Tianhong Yang

Coupled analysis of flow, stress and damage (FSD) in rock failure

Abstract Rock is a heterogeneous geological material that contains natural weakness of various scales. When rock is subjected to mechanical loading, these pre-existing weaknesses can close, open, grow or induce new fractures, which can in turn change the structure of the rock and alter its fluid flow properties. Experimental results provide strong evidence that rock permeability is not a constant, but a function of stresses and stress-induced damage. A flow-stress-damage (FSD) coupling model for heterogeneous rocks that takes into account the growth of existing fractures and the formation of new fractures is proposed herein. Implemented with the Rock Failure Process Analysis code (F-RFPA 2D ), this FSD model is used to investigate the behaviour of fluid flow and damage evolution, and their coupling action, in samples that are subjected to both hydraulic and biaxial compressive loadings. The modeling results suggest that the nature of fluid flow in rocks varies from material to material, and strongly depends upon the heterogeneity of the rocks.

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 analysis on scale effect of elasticity, strength and failure patterns of jointed rock masses

It is of great importance to study the failure process and scale effect of jointed rock mass in the field of rock mechanics and mining engineering. In the present paper, initially the uniaxial compression test on granite was performed and acoustic emission (AE) sequence was acquired during the compression process in laboratory. Results from numerical simulations using the particle flow code in two dimensions (PFC2D) were presented, and compared with experimental measurements. It was observed that the approach was reasonably good in predicting the real response of granite rock samples. The mechanical parameter of joint model was then calibrated based on PFC2D model with experimental results. Finally the mechanical properties of complex rocks with discrete fracture network (DFN) were studied and scale effects on the elasticity and strength were then investigated. The result showed that the failure pattern was similar when the ratio of joint contact bond strength (both shear and normal) to rock contact bond strength was in the range of 3~9%. The elastic modulus and strength parameters were changed with the sizes of rock sample for DFN models. Moreover, the variation of rock failure pattern under different sizes was also studied and finally the representative elementary volume (REV) size of the considered rock mass was estimated to be 9 × 9 m. It is suggested that the failure pattern analysis should be considered in the REV study of jointed rock mass.

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.

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.

Numerical Modeling of Non-Darcy Flow Behavior of Groundwater Outburst through Fault Using the Forchheimer Equation

AbstractIn a groundwater outburst through a fault, the velocity is generally so high that the relationship between the velocity and the hydraulic gradient deviates from the linear Darcy regime. Thi...