Chung-In Lee

Measurement of rock fracture toughness under modes I and II and mixed-mode conditions by using disc-type specimens

Rock fracture mechanics has been widely applied to blasting, hydraulic fracturing, mechanical fragmentation, rock slope analysis, geophysics, earthquake mechanics, hot dry rock geothermal energy extraction and many other practical problems. But a standard method to accurately determine fracture toughness of rocks, one of the most important parameters in fracture mechanics as an intrinsic property of rock, has not been yet well established. To obtain rock fracture toughness, disc-type specimens were used in this study. Rock fracture toughness under mixed-mode conditions was measured by using the straight-through crack assumption (STCA) applied to the cracked chevron-notched Brazilian disc (CCNBD) specimen and the semicircular bend (SCB) specimen. Size effects, in terms of specimen thickness, diameter and notch length on fracture toughness, were investigated. From the mixed-mode test results, fracture envelopes were obtained by applying various regression curves. The mixed-mode test results were also compared with the three well-known mixed-mode failure criteria.

Modeling of Fracture and Damage in Rock by the Bonded-Particle Model

ABSTRACT Since potential disturbance of a near-field host rock could adversely affect the stability of underground openings and could possibly change the properties of rock mass, it is very important to evaluate deformation and fracturing characteristics of rock. In this study, PFC2D modeling as well as laboratory experiments were carried out to study characteristics of deformation and fracturing of rocks under uniaxial and triaxial compressive conditions. Laboratory experiments include stress-strain measurements and acoustic emission measurements to carry out the moving point regression analysis. Damage thresholds showed linear relationships with confining pressures. In addition, the feasibility of the application of the bonded-particle model to reproduce mechanical behaviors of rocks was explored by comparing model behaviors to results from experiments at different confining pressures. The model could exhibit macroscopic behaviors such as strain softening, dilation and fracture that arise from extensive microcracking throughout a bonded assembly. However, axial stress increments with increasing confining pressures were much smaller than those from experiments.

Damage and Fracture Characteristics of Kimachi Sandstone in Uniaxial Compression

ABSTRACT Some of the key concerns regarding the design of underground openings include the implication in potential ground disturbance caused by the excavation method and the redistribution of in situ stress. Both of these factors are related to the extent of brittle fracture damage which could adversely affect the stability of excavation boundary and could possibly change the properties of the near-field host rock. Therefore, the evaluation of deformation characteristics of a damaged rock is very important in analyzing the stability of underground openings. The development of stress-induced micro-cracking in rock samples under uniaxial compression was investigated by using stress-strain data, the moving point regression technique and AE measurements. In addition, damage thresholds and deformation parameters were identified. Finally, empirically modeled deformation behaviors were compared with experimental data. It was shown that crack initiation and crack damage for Kimachi sandstone occurred at 0.578σUCS and 0.822σUCS, respectively. However, it was difficult to find the crack closure threshold due to intrinsic properties of Kimachi sandstone related to its high porosity. The appropriate interval for moving point regression technique should be at least 5% of total data set. Approximately 16% of the total axial strain occurred during crack closure and nearly 40% occurred before crack initiation. In contrast, only 5.6% of the total lateral strains were recorded during the crack closure interval. The largest proportion of total lateral strain was seen to be attributable to the coalescence and unstable crack propagation. Modelled behaviors of Kimachi sandstone coincided well with test data.

Performance Assessment of the Circular-Cut Blasting by Numerical Analysis

ABSTRACT The most important operation in the tunnel blasting procedure is to create an opening in the face in order to develop more free surface in the rock mass. This operation is highly dependent on the cut holes. In general, the burn-cut or V-cut is commonly adopted to blast the cut section. But, these traditional methods often raise problems. There are instances (a) that the actual advance is considerably less than the designed advance depending on the rock mass, (b) that they cannot sometimes enlarge the free surface efficiently and (c) that the blast vibration resulted from the cut blast exceeds the allowable vibration level. In order to resolve the problems, many different kinds of the cut blast can be considered. The objectives of this study are to suggest a new cut blast (named as ‘circular-cut’) using pre-splitting technique with an array of large empty holes and blast holes and to investigate the efficiency of the circular-cut. The distinct element computer program, particle flow code (PFC2D), was used to investigate the effect of the cut blast. By using the program, we could identify (a) the optimal spacing between the large empty holes and blast holes surrounding a central hole and (b) the optimal amount of explosive charge for the central hole. We could also evaluate the vibration attenuation rate of the cut blast obtained by pre-splitting. The optimal values of the above parameters favorably agreed with the field-measured data. In addition, a series of numerical simulations using the optimal values showed that the suggested method could reduce the blast vibration caused by the cut blast in comparison with the traditional methods.

Optimization of the Length of the Mist-eliminator Blades for Small Size Water Powered Scrubber

ABSTRACT The dust control in anthracite mines in Korea has been important in preventing pneumoconiosis, commonly known as the black lung disease among miners. Portable water-powered scrubber is one of the most effective devices used for dust collection in underground mines or tunnels in Korea. However, there is insufficient information about the design of mist eliminator blades or their performance under different operating conditions of a small size scrubber. In this study, we investigated the possibility of reducing the size of the present water-powered scrubber and the detailed performances of different types of mist-eliminator blades under various working conditions. A full-scale apparatus has been constructed to study the performance of each bend on the mist-eliminator blades. A special gravity-settling chamber and seventeen sets of mist-eliminator blades have been designed and constructed in the laboratory. The results show that the length of the mist-eliminator blades chamber can be reduced by using mist eliminator blades with 42° deflection angle, eight bends, and bend length of 3 cm. Blades having these specifications were the most effective in achieving high dust and coal collection efficiencies within a small distance along the mist eliminator blades. Based on these results, the length of the water powered scrubber can be reduced to 95 cm without significantly decreasing the dust and water collection efficiencies.