Feng Dai

Fracture Toughness Determination of Cracked Chevron Notched Brazilian Disc Rock Specimen via Griffith Energy Criterion Incorporating Realistic Fracture Profiles

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to measure the mode I fracture toughness of rocks, and has been widely adopted in laboratory tests. Nevertheless, a certain discrepancy has been observed in results when compared with those derived from methods using straight through cracked specimens, which might be due to the fact that the fracture profiles of rock specimens cannot match the straight through crack front as assumed in the measuring principle. In this study, the progressive fracturing of the CCNBD specimen is numerically investigated using the discrete element method (DEM), aiming to evaluate the impact of the realistic cracking profiles on the mode I fracture toughness measurements. The obtained results validate the curved fracture fronts throughout the fracture process, as reported in the literature. The fracture toughness is subsequently determined via the proposed G-method originated from Griffith’s energy theory, in which the evolution of the realistic fracture profile as well as the accumulated fracture energy is quantified by DEM simulation. A comparison between the numerical tests and the experimental results derived from both the CCNBD and the semi-circular bend (SCB) specimens verifies that the G-method incorporating realistic fracture profiles can contribute to narrowing down the gap between the fracture toughness values measured via the CCNBD and the SCB method.

Experimental Investigation of the Influence of Joint Geometric Configurations on the Mechanical Properties of Intermittent Jointed Rock Models Under Cyclic Uniaxial Compression

Intermittent joints in rock mass are quite sensitive to cyclic loading conditions. Understanding the fatigue mechanical properties of jointed rocks is beneficial for rational design and stability analysis of rock engineering projects. This study experimentally investigated the influences of joint geometry (i.e., dip angle, persistency, density and spacing) on the fatigue mechanism of synthetic jointed rock models. Our results revealed that the stress–strain curve of jointed rock under cyclic loadings is dominated by its curve under monotonic uniaxial loadings; the terminal strain in fatigue curve is equal to the post-peak strain corresponding to the maximum cyclic stress in the monotonic stress–strain curve. The four joint geometrical parameters studied significantly affect the fatigue properties of jointed rocks, including the irreversible strains, the fatigue deformation modulus, the energy evolution, the damage variable and the crack coalescence patterns. The higher the values of the geometrical parameters, the lower the elastic energy stores in this jointed rock, the higher the fatigue damage accumulates in the first few cycles, and the lower the fatigue life. The elastic energy has certain storage limitation, at which the fatigue failure occurs. Two basic micro-cracks, i.e., tensile wing crack and shear crack, are observed in cyclic loading and unloading tests, which are controlled principally by joint dip angle and persistency. In general, shear cracks only occur in the jointed rock with higher dip angle or higher persistency, and the jointed rock is characterized by lower fatigue strength, larger damage variable and lower fatigue life.

Experimental and Numerical Study on the Cracked Chevron Notched Semi-Circular Bend Method for Characterizing the Mode I Fracture Toughness of Rocks

The cracked chevron notched semi-circular bending (CCNSCB) method for measuring the mode I fracture toughness of rocks combines the merits (e.g., avoidance of tedious pre-cracking of notch tips, ease of sample preparation and loading accommodation) of both methods suggested by the International Society for Rock Mechanics, which are the cracked chevron notched Brazilian disc (CCNBD) method and the notched semi-circular bend (NSCB) method. However, the limited availability of the critical dimensionless stress intensity factor (SIF) values severely hinders the widespread usage of the CCNSCB method. In this study, the critical SIFs are determined for a wide range of CCNSCB specimen geometries via three-dimensional finite element analysis. A relatively large support span in the three point bending configuration was considered because the fracture of the CCNSCB specimen in that situation is finely restricted in the notch ligament, which has been commonly assumed for mode I fracture toughness measurements using chevron notched rock specimens. Both CCNSCB and NSCB tests were conducted to measure the fracture toughness of two different rock types; for each rock type, the two methods produce similar toughness values. Given the reported experimental results, the CCNSCB method can be reliable for characterizing the mode I fracture toughness of rocks.

Static and dynamic uniaxial compression tests on coal rock considering the bedding directivity

The mechanical properties and behavior of coal rock under both static and dynamic loading rates are of importance in the coal mining practices. In this study, both quasi-static and dynamic uniaxial compression tests are conducted on coal rock, considering the bedding directivity of coal rocks using a MTS hydraulic servo-control testing machine and split Hopkinson pressure bar (SHPB), respectively. The attained strain rates range from 10−5 to 10−2xa0s−1 for static tests and 20 to 100xa0s−1 for dynamic SHPB tests. For dynamic tests, pulse-shaping technique is utilized to achieve dynamic force balance and thus validate the quasi-static data reduction. A high-speed camera is used to capture the failure process in SHPB tests. The characteristics of failure mode, fracture strength, energy dissipation, and fractal dimension are investigated. A significant strain-rate-dependent behavior of coal rock is revealed, and the compressive strength, elasticity modulus and energy consumption increase with increasing strain rate. The bedding effect on the coal behavior at static strain rate is more prominent than that at dynamic strain rate. The measured strengths along different bedding directions exhibit distinct variations, featuring significant anisotropy. In addition, a sieving statistics analysis of the recovered fragments depicts obvious fractal; and the fracture dimension can be correlated to the fractal energy dissipation.

Microseismic Monitoring of the Left Bank Slope at the Baihetan Hydropower Station, China

Layered rock slopes are especially prone to collapse under excavation excitations due to the relatively weak strength characteristics of discontinuities such as joints, bedding planes and foliations (Liu et al. 2004, 2014). The stability analysis of layered rock slopes is of increased importance when subjected to continuous excavation. A systematic study and analysis of high rock slope stability has been performed in related studies (Stacey et al. 2004; Stead et al. 2006; Stead and Wolter 2015). However, approaches are rare that have already formed mature programmes and that have been effectively used in engineering practices. Compared with theoretical and numerical studies, field monitoring provides an actual and visual approach to the study of the deformation failure characteristics of rock slopes, especially for complicated cases (i.e., deep interlayer staggered zones and intraformational faulted zones) where theoretical or numerical solutions are difficult to obtain. Traditional measurement techniques, such as multiple-point extensometers, anchor stress gauges and global positioning system, are useful for monitoring the surface deformation. However, they are unrealistic to effectively monitor the microcracking activities in rock masses prior to the formation of macroscopic fractures on slope surface. In general, macro-fractures and deformation failure in the rock mass often lag behind the initiation, coalescences and propagation of microcracks. Thus, there must be an intrinsic correlation between micro-fractures (microseismicity) and rock slope instability (Xu et al. 2012). Although it is difficult to monitor the full-field stress, the response of the rock slope to stresses (i.e., microcracking) can be monitored. The variation of stress due to engineering excavation can be indirectly obtained through monitoring the microcracking evolution. The microseismic (MS) monitoring technique can help to achieve this goal (Kaiser 2009; Tang et al. 2011). More descriptions about the principle and applications of MS monitoring technique were detailed in related studies (Cai et al. 2001; Hirata et al. 2007; Hudyma and Potvin 2010; Trifu and Shumila 2010; Xu et al. 2014, 2016; Young et al. 2004).

Numerical Investigation of the Dynamic Properties of Intermittent Jointed Rock Models Subjected to Cyclic Uniaxial Compression

Intermittent jointed rocks, which exist in a myriad of engineering projects, are extraordinarily susceptible to cyclic loadings. Understanding the dynamic fatigue properties of jointed rocks is necessary for evaluating the stability of rock engineering structures. This study numerically investigated the influences of cyclic loading conditions (i.e., frequency, maximum stress and amplitude) and joint geometric configurations (i.e., dip angle, persistency and interspace) on the dynamic fatigue mechanisms of jointed rock models. A reduction model of stiffness and strength was first proposed, and then, sixteen cyclic uniaxial loading tests with distinct loading parameters and joint geometries were simulated. Our results indicate that the reduction model can effectively reproduce the hysteresis loops and the accumulative plastic deformation of jointed rocks in the cyclic process. Both the loading parameters and the joint geometries significantly affect the dynamic properties, including the irreversible strain, damage evolution, dynamic residual strength and fatigue life. Three failure modes of jointed rocks, which are principally controlled by joint geometries, occur in the simulations: splitting failure through the entire rock sample, sliding failure along joint planes and mixed failure, which are principally controlled by joint geometries. Furthermore, the progressive failure processes of the jointed rock samples are numerically observed, and the different loading stages can be distinguished by the relationship between the number of broken bonds and the axial stress.

Coupled DEM-CFD investigation on the formation of landslide dams in narrow rivers

Large-scale landslide dams can induce significant hazards to human lives by blocking the river flows and causing inundation upstream. They may trigger severe outburst flooding that may devastate downstream areas once failed. Thus, the advancement in understanding the formation of landslide dams is highly necessary. This paper presents 3D numerical investigations of the formation of landslide dams in open fluid channels via the discrete element method (DEM) coupled with computational fluid dynamics (CFD). By employing this model, the influence of flow velocity on granular depositional morphology has been clarified. As the grains settle downwards in the fluid channel, positive excess water pressures are generated at the bottom region, reducing the total forces acting on the granular mass. In the meantime, the particle sedimentations into the fluid channel with high impacting velocities can generate fluid streams to flow backwards and forwards. The coupled hydraulic effects of excess water pressure and fluid flow would entrain the solid grains to move long distances along the channel. For simulations using different flow velocities, the larger the flow velocity is, the further distance the grains can be transported to. In this process, the solid grains move as a series of surges, with decreasing deposit lengths for the successive surges. The granular flux into the fluid channel has very little influence on the depositional pattern of particles, while it affects the particle–fluid interactions significantly. The results obtained from the DEM-CFD coupled simulations can reasonably explain the mechanisms of granular transportation and deposition in the formation of landslide dams in narrow rivers.

Experimental Investigation on the Fatigue Mechanical Properties of Intermittently Jointed Rock Models Under Cyclic Uniaxial Compression with Different Loading Parameters

Intermittently jointed rocks, widely existing in many mining and civil engineering structures, are quite susceptible to cyclic loading. Understanding the fatigue mechanism of jointed rocks is vital to the rational design and the long-term stability analysis of rock structures. In this study, the fatigue mechanical properties of synthetic jointed rock models under different cyclic conditions are systematically investigated in the laboratory, including four loading frequencies, four maximum stresses, and four amplitudes. Our experimental results reveal the influence of the three cyclic loading parameters on the mechanical properties of jointed rock models, regarding the fatigue deformation characteristics, the fatigue energy and damage evolution, and the fatigue failure and progressive failure behavior. Under lower loading frequency or higher maximum stress and amplitude, the jointed specimen is characterized by higher fatigue deformation moduli and higher dissipated hysteresis energy, resulting in higher cumulative damage and lower fatigue life. However, the fatigue failure modes of jointed specimens are independent of cyclic loading parameters; all tested jointed specimens exhibit a prominent tensile splitting failure mode. Three different crack coalescence patterns are classified between two adjacent joints. Furthermore, different from the progressive failure under static monotonic loading, the jointed rock specimens under cyclic compression fail more abruptly without evident preceding signs. The tensile cracks on the front surface of jointed specimens always initiate from the joint tips and then propagate at a certain angle with the joints toward the direction of maximum compression.

Comprehensive evaluation of excavation-damaged zones in the deep underground caverns of the Houziyan hydropower station, Southwest China

The disturbance of a rock mass by blasting or stress redistribution can significantly influence the overall performance of an underground excavation. The characteristics of excavation-damaged zones (EDZs) during the excavation of underground caverns at the Houziyan hydropower station in Sichuan Province, China, were investigated using various in situ tests. This study presents a comprehensive evaluation of the evolution of EDZs in the surrounding rock mass in the underground powerhouse caverns using microseismic (MS) monitoring and conventional testing methods, including multi-point extensometers, acoustic wave testing and borehole TV. First, by analyzing a series of conventional testing and MS monitoring results, the deformation and failure characteristics of the surrounding rock mass were determined. Next, the formation mechanisms of fractures in the surrounding rock mass subjected to excavation-induced unloading in the underground powerhouse caverns were determined. The relationships between the EDZs of the surrounding rock mass, crack evolution and construction status were then analyzed to investigate the crack formation, development and coalescence processes. Finally, the thicknesses of the EDZs were quantitatively determined, the relationships between the fracture evolution and construction progress were established, and the EDZ formation and evolution mechanisms were evaluated. The results not only provide direct data for geological exploration but also contribute to optimizing excavation design and support for analyzing the deformation behaviors of underground powerhouse caverns.

Microseismicity and its time–frequency characteristics of the left bank slope at the Jinping first-stage hydropower station during reservoir impoundment

The instability risk associated with the left bank slope of the Jinping first-stage hydropower station in south-western China may increase as a result of the initial impoundment of the reservoir and the raising and lowering of the reservoir level. To improve our understanding of the rock slope stability and the real-time behaviour of the composite system of rock slope and dam, an existing microseismic (MS) monitoring system installed on May 2009 was improved in early 2014. This investigation represents the first time such a technique has been used to evaluate the stability of a high rock slope in China. The safety and maintenance of the rock slope during the reservoir impoundment were investigated through the development of fast and accurate real-time event location technique aimed at assessing the evolution and migration of the seismic activity and through the development of capabilities for forecasting the rock slope stability. Numerous MS events in the rock slope have been recorded using the MS monitoring system. Zones of damaged rock mass were identified based on a spatio-temporal analysis of the seismic activity during the impoundment period. Moreover, a waveform decomposition method based on the S transform was proposed for processing the seismic waveforms during the construction and impoundment periods of the left bank slope, and the time–frequency characteristics of seismic events during these two periods were thus compared. The understanding of the relationships between the time–frequency parameters variations in the seismic signals and the rock mass damage can provide a basis for evaluating the stability of similar rock slopes.