Hongyuan Liu

Numerical simulation of the rock fragmentation process induced by indenters

Rock fragmentation processes induced by single and double indenters were examined by a numerical method. The simulated results reproduce the progressive process of rock fragmentation in indentation. Rock deforms elastically at the initial loading stage. Then tensile cracks are initiated around the two corners of the truncated indenter and propagate in the well-known conical Hertzian manner. The rocks immediately under the indenter are in a highly tri-axial stress state, and some of them fail in the ductile cataclastic mode with the stress satisfying the ductile failure surface of the double elliptic strength criterion. With the tensile cone cracks and ductile cataclastic failure releasing the confining pressure, the rocks under the indenter are compressed into failure and the crushed zone gradually comes into being. With increasing loading displacement, the re-compaction behaviour of the crushed zone occurs. Side cracks initiated from the crushed zone or bifurcated from cone cracks are driven by tensile stress associated with the crushed zone to propagate in a curvilinear path and finally intersect with the free surface to form chips. It is pointed out that the curvilinear path is caused by heterogeneity. The simulated force-penetration curve is in fact the indication of the propagation of cracks, the crushing of microstructural grains and the formation of chips. It is found that the confining pressure has an important influence on the indentation results. With decreasing confining pressure, there is a decrease in the indentation strength and a change in the rock failure process from the formation of rock chips to a vertically axially splitting failure. The simulated fragmentation process in the double indenter test reproduces the side cracks, which are induced by two indenters, propagate, interact and finally coalesce, chipping the rock between the indenters. The line spacing is an important factor that affects the fragmentation efficiency in multiple indenter tests. It is pointed out that simultaneous loading with multiple indenters with an appropriate line spacing seems to provide a possibility of forming larger rock chips, controlling the direction of subsurface cracks and consuming a minimum total specific energy. According to the simulated results, it is believed that the numerical simulation method will contribute to an improved knowledge of rock fragmentation in indentation, which will in turn help to enhance mining and drilling efficiency through the improved design of mining tools and equipment.

Numerical investigation of particle breakage as applied to mechanical crushing : Part I, Single-particle breakage

Abstract A numerical approach to particle breakage is applied using the Rock Failure Process Analysis code, RFPA 2D . The numerical tool is validated by simulating the Brazilian test with a two-dimensional disk sample. Then two irregularly shaped particles with an exact geometry and exact mechanical properties are numerically modelled to investigate their breakage behaviour under unconfined and confined loading conditions. The numerical results indicate that the dominant mode of failure is catastrophic splitting and progressive crushing, which mainly depends on the loading conditions with respect to confinement. The analysis of the load–displacement curves obtained from the simulations suggests a brittle-ductile transition between the two cases. The lateral constraint increases the initial stiffness and the maximum breakage strength of the particle. Most of the energy released during the failure process comes from the crushing of highly stressed areas, particularly, in the vicinity of the contact points where a crushed zone forms. It is also found that the particle shape governs the breakage strength in addition to the material properties themselves, and that the heterogeneity of the particles governs the fracture propagation paths.

A Mesostructure-based Damage Model for Thermal Cracking Analysis and Application in Granite at Elevated Temperatures

Thermal stress within rock subjected to thermal load is induced due to the different expansion rates of mineral grains, resulting in the initiation of new inter-granular cracking and failure at elevated temperatures. The heterogeneity resulting from each constituent of rock should be taken into account in the study of rock thermal cracking, which may aid the better understanding of the thermal cracking mechanisms in rock. In this paper, a mesostructure-based numerical model for the analysis of rock thermal cracking is proposed on the basis of elastic damage mechanics and thermal–elastic theory. In the proposed model, digital image processing (DIP) techniques are employed to characterize the morphology of the minerals in the actual rock structure to build a numerical specimen for the rock. In addition, the damage accumulation induced by thermal (T) and mechanical (M) loads is considered to modify the elastic modulus, strength and thermal properties of individual elements with the intensity of damage. The proposed model is implemented in the well-established rock failure process analysis (RFPA) code, and a DIP-based RFPA for the analysis of thermally induced stress and cracking of rock (abbreviated as RFPA-DTM) is developed. The model is then validated by comparing the simulated results with the well-known analytical solutions. Finally, taking an image from a granite specimen as an example, the proposed model is used to study the thermal cracking process of the granite at elevated temperatures and the effects of temperature on the physical–mechanical behaviors of the granite are discussed. It is found that thermal cracks mostly initiate at the location of mineral grain boundaries and propagate along them to form locally closed polygons at the elevated temperatures. Moreover, the effects of temperature on the uniaxial compressive strength and elastic modulus of the granite are quite different. The uniaxial compressive strength decreases consistently with increasing temperature, but there exists a threshold temperature for elastic modulus which starts to decrease as the temperature increases after it exceeds the threshold.

Study on Optimal Grouting Timing for Controlling Uplift Deformation of a Super High Arch Dam

AbstractA grouting model is developed for use during the grouting of the complex foundation of a super high arch dam. The purpose as to determine the optimal grouting timing and appropriate grouting pressure involved in controlling the uplift deformation of the dam. The model determines the optimal grouting time as the height of the arch dam increases with the concrete pouring, by checking the tensile stresses in the dam against standard specifications. The appropriate grouting pressures are given on the basis of the actual grouting pressures monitored during the upstream riverbed foundation grouting. An engineering procedure, applying the model, was then proposed and used during foundation grouting under the toe block of the Xiluodu super high-arch dam in south-western China. The quality of the foundation grouting was evaluated against the results from pressurized water permeability tests, acoustic wave velocity tests, elastic modulus tests and panoramic photographing of the rockmass on completion of the foundation grouting. The results indicated that the proposed grouting model can be applied to effectively reduce the uplift deformation and associated cracking risk for super high arch dams, and it can be concluded that the proposed engineering grouting procedure is a valuable tool for improving foundation grouting under the toe blocks of a super high arch dam.

Effects of Outlets on Cracking Risk and Integral Stability of Super-High Arch Dams

In this paper, case study on outlet cracking is first conducted for the Goupitan and Xiaowan arch dams. A nonlinear FEM method is then implemented to study effects of the outlets on integral stability of the Xiluodu arch dam under two loading conditions, i.e., normal loading and overloading conditions. On the basis of the case study and the numerical modelling, the outlet cracking mechanism, risk, and corresponding reinforcement measures are discussed. Furthermore, the numerical simulation reveals that (1) under the normal loading conditions, the optimal distribution of the outlets will contribute to the tensile stress release in the local zone of the dam stream surface and decrease the outlet cracking risk during the operation period. (2) Under the overloading conditions, the cracks initiate around the outlets, then propagate along the horizontal direction, and finally coalesce with those in adjacent outlets, where the yield zone of the dam has a shape of butterfly. Throughout this study, a dam outlet cracking risk control and reinforcement principle is proposed to optimize the outlet design, select the appropriate concrete material, strengthen the temperature control during construction period, design reasonable impounding scheme, and repair the cracks according to their classification.

Rheological Characteristics of Weak Rock Mass and Effects on the Long-Term Stability of Slopes

The creep deformation behavior of the northern slope of an open-pit mine is introduced. Direct shear creep tests are then conducted for the samples taken from the northern slope to study the rheological characteristics of the rock mass. The experimental results are analyzed afterwards using an empirical method to develop a rheological model for the rock mass. The proposed rheological model is finally applied to understand the creep behavior of the northern slope, predict the long-term stability, and guide appropriate measures to be taken at suitable times to increase the factor of safety to ensure stability. Through this study, a failure criterion is proposed to predict the long-term stability of the slope based on the rheological characteristics of the rock mass and a critical deformation rate is adopted to determine when appropriate measures should be taken to ensure slope stability. The method has been successfully applied for stability analysis and engineering management of the toppling and slippage of the northern slope of the open-pit mine. This success in application indicates that it is theoretically accurate, practically feasible, and highly cost-effective.

Microstructural modeling approach applied to rock material

The importance of the microstructural parameters in rock mechanical behavior has been investigated by several authors. Moreover, the Weibull statistical model has been used to characterize the heterogeneity of several materials on the basis of the concept that the microscopic defects within the material determine their mechanical strength. The modeling of different rocks is a topic that is fundamental for the prediction of rock fragmentation. In this article, the analysis of rock microstructure is performed using the microstructural modeling approach, which consists of the simplification, quantification, and modeling of the main properties of rock microstructure. The grain size, grain shape, and microcracks are modeled by means of statistical density functions, namely, Cauchy, chi-squared, exponential, extreme value, gamma, Laplace, normal, uniform, and Weibull. It is found that the Weibull distribution is the most appropriate statistical model of the grain size and grain shape, when compared with the other eight statistical models. Regarding microcracks, the results show that the gamma distribution is the most appropriate model. The Weibull and gamma distributions are then used to analyze the heterogeneity of the microstructure. This is done by comparison of the statistical models of each microstructural property evaluated in several thin sections of the same rock. It is found that with respect to grain size and grain shape, the rock is homogeneous, while the size distribution of the microcracks shows a clear trend toward less homogeneity. The microstructural modeling approach is important for modeling, characterizing, and analyzing the microstructure of rock material. Among other applications, it can be used to explain differences in the mechanical behavior obtained in testing several specimens.

A case study on the strata movement mechanism and surface deformation regulation in Chengchao underground iron mine

The regular pattern of surface deformation and the mechanism of underground strata movement, especially in iron mines constructed with the block caving method, have a great influence on infrastructure on the surface, so they are an important topic for research. Based on the engineering geology conditions and the surface deformation and fracture features in Chengchao Iron Mine, the mechanism of strata movement and the regular pattern of surface deformation in the footwall were studied by the geomechanical method, and the following conclusions can be drawn: I. The surface deformation process is divided into two stages over time, i.e., the chimney caving development stage and the post-chimney deformation stage. Currently, the surface deformation in Chengchao Iron Mine is at the post-chimney deformation stage. II. At the post-chimney deformation stage, the surface deformation and geological hazards in Chengchao Iron Mine are primarily controlled by the NWW-trending joints, with the phenomenon of toppling deformation and failure on the surface. Based on the surface deformation characteristics in Chengchao Iron Mine, the surface deformation area can be divided into the following four zones: the fracture extension zone, the fracture closure zone, the fracture formation zone and the deformation accumulation zone. The zones on the surface can be determined by the surface deformation characteristics. III. The cantilever beams near the chimney caving area, caused by the NWW-trending joints, have been subjected to toppling failure. This causes the different deformation and failure mechanisms in different locations of the deep rock mass. The deep rock can be divided into four zones, i.e., the fracture zone, fracture transition zone, deformation zone and undisturbed zone, according to the different deformation and failure mechanisms. The zones in the deep rock are the reason for the zones on the surface, so they can be determined by the zones on the surface. Through these findings, the degree of damage to the infrastructure in different locations can be determined based on the surface deformation zones. As the mining continues deeper, the development regulation of the zones on the surface and in deep rock mass can be further studied based on the zones in the deep rock.

Microseismic Monitoring and 3D Finite Element Analysis of the Right Bank Slope, Dagangshan Hydropower Station, during Reservoir Impounding

The right bank slope of Dagangshan hydropower station in China has complex geological conditions and is subjected to high in situ stress. Notably, microseismic activities in the right bank slope occurred during reservoir impounding. This paper describes the microseismic monitoring technology, and three-dimensional (3D) finite element analysis is used to explore the microseismic activities and damage mechanisms in the right bank slope during reservoir impounding. Based on data obtained from microseismic monitoring, a progressive microseismic damage model is proposed and implemented for 3D finite element analysis. The safety factor for the right bank slope after reservoir impoundment obtained from the 3D finite element analysis, which included the effects of progressive microseismic damage, was 1.10, indicating that the slope is stable. The microseismic monitoring system is able to capture the slope disturbance during reservoir impounding in real time and is a powerful tool for qualitatively assessing changes in slope stability over time. The proposed progressive microseismic damage model adequately simulates the changes in the slope during the impoundment process and provides a valuable tool for evaluating slope stability.

Effect of the impounding process on the overall stability of a high arch dam: a case study of the Xiluodu dam, China

Effects of the first filling process of the Xiluodu super-high arch dam are studied by both field monitoring and numerical modelling methods. The numerical modelling is conducted by the three-dimensional nonlinear finite element method, which takes into account the nonlinear contact of the transverse joints between dam monoliths and adopts both the design and feedback parameters of the dam. The results obtained from the various numerical analysis cases for different impounded water levels are comprehensively compared with those obtained from the field monitoring. Throughout this study, it is concluded that: (1) changes of the hydrostatic load resulting from the filling process have a significant effect on the deformation and stress states of the dam and foundation. The foundation settles as a consequence of consolidation of the reservoir bed under the effect of the water mass. During the filling process, the compressive stress of the dam in the sealed region increases in the arch direction while that in the cantilever direction decreases, which reveals an enhancement of dam thrust into the two banks. (2) The transverse joints between the dam monoliths are compressed almost over the entire height of the poured dam, and their aperture gradually decreases as the impounded water level increases. (3) The overall deformation of the Xiluodu dam is obviously affected by the uneven foundation settlement due to the complex reservoir basin. The tensile stress concentration appears at the crossing zone of the top of the dam’s sealed region and the dam–foundation interface. Thus, reasonable layout of steel bars and suitable jointing design are essential in these tensile stress concentration regions. (4) The dam behaviour predicted using the three-dimensional numerical modelling is in good agreements with that from the field measuring in terms of the stress and strain distribution characteristics in the dam and foundation, especially when the feedback parameters are adopted for the numerical analysis. The results from both the field monitoring and numerical modelling reveal that overall stability of the dam during the first filling process meets the dam operational requirements.