Search

Normal stress changes occur commonly during fault and rock joint rupture, and play a key role in determining frictional behavior and shear stability of these discontinuities. Previous experimental studies of the direct shear test under cyclic normal loads confirm that there exists a phase shift between peak normal stress and peak shear stress, as well as between peak friction coefficient and peak shear stress with shear stress and friction coefficient lagging. However, the underlying physics of this finding is poorly understood. Here, we present a numerical study to investigate the effect of cyclic normal loads on the friction of smooth joints. Our simulations show reasonable agreements with experimental observations, verifying the capability of the proposed model based on the discrete element method. We also investigate the effect of normal loading rate, dynamic amplitude, static normal stress level, shear velocity, and joint stiffness on the frictional behavior of the joint. From a microscopic point of view, we focus on the underlying processes of the phase shift between the peak shear stress (friction coefficient) and peak normal loads. We find that phase shift is related to the changes of the population of slipping and frozen contacts, and also the evolutions of shear force, shear velocity, and shear displacement of individual contacts, in which the macroscopic shear velocity plays a significant role. Inhomogeneous stress distributions near the joint indicate that damage or failure should occur in some contact-scale regions, although which is not directly verified due to the limitations of the presented model. Our work improves the understanding of the physics on how normal load perturbations affect the shear behavior of smooth rock joints.

Being physical mechanical parameters of joints, normal and shear stiffnesses are indispensable components of the numerical simulation and theoretical analysis of the behavior of joints. The objective of this work is to put forward an experimental method to determine joint stiffness under constant normal load (CNL) conditions. For this purpose, joint closure and direct shear tests under CNL conditions were conducted. Normal stiffness was determined by the ratio of normal stress increment and the corresponding normal displacement; the shear stiffness was calculated by the ratio of shear stress increment to the relative shear displacement. The average and instantaneous shear stiffness were distinguished. Experimental results showed that joint normal and shear stiffness are time and spatially varying parameters during direct shearing. With increasing normal stress, average shear stiffness, maximum instantaneous shear stiffness, and normal stiffness increases. Normal stiffness was about 26–28 times higher than the average shear stiffness under one normal stress level. Average shear stiffness was little influenced by shear velocity. Instantaneous shear stiffness shows the velocity-dependent behavior. Maximum instantaneous shear stiffness decreases rapidly with increasing shear rate. At lower shear velocity, maximum instantaneous shear stiffness is higher than normal stiffness; minimum instantaneous shear stiffness can be negative and the absolute value also decreases with faster shear velocity. These findings provide a reference for selecting the appropriate value of normal and shear stiffness for evaluation of the mechanical response of interface.

Changes in shear velocity can strengthen or weaken the frictional resistance of joints/faults in natural systems, but the mechanism remains unclear. We investigated the shear behavior of a rough basalt fracture in well-controlled, repeatable shear tests under constant and dynamic normal load conditions at different shear velocities. Normal load vibrations, simulating a dynamic normal load, were applied to the upper block of a fractured basalt sample. Simultaneously, a shear load was applied to the bottom block, providing a constant shear velocity. The peak shear strength increased with increasing shear velocity under constant normal load conditions. The peak shear strength decreased at a lower shear velocity under normal load vibrations. When the shear velocity exceeded the critical value, vc, the peak shear strength increased. The apparent coefficient of friction reduced under normal load vibrations. The reduction in the dynamic coefficient of friction increased with increasing shear velocity. We identified a phase shift between the peak normal load and peak shear load with peak shear load delay (D1) and a phase shift between peak normal load and the peak coefficient of friction with the peak coefficient of friction delay (D2). D1 and D2 were dependent on the quasi-static coefficient of friction and shear velocity, and both decreased with increasing shear velocity. D1 decreased with the increasing quasi-static coefficient of friction, while D2 was almost constant with changes in the quasi-static coefficient of friction. A new shear strength criterion was proposed for a rough joint under a constant shear velocity and normal load vibrations.

An accurate quantification of the frictional behaviour of joints under cyclic normal load conditions during the cyclic shear process is important to characterize the joint and fault interactions during earthquakes and rock bursts. We conducted experimental studies and numerical simulations to investigate the cyclic frictional responses of planar joints subjected to cyclic changes of normal loads. Experiments were conducted on artificial rock-like planar joints using a large shear box device (GS-1000), with different vertical and horizontal impact frequencies, vertical impact load amplitudes, horizontal shear displacement amplitudes, and normal load levels. The average normal displacement of the upper block increased with decreasing normal load and decreased with increasing normal load during each cycle. The normal displacement decreased gradually with increasing number of shear cycles due to damage to the micro-asperities at the contact surface. Shear force and the apparent coefficient of friction (k = FShear/FNormal) changed cyclically with a change in shear direction, where k followed a square wave curve with the same peak value at the stable shear stage. The cyclic normal load amplitudes, horizontal shear displacement amplitudes, cyclic normal load frequencies, cyclic horizontal shear frequencies, and static normal force levels had little influence on the peak values of k. Numerical simulations proved that the spatial movement pattern of the loading plate and upper block of the specimen rotated clockwise or anti-clockwise at different shear displacements. Due to the rotation of the upper block, shear and normal stresses distributed at the contact surface were inhomogeneous, which generated a stress gradient along the interface. Consequently, the samples were damaged at the two edges due to the high local stresses. Finally, a mathematical equation is proposed, which can be used for predicting the shear strength of planar joints under cyclic changes of shear velocity and normal load.

Under certain extreme conditions in rock engineering works, fast change in temperature in the load-bearing rocks can happen. Known as thermal shock (TS), such process involves rapid temperature rise or drop, which causes fracturing in the rock material and thus can pose as a threat to the stability of the rock structures. To investigate the influence of thermal shock caused by fast cooling on the mechanical property of rock, laboratory tests are performed on heated granite which are cooled with different methods, with the highest cooling rate reaching 167.4 °C/min. The dynamic loading tests are performed on the heated granite specimens utilizing the split Hopkinson pressure bar (SHPB) system. The test results show that the dynamic compressive strength drops with the increase in heating level or cooling rate. This pattern is explained by the nuclear magnetic resonance (NMR) test data that the pores inside the heated granite increase both in size and quantity as heating level or cooling rate rises. Damage patterns of the tested granite specimen fragments are analyzed based on the observation with scanning electron microscope (SEM), and the mechanisms of thermal shock in granite are also discussed.

In this study, a 3D DEM model containing a mortar-bolt interface subjected to shearing was established in the context of the simplified rock bolt model (SRBM) proposed in a companion paper. The DEM model was calibrated against a series of laboratory experiments to reproduce the mechanical characteristics of a cement mortar with a uniaxial compressive strength of 30 MPa. The DEM simulation has led to a detailed observation and an in-depth understanding of the mode II progressive debonding of the mortar-bolt interface and subsequent mortar rupture (due to mechanical interlocking). In addition, the effects of particle size of mortar and profile configurations of rebar bolts (i.e., different rib spacings and rib heights) on simulation results were discussed. The numerical findings in the study were validated against laboratory measurements and a broad agreement was observed.

The problem of how dynamic friction of faults influences the observed earthquakes remains largely unsolved. Our paper presents results of experiments aimed to understand the complex frictional behavior of faults under dynamic normal load (DNL) conditions under consideration of horizontal slip velocity, dynamic normal force amplitude and normal impact frequency using a dynamic shear box device. Data were obtained by continuous measurements of shear and normal forces as well as corresponding displacements. We identified a phase shift between peak normal force and peak shear force with peak shear force lagging. Consequently, the dynamic friction coefficient shows cyclic behavior and the minimum value of the dynamic friction coefficient increases with increasing of slip velocity which is in conflict to the traditional views under constant normal load (CNL) conditions. We found, that dynamic fault friction coefficient during earthquakes may increase or decrease depending on velocity-frequency (VF), velocity-amplitude (VA) and velocity-frequency-amplitude (VFA) parameter. These findings have significant implications for seismic hazard assessment and reliable forecasting of earthquakes.

In this paper, we report a new multi-function servo control dynamic shear test apparatus, DJZ-500. This apparatus consists of five parts, including power system, loading frame, servo control unit, cooling system and PC console. The normal load and shear load are measured by load cells with an accuracy of 0.1 kN; normal and shear displacement are measured by LVDTs with an accuracy of 0.1 μm. DJZ-500 has the dynamic and static loading modules, which can perform the force and displacement servo control experiments in both vertical and horizontal directions. In addition, the dynamic loading modules can execute the custom waveforms, which can be utilized to investigate the seismic response of rock and soil masses. We presented six sets of representative tests on sands to demonstrate these functions. Moreover, DJZ-500 can also preform the coupled shear-flow test with the pumping system to investigate the flow behavior in sheared rock fractures. This device makes up for the deficiency of the existing equipment, and promotes the study of frictional properties of geomaterials.

Understanding the frictional characteristics of granular materials (e.g., fault gouge and sand foundations) exposed to dynamic normal load are important for investigations of engineering geology disasters and safety assessment. Direct shear tests on granular materials under dynamic normal load were performed in the laboratory using a dynamic shear box device. Experimental results showed that the shear strength and normal displacement varied with variations of normal load, identified by a phase shift and normal stress loading peak. The peak shear stress decreased with increasing normal load frequency and shear velocity, in particular, the peak shear stress changed nonlinearly with the increase of normal load amplitude, characterized by a critical point. The influence of normal load amplitude on the shear strength and normal displacement was analyzed in detail. Furthermore, the results showed that dynamic normal load can both enhance and reduce the shear strength on granular material, and the frictional behaviors were velocity-amplitude-frequency dependent, which can be well interpreted by the Rate and State Friction constitutive law. These findings provide guidance in the design of geotechnical engineering projects when dynamic normal loads are encountered (e.g., blasting, earthquakes, earth tides and reservoir effects).

Understanding the flow characteristics in sheared rock fractures is with significance for many engineering projects (e.g., geothermal reservoirs, dams, oil or gas explorations, etc.). This study reports a FLAC3D FEM-based fluent CFD - methodology to investigate the fluid flow patterns in sheared rock joints exposed to complex stress states. First, a direct shear experiment of a rough basalt fracture, subjected to dynamic normal force is performed. A FLAC3D numerical model is setup to simulate the experimental process. Using a self-designed algorithm, the fracture contact area, aperture size, and fracture volume are analyzed at different shear displacements in the numerical model. The two-dimensional apertures, obtained from the FLAC3D model, are applied to FEM-based fluent CFD to analyze variation patterns of the relevant fluid flow during the shear process. Results show that the flow characteristics including the generation and development of eddy currents, and the estimated value of the Reynolds number, flow rate, and apparent permeability, are associated with both the shearing and the dynamic normal force. These findings provide a new horizon on features of aperture flow in rock mass under complex geological stresses.

In deep rock engineering, the rock mass can be subjected to thermal stress caused by sudden changes in temperature, which is referred to as thermal shock (TS). To study the effect of TS on heated sandstone, three cooling methods are used to provide different cooling rates. Then the coupled dynamic and static loading tests are carried out on the heated sandstone by means of a modified split Hopkinson pressure bar (SHPB) system. The test results show that as the heating level increases, the dry density, P-wave velocity, and the dynamic combined strength of the heated sandstone decrease, while specimen porosity increases. Particularly, a sharp change in the physical properties of sandstone can be observed at 650 °C, which is believed to be caused by the α-β transition of quartz at 573 °C. At each heating level of the test, the damage caused by the higher cooling rate to the heated sandstone is more than that caused by the lower cooling rate. The different failure modes of sandstone with increasing temperature are analyzed. The mechanism of TS acting on heated sandstone is discussed, and two typical fracture patterns reflecting the action of TS are identified through SEM.

The prior aim of lab testing is the determination of parameters. However, the measurements do not only reflect the rock or soil properties, but contain also, at least to some extent, the influence of the test equipment. Even if measurements are designed to minimize the influence of the test equipment often some measurement values are effected by an unknown amount. Therefore, it is proposed to build detailed numerical models of the test device including the specimen and to duplicate the lab test completely. This paper presents experimental and numerical tests performed with a shear box device. Multi-stage direct shear tests were conducted on smooth plane joints in artificial rock-like material under constant normal loading (CNL) conditions. A 3-dimensional numerical model of the shear box device including the sample was built and the laboratory tests were simulated. The numerical model comprises the loading frame, the hydraulic pistons, the shear boxes and the sample and considers the measuring devices. The numerical model was able to reproduce the non-uniform stress distribution at the joint connected with sample rotation, which in turn produces unwanted frictional forces at the upper shear box. Consequently, only 91–94% of the applied and measured normal force really acts at the joint. Moreover, numerical simulations also shows, to what extend the different parts of the loading frame deform.

The effect of dynamic normal loading (DNL) on the direct shear behavior of a plane joint has been first time investigated using a dynamic direct shear box device. Multi-stage normal loading tests with superimposed normal cyclic excitation at different frequencies were conducted at constant horizontal shear velocity of 3000 μm/min. Experimental results show, that peak shear force increases with increase of normal stress, and amplitude ratio between shear and normal stress decreases with the increase of normal stress. A significant phase shift between peak normal stress and peak shear stress is observed, which decreases with increasing normal load, while the relative time shift between peak normal stress and peak friction coefficient is nearly constant. Impact frequency has little effect on relative time shift. The dynamic normal stiffness is significantly higher than the static one. Finally, a new shear strength criterion is proposed for a joint under constant shear velocity and sinusoidal normal loading.

The mine improvement of rock mass rating (M-IRMR) evaluation method was put forward based on the existing theory of the rock quality classification and the stability evaluation in the undersea deposit of Sanshandao Gold Mine, China. The M-IRMR evaluation method includes 9 evaluation indexes which are rock compressive strength, rock quality index RQD, joint spacing, joint state, groundwater state, joint direction, ground stress, blasting vibration, and exposed area, respectively. During the evaluation process, according to the special features of the undersea deposit, the rock mass rating (RMR) method was used as the foundation, and the four geological parameters (i.e., rock compressive strength, rock quality index, joint spacing, and ground stress) were modified. The computing methods of the two engineering factors (blasting vibration and exposed area) were presented. The M-IRMR rock quality classification and stability evaluation method was applied in the level of −420 to −690 m in the undersea deposit of Sanshandao Gold Mine and the classification results were consistent with the actual situations, which can provide a scientific basis for choosing the suitable mining method and stope support system of the undersea deposit.

Understanding the shear-induced evolution of aperture size is critical for controlling fracture permeability in rock engineering projects. This study reports a robust procedure to evaluate the change of aperture size and its effect on fluid flow. A Finite Element Model is first calibrated based on the direct shear tests on a rough fracture to reproduce the uneven deformation of the fracture. A Fluent Computational Fluid Dynamics analysis is subsequently used to illuminate flow characteristics in the fracture without and with asperity deformation. The results indicate that the aperture size of the fracture increases dramatically with increasing shear displacement. However, the aperture size can be reduced slightly due to asperity deformation. Both the shear displacement and asperity deformation jointly influence flow characteristics in the sheared fracture (e.g., the development of eddy flow regions) and the accurate estimation of fracture permeability.

According to the actual conditions in Sanshandao Gold Mine, the cancelling ore pillars mining method was researched. Firstly, a series of tests for the physical and mechanical characteristics of rock mass were carried out and a quality classification system of rock mass applied in coastal metal deposit was established. Secondly, the reasonable demarcation depth of cancelling ore pillars was simulated using the finite element method, and the simulation results show that the ore pillars can be cancelled below the level of −555 m. Thirdly, a novel layer-backfill mining method of room-pillar alternation was designed to reduce the disturbance and settlement of terrain in mining area. Engineering practice shows that the new mining method enhanced the mining output and relieved rock disturbance. Furthermore, the settlement of the roof strata was small and no disaster occurred. The new mining technology effectively controlled the deformation of the terrain, indicating that the mining of the large-scale gold coastal deposit in Sanshandao Gold Mine was achieved safely, efficiently, and with a low loss rate.

Filling the goaf with phosphogypsum can reduce damage to the environment due to phosphate mining. Research was conducted to test the feasibility of using phosphogypsum as a backfill aggregate, concentrating on the mixture ratio of filling materials and the filling process. First, the physical and chemical characteristics of phosphogypsum, fly ash and yellow phosphorus slag were determined. Then, two types of cement strength tests were conducted. One type of cementation was made up of phosphogypsum, cement and fly ash; the other was made up of phosphogypsum, yellow phosphorus slag, alkali activator and composite water reducer. The test results show that by adding fly ash, the characteristics of phosphogypsum change making it usable as a backfill aggregate. In addition, the curing agent of cement can be replaced by the yellow phosphorus slag, and a reasonable mixture ratio for backfill materials can be obtained. The new cemented backfill technology (in which the phosphogypsum is used as the main fil...

The exploration of undersea resources becomes popular as land resources decrease. Researches were conducted with emphasis on the safety and efficiency of undersea mining of the large gold deposit in Xinli District of Sanshandao Gold Mine. A series of tests for the physical and mechanical characteristics of rock mass were carried out, and the three-dimensional geo-stress distribution was tested in the mining area. Further, a similar experimental simulation platform, which revealed the mechanism of water inrush and ascertained the reasonable thickness of the safety isolate layer, was established for the undersea mining. Meanwhile, the feasibility of cancelling the ore pillars and the safety conditions was checked by numerical simulation. The simulation results show that it is safe to exploit the ore body below the −85 m level (presently, the exploitation level is below −160 m in Xinli District), and the ore pillars can be cancelled below the −560 m level. Furthermore, a novel backfill method was designed to reduce the rock strata disturbance and settlement, and the settlement of roof strata was monitored during the mining process. Engineering practice shows that the settlement of roof strata was small and that no disaster happened. This indicates that the undersea safety mining technology of the large gold deposit is achieved in Xinli District.