Qinglei Yu

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.

Microseismicity Induced by Fault Activation During the Fracture Process of a Crown Pillar

Shirengou iron mine in Hebei Province, China is now under transition from open pit to underground mining. During this process, the unstable failure risk of crown pillar is growing as a result of underground mining, fault activation and water seepage. To monitor the stability of the crown pillar, a microseismic monitoring system was equipped in 2006. Based on temporal and spatial distribution of microseismic events and deformation mechanism, it was found that it is the propagation of the buried fault F15 that causes the failure of the crown pillar, resulting in increased water seeping into the underground drifts. By analyzing the temporal changes in multiple microseismic parameters during the fracture process of the crown pillar, it was found that several distinct abnormalities in the microseismic data such as a rapid decrease in the b value, a sharp increase in energy release, an abnormal increase in apparent stress and a low dominant frequency, could be judged as the signal of an increasing risk. Therefore, the microseismic monitoring has been proven to be a suitable method for understanding damage and fracture process of the crown pillar during the transition from open pit to underground mining.

A Model of Anisotropic Property of Seepage and Stress for Jointed Rock Mass

Joints often have important effects on seepage and elastic properties of jointed rock mass and therefore on the rock slope stability. In the present paper, a model for discrete jointed network is established using contact-free measurement technique and geometrical statistic method. A coupled mathematical model for characterizing anisotropic permeability tensor and stress tensor was presented and finally introduced to a finite element model. A case study of roadway stability at the Heishan Metal Mine in Hebei Province, China, was performed to investigate the influence of joints orientation on the anisotropic properties of seepage and elasticity of the surrounding rock mass around roadways in underground mining. In this work, the influence of the principal direction of the mechanical properties of the rock mass on associated stress field, seepage field, and damage zone of the surrounding rock mass was numerically studied. The numerical simulations indicate that flow velocity, water pressure, and stress field are greatly dependent on the principal direction of joint planes. It is found that the principal direction of joints is the most important factor controlling the failure mode of the surrounding rock mass around roadways.

Numerical Modeling of Jointed Rock Under Compressive Loading Using X-ray Computerized Tomography

As jointed rocks consist of joints embedded within intact rock blocks, the presence and geometrical fabric of joints have a great influence on the mechanical behavior of rock. With consideration of the actual spatial shape of joints, a numerical model is proposed to investigate the fracture evolution mechanism of jointed rocks. In the proposed model, computerized tomography (CT) scanning is first used to capture the microstructure of a jointed sandstone specimen, which is artificially fabricated by loading the intact sample until the residual strength, and then digital image processing (DIP) techniques are applied to characterize the geometrical fabric of joints from the CT images. A simple vectorization method is used to convert the microstructure based on a cross-sectional image into a layer of 3-D vectorized microstructure and the overall 3-D model of the jointed sandstone including the real spatial shape of the joints is established by stacking the layers in a specific sequence. The 3-D model is then integrated into a well-established code [three-dimensional Rock Failure Process Analysis, (RFPA3D)]. Using the proposed model, a uniaxial compression test of the jointed sandstone is simulated. The results show that the presence of joints can produce tensile stress zones surrounding them, which result in the fracture of jointed rocks under a relatively small external load. In addition, the spatial shape of the joints has a great influence on the fracture process of jointed rocks.

Numerical simulation on slope stability analysis considering anisotropic properties of layered fractured rocks: a case study

Anisotropy is one of the natural properties of layered fractured rock, and it plays an important role in slope stability analysis, which is a vital problem in the geotechnical engineering. However, in the present engineering design, rock mass is simply treated as isotropic material, which fails to take into account the anisotropic properties. This work begins with formulizations of the anisotropic seepage-stress coupled model for the layered fractured rock slope stability analysis based on the equivalent continuum theory. Next, the model is applied in the numerical simulation of the southern slope of an open-pit mine in China to understand the mechanism of the existing failure strain as well as the failure mechanism of the potential landslide. The computed water table and damage zone have been compared with the field measurements and found to be in good agreement with field observations. Finally, the effective measures to prevent the slope failure and strengthen the slope stability have been suggested. The proposed model successfully applied in the case study indicated that it is much more feasible and efficient comparing with using the traditional isotropic coupled model in such layered fractured rock slopes stability problems.

A comparative study of hydraulic fracturing with various boreholes in coal seam

Comparative numerical study on hydraulic fracturing with various boreholes in coal seam combined with in situ experiments was carried out to investigate the fracturing mechanism and loosening effect of hydraulic fracturing in coal seam. Hydraulic fracturing models with single-borehole, three-borehole and fiveborehole were built based on in situ tests in Chengshan coalmine, Jixi city, Heilongjiang province, China and the changes of water pressure and shear stress around boreholes during hydraulic fracturing were analyzed. The influence of hydraulic fracturing with controlling borehole on crack initiation and propagation was discussed. According to the in situ testing results, it is found that controlling boreholes in hydraulic fracturing not only can control the direction of crack propagation, but also can judge the effect of crack initiation and breakdown. The work in this paper is of great importance for the design of hydraulic fracturing technology and the alternative of the parameters in theory and practice.

Deformational behavior of underground opening using a stress-seepage coupled model considering anisotropic characteristics

Fractures have a dominant effect on the mechanical response of a rock mass subjected to mining operations. A numerical finite element model describing the anisotropic property of elasticity and seepage was used in this study to assess the drift spacing at the Heishan metal mine in Hebei Province, China. The fractured rock mass is assumed to be transversely isotropic, and the influence of joint orientation on the anisotropic properties of elasticity of the surrounding rock mass around roadways was investigated. The associated stress field and fracture pattern of the surrounding rock mass were also numerically studied. The results indicate that the existence of joint planes greatly affects the stress field and fracture patterns, and the principal direction of joints can control the failure pattern of the surrounding rock mass around drifts.

A NEW NUMERICAL APPROACH FOR STUDYING SELF-ORGANIZED CRITICALITY BEHAVIOUR IN ROCK FAILURE PROCESS

Self-Organization in rock failure progress is simulated by introducing a new nu- merical approach, rock failure progress analysis (RPPA 2D ). Through the analysis of acoustic emission (AE) event time series and the frequency distribution of dam- age group size by correlation function and rescaled range (R/S) analysis method, it is found that the frequency distribution of damage group size complies with a power law (fractal geometry configuration) and the time series of AE event ex- hibits the similar scale-invariant properties and temporal long-range correlation. These fractal geometry configuration properties and long-range correlations are two fingerprints of self-organized criticality, which denonstrates the occurrence of SOC.

Study of the Rock Mass Failure Process and Mechanisms During the Transformation from Open-Pit to Underground Mining Based on Microseismic Monitoring

To quantitatively understand the failure process and failure mechanism of a rock mass during the transformation from open-pit mining to underground mining, the Shirengou Iron Mine was selected as an engineering project case study. The study area was determined using the rock mass basic quality classification method and the kinematic analysis method. Based on the analysis of the variations in apparent stress and apparent volume over time, the rock mass failure process was analyzed. According to the recent research on the temporal and spatial change of microseismic events in location, energy, apparent stress, and displacement, the migration characteristics of rock mass damage were studied. A hybrid moment tensor inversion method was used to determine the rock mass fracture source mechanisms, the fracture orientations, and fracture scales. The fracture area can be divided into three zones: Zone A, Zone B, and Zone C. A statistical analysis of the orientation information of the fracture planes orientations was carried out, and four dominant fracture planes were obtained. Finally, the slip tendency analysis method was employed, and the unstable fracture planes were obtained. The results show: (1) The microseismic monitoring and hybrid moment tensor analysis can effectively analyze the failure process and failure mechanism of rock mass, (2) during the transformation from open-pit to underground mining, the failure type of rock mass is mainly shear failure and the tensile failure is mostly concentrated in the roof of goafs, and (3) the rock mass of the pit bottom and the upper of goaf No. 18 have the possibility of further damage.