Cunbao Li

Bio: Cunbao Li is an academic researcher from Shenzhen University. The author has contributed to research in topics: Geology & Materials science. The author has an hindex of 12, co-authored 45 publications receiving 565 citations. Previous affiliations of Cunbao Li include Sichuan University & Northwestern University.

Acoustic Emission Characteristics and Damage Evolution of Coal at Different Depths Under Triaxial Compression

Abstract: To obtain a comprehensive understanding of the difference between deep and shallow rocks, and the damage evolution laws of a rock mass at different depths, a case study was conducted on the Pingdingshan Coal Mine. Mechanical behavior and real-time acoustic emission (AE) testing were carried out to investigate the spatiotemporal evolution of the damage in coal at different depths under triaxial compression conditions. Coal samples from different depths (300, 600, 700, 850, and 1050 m) of the same coal seam were collected, and the geostresses measured at each depth were considered in the two-factor simulation method. The AE characteristics and the spatiotemporal evolution of the coal damage at different depths were also obtained. The testing results show that under the influence of the confining pressure caused by increased geostress, the AE activity and the average scale of the cracks in the coal decreased with increasing depth. The deeper coal developed more small cracks and more plastic strain. With increasing depth, the fractal dimension reduction mode of the spatial distribution of AE under continuous loading changed from dropping suddenly from a higher dimension level (2.97) at a higher stress level (70%) to dropping slowly from a lower dimension level (2.63) at a lower stress level (20%). The generation of AE events was more uniform in the time dimension, while the spatial distribution became more uneven and clustered. With continuous loading, the number of AE events and damage in the shallow coal increased, and the dimension of the spatial distribution of AE decreased sharply during the failure stage before the peak stress was reached. With increasing depth, the damage initiation of coal shifted to an earlier time, while the damage evolution process was more stable and orderly, and ended with a higher damage degree. The sudden damage increase of deep coal was not obvious when approaching to failure. The shallow coal was more brittle and prone to sudden failure with the centralized release of AE energy after reaching the peak stress. However, the deeper coal exhibited a more plastic behavior and the plastic deformation became more obvious during the loading process with the gradual development of damage. These research results deepen the understanding of rock mechanics at different depths, promote the study of the difference in the damage laws of deep and shallow rock masses, and provide a meaningful reference for microseismic monitoring and disaster prevention in deep rock engineering.

Numerical investigation of geometrical and hydraulic properties in a single rock fracture during shear displacement with the Navier–Stokes equations

Abstract: Extensive research has shown that fluid flow through rock fractures is greatly influenced by surface roughness. For a single rock fracture, the roughness of the upper and bottom surfaces is the same in the initial condition and then its deformation occurs with normal stress and shear stress imposed on the natural rock. Previous researchers have paid considerable attention to describing the roughness of the single fracture and its effects on fluid flow. However, few studies have explained the fluid flow with shear displacement and the direction of the fluid flow velocity field. In this work, a more detailed 2D numerical model was developed using a laser scanner system with a spacing grid of 0.1 mm. To investigate the influence of shear displacement accurately, the COMSOL multiphase codes were applied. By applying the Navier–Stokes equations, the results of the procedure for shear displacement simulation illustrate the distribution of the absolute velocity and pressure drop under the constant pressure gradient. The velocities predicted at the vertical profiles of the inlet were similar to the parabolic velocity curve defined by the cubic laws. The mean mechanical aperture was usually larger than the hydraulic aperture from the measured flow rates, and a devised empirical equation was proposed to describe the relationship between the mechanical aperture and the hydraulic aperture values. The recirculation zones observed in directional fluid flow during shear explain the anisotropy of roughness of a single fracture, and the phenomenon argues the applicability of local cubic laws which overestimate the total fluid flow rate.

VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

Abstract: The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

Mechanical behavior of coal under different mining rates: A case study from laboratory experiments to field testing

Abstract: During deep mining, the excavation disturbance stress path is the domination factor for the stability of the surrounding rock mass as well as the ground pressure. One of the important parameters of the stress path is the loading or unloading rate of the disturbed rock or coal, which depended on the mining rate. To achieve a well understanding of the mining rate and its effect on the coal behavior, a preliminary case investigation of the mechanical properties of the coal at the various mining rates in both the laboratory scale and field scale was performed. Based on the uniaxial compression test and the digital image correlation (DIC) method, the mechanical behavior of the coal samples, such as the evolution of the strength, surface deformation, crack propagation, and elastic strain energy of the coal under the various loading rates were analyzed. A threshold range of the loading rate has been observed. The uniaxial compressive strength (UCS) and releasable elastic strain energy (Ue) increase with increasing loading rate when the loading rate is below the threshold. Otherwise, the UCS and Ue may decrease with the loading rate. Under the low loading rate (≤0.05 mm/min), the tensile deformation of the original defects could result in crack coalescence, whereas failure of the coal matrix is the key contributor to the crack coalescence under the high loading rate (greater than0.05 mm/min). Afterwards, with the consideration of the bearing capacity (UCS) and energy release of the mining-disturbed coal mass (Ue), a power exponential relationship between the mining rate (MR) in the field and the critical loading rate (vc) in the laboratory was proposed. The application potential of the formulas was then validated against the field monitored data. Finally, based on the critical loading rate, the released strain energy, and the monitored pressure on the roof supports, a reasonable mining rate MR for the Ji15-31030 working face was determined to be approximately 3 m/d.