Effect of joint angle in coal on failure mechanical behaviour of roof rock–coal combined body

Abstract

Geological dynamic hazards during coal mining can be caused by the structural instability of a composite system of roof rock and coal layers, and joints in coal play a vital role in this structural instability. In this paper, uniaxial compression simulation tests on roof rock–coal combined bodies with a single joint at different angles in coal were conducted using PFC2D software. In particular, the effects of joint angle on the uniaxial compressive strength (UCS), acoustic emission (AE) and failure characteristics in a combined body were analysed. The results show the following. (1) The joint in coal causes a decrease in the UCS and crack initiation stress, and has promoting effects on the AE occurrence of the combined body. With an increase in the included angle α between the loading direction and joint plane direction, the UCS, AE occurrence time and crack initiation stress first decrease and then increase, presenting ‘V-shaped’ curves. The UCS and crack initiation stress at α of 45° are the lowest, and the AE occurrence time at α of 30° is the earliest. (2) The AE characteristics at α of 0°, 75° and 90° are basically consistent with those of the intact combined body, whereas at α between 15° and 60°, the intensity and frequency of AE events in the macro-failure stage are reduced by the joint in coal. (3) The failure of the combined body mainly occurs within the coal, whereas no apparent failure is observed for the roof rock. Only when α is between 15° and 45°, the roof rock near the bedding plane is destroyed by crack propagation in the coal. (4) Three typical failure modes (i.e. V-shaped shear failure cutting across the coal, shear failure partly along the joint plane direction and shear failure along the joint plane direction) are found for combined bodies with a single joint in coal.

Simulation Study on Strength and Failure Characteristics for Granite with a Set of Cross‐Joints of Different Lengths

“…Particle Flow Code. e particle flow code (PFC) is an effective method to study the macromechanics problems of an analytic object (including construction and rock mass) at the microlevel [22][23][24][25][26], which has been widely applied in simulation tests of uniaxial compression, biaxial compression, and triaxial compression for the rock specimen [19,[27][28][29][30]. ere are two bonding models in PFC 2D software, including the contact bond model and parallel bond model [26][27][28][29][30].…”

“…According to the previous studies, the microparameters of the joint plane were weakened and generally set as very small values [19,[26][27][28][29]. In this study, the microparameters of the main joint plane were taken the same as that of the secondary joint plane.…”

“…is is principally because that there is no initial compression stage in PFC 2D software, which is di erent form the physical test [35]. And, this phenomenon can be found in many simulation studies using PFC 2D software [19,[26][27][28][29]. Figure 5 shows the variation regularities of the uniaxial compressive strength with α and β, respectively.…”

“…Morgan et al studied the cracking processes in Barre granite, including the fracture process zones and crack coalescence [18]. Also, the effects of joint angle in coal on the failure mechanical behavior of a roof rock-coal combined body were studied by Yin et al [19].…”

Field and simulation study of the rational coal pillar width in extra‐thick coal seams

“…Medhurst and Brown pointed out that the fractures and joints could affect the strength of the rock mass, and the mechanical properties of intact rock samples obtained from laboratory tests are much larger than those of the in situ rock masses. And parameters obtained for the intact rock by laboratory tests cannot directly be used in the numerical simulation . Therefore, rock mass strength analysis software (RocLab 10.0) based on the Hoek‐Brown failure criterion was applied to determine the parameters of the rock mass: where is the uniaxial compressive strength of the intact rock; and are the maximum principal stress and minimum principal stress, respectively; and , s , and a are rock mass constants, which can be calculated as follows: where m ci is the constant of the intact rock, D is the disturbance factor, and GSI is the constant evaluating the fractured rock mass.…”