecting both loading points increases, which requires many high-strength force chains. The load level rises, leading to an increase in the tensile strength of the numerical sample.

This paper proposes a methodology to construct logs of rock strength from the cutting force signal recorded in scratch tests conducted in the ductile regime. The approach, which is based on the application of discrete wavelet transforms, recognizes the existence of two length scales ℓc and ℓr. The strength length scale ℓc represents the length over which it is meaningful to measure strength, while the repeatability length scale ℓr is related to the resolution at which the force signal must be observed to become insensitive to the stochastic micro-failure events triggered by the motion of the cutter. It is postulated that the original cutting force signal, assumed to be sampled at a high enough frequency, can be decomposed into a deterministic signal intrinsic to the rock and a stochastic one resulting from discrete rock failure events. The technique of multiresolution analysis based on the maximal overlap discrete wavelet transform is applied as a low-pass filter to the original cutting force signals so as to wipe out the high-frequency components associated with the stochastic rock failure events. A criterion to determine the optimum cutoff frequency of the low-pass filter and the corresponding repeatability length scale is discussed in terms of the correlation coefficients between different filtered signals. It is shown that the low-pass filtered signals obtained at the optimum cutoff frequency have two salient features: (i) repeatability over different tests conducted at the same depth of cut on the same sample, and (ii) variability along the cutting distance. The excellent repeatability reveals that the deterministic background trend of the original force signals is relevant to the rock strength property, and the variability of the background trend captures the spatial variation of the rock strength.

Water is the most abundant molecule found on the earth's surface and is a key factor in multiscale rock destruction. However, given the fine-grained nature of rock and the complexity of its internal structure, the microstructural evolution of rock under the action of water has not yet been elucidated in detail, and little is understood about the relationship between the rock structure and solid–liquid unit. A variety of techniques were used in this study to track the mechanical properties, pore and crack characteristics, and mineral structure degradation characteristics of sandstone at different stages under the action of deionized water, and the evolution mechanisms of the microstructure were analyzed at the molecular scale. The results showed that during the water–rock interaction process, water was adsorbed onto the surface of dolomite minerals and the hydrophilic surface of clay minerals, forming a high-density hydrogen bond network. However, different mineral surface structures had different water adsorption structures, resulting in the strain of the dense clay mineral aggregates under expansion action. Stress concentrated at crack tips under the capillary force of dolomite minerals (very weak dolomite dissolution). These effects resulted in a substantial increase in the number of small pores and enhancements in pore–crack connectivity, and the rock strength exhibited varying degrees of decline at different stages of wet-dry cycles. In general, the results of this paper will help to further elucidate the internal connections between molecular-scale and macroscale processes in rock science.

Study on crack propagation process of brittle rock is of most significance for cracking-arrest design and cracking-network optimization in rock engineering. Phase-field model (PFM) has advantages of simplicity and high convergence over the common numerical methods (e.g. finite element method, discrete element method, and particle manifold method) in dealing with three-dimensional and multi-crack problems. However, current PFMs are mainly used to simulate mode-I (tensile) crack propagation but difficult to effectively simulate mode-II (shear) crack propagation. In this paper, a new mixed-mode PFM is established to simulate both mode-I and mode-II crack propagation of brittle rock by distinguishing the volumetric elastic strain energy and deviatoric elastic strain energy in the total elastic strain energy and considering the effect of compressive stress on mode-II crack propagation. Numerical solution method of the new mixed-mode PFM is proposed based on the staggered solution method with self-programmed subroutines UMAT and HETVAL of ABAQUS software. Three examples calculated using different PFMs as well as test results are presented for comparison. The results show that compared with the conventional PFM (which only simulates the tensile wing crack but not mode-II crack propagation) and the modified mixed-mode PFM (which has difficulty in simulating the shear anti-wing crack), the new mixed-mode PFM can successfully simulate the whole trajectories of mixed-mode crack propagation (including the tensile wing crack, shear secondary crack, and shear anti-wing crack) and mode-II crack propagation, which are close to the test results. It can be further extended to simulate multi-crack propagation of anisotropic rock under multi-field coupling loads.

A reasonable evaluation of unloading deformation characteristics is of great significance for the effective analysis of deformation and stability of surrounding rocks after underground excavation. In this study, the damage-controlled cyclic triaxial loading tests were conducted to investigate the pore compaction mechanism and its influences on the unloading deformation behavior of red sandstone, including Young's modulus, Poisson's ratio, volumetric strain, and irreversible strain. The experimental results show that the increases of volumetric and irreversible strains of rocks can be attributed to the compaction mechanism, which almost dominates the entire pre-peak deformation process. The unloading deformation consists of the reversible linear and nonlinear strains, and the irreversible strain under the influence of the porous grain structure. The pre-peak Young's modulus tends to increase and then decrease due to the influence of the unloading irreversible strain. However, it hardly changes with the increasing volumetric strain compaction under the influence of reversible nonlinear strain. Instead, the initial unloading tangent modulus is highly related to the volumetric strain, and clearly reflects the compaction state of red sandstone. Furthermore, both the reversible nonlinear and irreversible unloading deformations are independent of confining pressure. This study is beneficial for the theoretical modeling and prediction of cyclic unloading deformation behavior of red sandstone.

Fault activation has been the focus of research community for years. However, the studies of fault activation remain immature, such as the fault activation mode and its major factors under constant normal stiffness (CNS) conditions associated with large thickness of fault surrounding rock mass. In this study, the rock friction experiments were conducted to understand the fault activation modes under the CNS conditions. Two major parameters, i.e. the initial normal stress and loading rate, were considered and calibrated in the tests. To reveal the response mechanism of fault activation, the local strains near the fault plane were recorded, and the macroscopic stresses and displacements were analyzed. The testing results show that the effect of displacement-controlled loading rate is more pronounced under the CNS conditions than that under constant normal load (CNL) conditions. Both the normal and shear stresses drop suddenly when the stick-slip occurs. The decrease and increase of the normal stress are synchronous with the shear stress in the regular stick-slip scenario, but mismatch with the shear stress during the chaotic stick-slip process. The results are helpful for understanding the fault sliding mode and the prediction and prevention of fault slip.

Microbial-induced carbonate precipitation (MICP) and enzyme-induced carbonate precipitation (EICP) are two bio-cementation techniques, which are relatively new methods of ground improvement. While both techniques share some similarities, they can exhibit different overall behaviours due to the differences in urease enzyme sources and treatment methods. This paper presented 40 unconfined compressive strength (UCS) tests of MICP and EICP treated sand specimens with similar average calcium carbonate (CaCO3) content subjected to cycles of wetting-drying (WD), freezing-thawing (FT) and elevated temperature (fire resistance test – FR and thermogravimetric analysis – TG). The average CaCO3 content after a certain number of WD or FT cycles (ACn) and their corresponding UCS (qn) reduced while the mass loss increased. The EICP treated sand specimens appeared to exhibit a lower resistance to WD and FT cycles than MICP treated specimens possibly due to the presence of unbonded or loosely bonded CaCO3 within the soil matrix, which was subsequently removed during the wetting (during WD) or thawing (during FT) process. FR test and TG analysis showed a significant loss of mass and reduction in CaCO3 content with increased temperatures, possibly due to the thermal decomposition of CaCO3. A complete deterioration of the MICP and EICP treated sand specimens was observed for temperatures above 600 °C. The observed behaviours are complex and theoretical understanding is far behind to develop a constitutive model to predict qn. Therefore, a multi-objective evolutionary genetic algorithm (GA) that deals with pseudo-polynomial structures, known as evolutionary polynomial regression (EPR), was used to seek three choices from millions of polynomial models. The best EPR model produced an excellent prediction of qn with a minimum sum of squares error (SSE) of 2.392, mean squared error (MSE) of 0.075, root mean square error (RMSE) of 0.273 and a maximum coefficient of determination of 0.939.

This study presents a hybrid framework to predict stability solutions of buried structures under active trapdoor conditions in natural clays with anisotropy and heterogeneity by combining physics-based and data-driven modeling. Finite-element limit analysis (FELA) with a newly developed anisotropic undrained shear (AUS) failure criterion is used to identify the underlying active failure mechanisms as well as to develop a numerical (physics-based) database of stability numbers for both planar and circular trapdoors. Practical considerations are given for natural clays to three linearly increasing shear strengths in compression, extension, and direct simple shear in the AUS material model. The obtained numerical solutions are compared and validated with published solutions in the literature. A multivariate adaptive regression splines (MARS) algorithm is further utilized to learn the numerical solutions to act as fast FELA data-driven surrogates for stability evaluation. The current MARS-based modeling provides both relative importance index and accurate design equations that can be used with confidence by practitioners.

Previous studies have demonstrated the effectiveness of a novel three-layer landfill cover system constructed with recycled concrete aggregates (RCAs) without geomembrane in both laboratory and field. However, no systematic investigation has been carried out to optimize the combination of the particle sizes for fine-grained RCAs (FRC) and coarse-grained RCAs (CRC) that can be used for the three-layer landfill cover system. The aim of this paper is to assist engineers in designing the three-layer landfill cover system under a rainfall of 100-year return period in humid climate conditions using an easily controlled soil parameter D10 of RCAs. The numerical study reveals that when D10 of FRC increases from 0.05 mm to 0.16 mm, its saturated permeability increases by 10 times. As a result, a larger amount of rainwater infiltrates into the cover system, causing a higher lateral diversion in both the top FRC and middle CRC layers. No further changes in the lateral diversion are observed when the D10 value of FRC is larger than 0.16 mm. Both the particle sizes of FRC and CRC layers are shown to have a minor influence on the percolation under the extreme rainfall event. This implies that the selection of particle sizes for the FRC and CRC layers can be based on the availability of materials. Although it is well known that the bottom layer of the cover system should be constructed with very fine-grained soils if possible, this study provides an upper limit to the particle size that can be used in the bottom layer (D10 not larger than 0.02 mm). With this limit, the three-layer system can still minimize the water percolation to meet the design criterion (30 mm/yr) even under a 100-year return period of rainfall in humid climates.

It is well known that fabric of sand may significantly affect mechanical behaviors and liquefaction resistance of sand. Various optical techniques are currently utilized to visualize the fabric, especially the distribution of the long axis of soil particles. However, none of these methods provides an ideal solution in laboratory tests and in situ observation. In this study, anisotropy of magnetic susceptibility (AMS) was first proposed as a convenient and efficient way to evaluate the liquefaction of clean sand. At first, investigations with scanning electron microscopy (SEM) and AMS were simultaneously conducted on two groups of soil specimens with different initial fabrics to verify the feasibility of the AMS technique. Then, 80 in situ samples were collected to analyze the feature of liquefied and non-liquefied sand layers through AMS tests. It is clearly known from the test results that the natural sedimentary fabric was destroyed during liquefaction and the fabric anisotropy was greatly changed after liquefaction. The feasibility of evaluating soil fabric using the AMS survey was verified by the laboratory tests. Furthermore, the applicability of AMS in detecting liquefied layer in situ was confirmed for the first time.

This paper presents an undrained semi-analytical elastoplastic solution for cylindrical cavity expansion in anisotropic soil under the biaxial stress conditions. The advanced simplified SANICLAY model is used to simulate the elastoplastic behavior of soil. The cavity expansion is treated as an initial value problem and solved as a system of eight first-order ordinary differential equations including four stress components and four anisotropic parameters. The results are validated by comparing the new solutions with existing ones. The distributions of stress components and anisotropic parameters around the cavity wall, the expansion process, the stress yield trajectory of a soil element and the shape and size of elastoplastic boundary are further investigated to explore the cavity expansion response of soils under biaxial in situ stresses. The results of extensive parameters analysis demonstrate that the circumferential position of the soil element and the anisotropy of the soils have noticeable impacts on the expansion response under biaxial in situ stresses. Since the present solution not only considers the anisotropy and anisotropy evolution of natural soil, but also eliminates the conventional assumption of uniform radial pressure, the solution is better than other theoretical solutions to explain the pressure test and pile installation effect of shallow saturated soil.

A typical blasting vibration wave is a composite wave, and its attenuation law is affected by the type of dominant wave component. The purpose of the present study is to establish an attenuation equation of the peak particle velocity (PPV), taking into account the attenuation characteristics of P-, S- and R-waves in the blasting vibration wave. Field blasting tests were carried out as a case to specifically apply the proposed equation. In view of the fact that the discrete properties of rock mass will inevitably cause the uncertainty of blasting vibration, we also carried out a probability analysis of PPV uncertainty, and introduced the concept of reliability to evaluate blasting vibration. The results showed that the established attenuation equation had a higher prediction accuracy, and can be considered as a promising equation implemented on more complex sites. The adopted uncertainty analysis method can comprehensively take account of the attenuation law of blasting vibration measured on site and discrete properties of rock masses. The obtained distribution of the PPV uncertainty factor can quantitatively evaluate the reliability of blasting vibration, which is a powerful and necessary supplement to the PPV attenuation equation.

Rockburst is becoming a huge challenge for the utilization of deep underground space. Extensive efforts have been devoted to investigating the rockburst behavior and mechanism experimentally, theoretically, and numerically. The aim of this review is to discuss the novel development and the state-of-the-art in experimental techniques, theories, and numerical approaches proposed for rockburst. The definition and classification of rockburst are first summarized with an in-depth comparison among them. Then, the available laboratory experimental technologies for rockburst are reviewed in terms of indirect and direct approaches, with the highlight of monitoring technologies and data analysis methods. Some key rockburst influencing factors (i.e. size and shape, rock types, stress state, water content, and temperature) are analyzed and discussed based on collected data. After that, rockburst theories and mechanisms are discussed and evaluated, as well as the microscopic observation. The simulation approaches of rockburst are also summarized with the highlight of optional novel numerical methods. The accuracy, stability, and reliability of different experimental, theoretical and numerical approaches are also compared and assessed in each part. Finally, a summary and some aspects of prospective research are presented.