Guan Rong

Effect of Particle Shape on Mechanical Behaviors of Rocks: A Numerical Study Using Clumped Particle Model

Since rocks are aggregates of mineral particles, the effect of mineral microstructure on macroscopic mechanical behaviors of rocks is inneglectable. Rock samples of four different particle shapes are established in this study based on clumped particle model, and a sphericity index is used to quantify particle shape. Model parameters for simulation in PFC are obtained by triaxial compression test of quartz sandstone, and simulation of triaxial compression test is then conducted on four rock samples with different particle shapes. It is seen from the results that stress thresholds of rock samples such as crack initiation stress, crack damage stress, and peak stress decrease with the increasing of the sphericity index. The increase of sphericity leads to a drop of elastic modulus and a rise in Poisson ratio, while the decreasing sphericity usually results in the increase of cohesion and internal friction angle. Based on volume change of rock samples during simulation of triaxial compression test, variation of dilation angle with plastic strain is also studied.

A new shear strength criterion of rock joints based on cyclic shear experiment

In mining, dam foundations, underground caverns and petroleum engineering, engineers often face problems associated with jointed rock mass. The prime objective of this study is to improve our understanding of the shear behaviour of rock joints. Shear tests of several tensile rock joint samples under different normal loads have been conducted in order to relate the peak shear strength of a rock joint with the three-dimensional (3D) surface morphology. In the study, two parameters, which are termed as the average inclination angle and the maximum contact area ratio A0, are used to characterise the surface of a rock joint, and the parameters can be easily measured using a morphology scanner system. A rational function of dilatancy angle is proposed based on the variation of the parameters which can be easily measured. Then, a new empirical peak shear strength criterion of Mohr–Coulomb type which has the capability of estimating the peak shear strength at the laboratory scale is proposed. Finally, a comparison among the proposed criterion, Barton’s criterion, Grasselli’s criterion and Xia’s criterion are made from the perspective of both the reasonableness of the formula and the prediction accuracy. The advantages of the proposed criterion were analysed in detail. The proposed criterion was suitable for smooth horizontal rock joints. In addition, the dilatancy angle under zero and infinite normal stress was taken into consideration. Through the comparison, we could say the proposed criterion, which was easier and more intuitive from an engineering point of view, predicted the peak shear strength of rock joints with accurate precision.

Author’s Reply to Discussion of the Paper “An Empirical Failure Criterion for Intact Rocks” by Peng et al. (2013)

First of all, we welcome the discussion by Bewick and Kaiser (2013) (in which the following will be referred to as ‘‘the Discussion Paper’’) on our paper entitled ‘‘An Empirical Failure Criterion for Intact Rocks’’ (Peng et al. 2013). Healthy discussion is good for advancing science. The Discussion Paper provides a review of the Hoek– Brown failure criterion and analyzes the triaxial test data we utilized to develop our model. The Hoek–Brown failure criterion is an empirical failure criterion developed by fitting triaxial test data of intact rocks, and it is one of the most widely used failure criteria in rock mechanics and rock engineering. One major contribution of the Discussion Paper is that it emphasizes that the Hoek–Brown failure criterion should be used with its applicability condition in mind. We completely agree with this viewpoint. What is the applicability condition of the Hoek–Brown failure criterion? The Discussion Paper emphasizes that the Hoek–Brown failure criterion should be used for data in the confining stress range 0 \ r3 \ 0.5rc, where rc is the uniaxial compressive strength (UCS). Hoek and Brown (1997) stated that ‘‘the range of the confinement (r3) values over which these tests are carried out is critical to determine reliable mi and rc values’’. According to Hoek and Brown (1997), ‘‘in deriving the original values of rc and mi, Hoek and Brown (1980) used the range 0 \r3 \ 0.5rc and, in order to be consistent, it is essential that the same range be used in any laboratory triaxial tests on intact rock specimens’’. Hence, 0 \r3 \ 0.5rc can be considered as an applicability condition for using the Hoek–Brown failure criterion. As shown in Fig. 1, laboratory test data of sandstone investigated by Hoek and Brown (1980) show a good fit of all data in a confinement range up to 1.0rc. If that is the case, what is the other applicability condition for the Hoek–Brown failure criterion? The answer has been given by Hoek (1983), who stated that ‘‘A rough rule-of-thumb used by this author is that the confining pressure r3 must always be less than the unconfined compressive strength rc of the material for the behavior to be considered brittle’’. He further commented that ‘‘In the case of materials characterized by very low values of the constant mi, ..., the value of r3 = rc may fall beyond the brittle–ductile transition’’. Although not explicitly stated by Hoek (1983), it is clear that the confinement should be less than the brittle–ductile transition boundary, which is defined by r1/r3 = constant. Hence, rocks should behave in a ‘‘brittle’’ manner is another applicability condition of the Hoek–Brown failure criterion. The constant that defines the brittle–ductile transition boundary is between 3 and 5 (Hoek 1983). In the following discussion, we follow the Discussion Paper and use 3.4 as suggested by Mogi (1966). We will show that ensuring data points be on the left side of the brittle–ductile transition boundary is a looser applicability condition than the condition of 0 \ r3 \ 0.5rc. In our reply, we will first further explain why we developed our new empirical model, followed by a further discussion on the Hoek–Brown model and our model through some additional examples. We hope that this J. Peng (&) G. Rong C. Zhou X. Wang State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China e-mail: pengiun2010@gmail.com

Fluid Flow Through Single Fractures With Directional Shear Dislocations

This paper numerically investigates the fluid flow behavior through single fractures with directional shear dislocations. Synthetic fractures are generated with directional shear dislocations, and the lattice Boltzmann method is used to simulate the fracture flow. With an ignorance of tortuosity effect, a notable overestimation of hydraulic conductivity is observed when the simplified local cubic law is used. During the closure process, the decreasing rate of conductivity is found to be highly related to the roughness of fractures. The conductivity of smoother fractures decreases faster than that of rougher fractures. By conducting simulations on fractures with a constant shear displacement, the effective conductivity is found to vary with the shear directions. The results show that the conductivity of rougher fractures is less sensitive to the shear directions than that of smoother fractures. As fracture surfaces come into contact, a sharp decrease in effective conductivity is observed and the decreasing trend flattens as the contact ratio continues to increase. A new model is proposed based on the bottleneck model to predict the conductivity of sheared fractures. By integrating the tortuosity and channeling effects into the original model, the proposed new model shows a better performance in predicting the conductivity, especially for fractures with rougher surfaces.

Acoustic emission characteristics of a fine-grained marble with different thermal damages and specimen sizes

Acoustic emission (AE) is a technique which has been widely used in geomechanics to study the progressive micro-cracking behavior of rocks in response to different loadings. However, the study of the combined effects of thermal damage and specimen size on the performance of rocks using the AE technique is still limited, which needs further investigation. This study experimentally investigated the AE characteristics of a fine-grained marble with different thermal damages and specimen sizes. The variation of AE counts in response to the rock deformation can divide the stress−strain relation into several stages. The AE activity is limited in the initial deformation stage and multiplies at a stress level about 0.7 to 0.8 times the peak stress. However, the AE signals in the initial stage become more prominent as the treatment temperature increases. The accumulated AE parameters (i.e., AE counts, AE hits, and AE energy) are found to decrease with the increase in the treatment temperature. The b-value, which generally decreases as the stress approaches the peak strength, correlates well with the stress−strain relation. It is also found that the b-value generally increases as the treatment temperature gradually increases, which is mainly attributed to the initially generated thermal micro-cracks in the rock specimen. The real-time spatial distribution of AE events is in considerable agreement with the failure mode observed in laboratory tests. Overall, the results in this study reveal that the AE technique is capable of studying the micro-cracking behavior involved in the deformation process of rocks possessing different degrees of thermal damage and with different specimen sizes.