Soo-Ho Chang

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.

Performance estimation of conical picks with slim design by the linear cutting test (I): depending on attack angle variation

In this study, the cutter acting forces were measured by 3-directional load cell at two different skew angles and various S/d ratios during a series of linear cutting tests using a slim conical pick. The analysis for cutting performance were carried out after calculating average values of the measured results. The increase of penetration depth results in the decrease of specific energy. And the variations of the cutter acting forces depending on penetration depth in the case of 6 degree skew angle were smaller than in the case of 0 degree skew angle. From this results, 6 degree skew angle is more effective than 0 degree skew angle in designing optimal specifications of cutting head. In addition, under the setting of 6 degree skew angle was smaller than under the setting of 0 degree skew angle. However, it should be considered that the increase of cutter acting force in the cutting direction accompanied the increase of driving force in the case of the setting for 6 degree skew angle.

Characterization of the deformation of a disc cutter in linear rock cutting test

Disc cutter is a key cutting tool for rock excavation by TBMs. This study aimed to characterize the deformation of a cutter ring by strain measurement as well as infrared thermal camera measurement during a series of linear cutting tests for a hard rock. The strain measurement results indicated that the cutter ring clearly showed a linear elastic behavior. The data obtained from the infrared thermal camera measurement demonstrated that the maximum temperature increase in the cutter ring was below 14.4℃. The deformation and temperature increase of the cutter ring during rock cutting were insignificant in a given cutting test condition of this study.

A numerical study on evaluation of unsupported pillar strength in the room and pillar method

This study aims to evaluate the mechanical behaviors of unsupported rock pillars in a room-and-pillar underground structure by a series of numerical analyses. In addition, rock pillar strengths estimated by a few empirical equations proposed for underground mines are compared with those from numerical analyses. Based on the results from the numerical analysis, the ratio of pillar strength to rock mass strength increases as the ratio of the width of a pillar to its height becomes bigger. It means that higher ratio of pillar width to its height is much more favorable for stabilizing a room-and-pillar underground structure. Especially, unsupported pillar strengths estimated from numerical analyses are higher than rock mass strength when the ratio of pillar width to height is approximately over 1.5. It is also found that the choice of an empirical equation appropriate for a given geometric condition of a pillar is important for its feasible application to the stability analysis of a pillar in the room-and-pillar method.

Development of a Powder-type Thin Spray-on Liner and Its Performance Evaluation at Different Curing Ages

Thin Spray-on Liner (TSL) has been considered as a new rock support to replace shotcrete as well as wire mesh. However, the development of its original production technology is highly in demand since it is not open to the public. Therefore, two kinds of powder-type TSL prototypes were developed as the first development stage. Then, their mechanical properties were experimentally compared with those of a two-component foreign TSL material including both of liquid and powder components. From a series of experiments, the first TSL prototype mixing condition satisfied every TSL performance requirements specified by EFNRAC (2008), and showed much higher tensile and bond strengths than those of the two-component foreign TSL, even though the other TSL prototype cannot be used as a support member since its elongation at break is much lower than its corresponding EFNARC (2008) performance criterion. In addition, a further study to increase the ductility of the first TSL prototype might be necessary to guarantee its higher applicability to field conditions.

A experimental study on the loads and temperature acting on the shaft of a disc cutter during linear rock cutting test

This study aimed to estimate the axial stress and torque on a shaft in a disc cutter. The corresponding inner temperature and the surface temperature of a cutter ring were also measured by using strain gauges and thermocouples during the linear cutting tests. The maximum values of the axial stress and torque were recorded to 11.3 MPa, 171 kN· m respectively. They have higher correlations with normal force rather than rolling force. The results of temperature measured by thermocouples during a linear cutting test showed that the rate of increase in temperature was below 0.2℃. When the cutter spacing is set to be 70 mm, the inner temperature and surface temperature of a disc cutter were 0.1℃/m, 0.15~0.17℃/m respectively. Similarly, when the cutter spacing is 90 mm, the temperature values were 0.09℃/m, 0.13~0.23℃/m

A study on conceptual evaluation of structural stability of room-and-pillar underground space

In this study, in order to evaluate stability of the room-and-pillar underground structure, a series of preliminary numerical analyses were performed. Design concept and procedure of an underground structure for obtaining a space are proposed, which should be different from structural design for the room-and-pillar in mine. With assumed material properties, a series of numerical analyses were performed by varying size ratios of room and pillar and then the failure modes and location at yielding initiation were investigated. From the results, relationship between the ratio of pillar width to the roof span (w/s) and overburden pressure at failure initiation shows a relatively linear relation, and the effect of w/s on structural stability is much more critical than the ratio of pillar width and height (w/H) which is a crucial parameter in design of the room-and-pillar mining. It means that roof tensile failure and shear failure at shoulder and pillar are necessary to be considered together for confirming overall structural stability of the room-and-pillar structure, rather than considering the pillar stability only in mining. Failure modes and location at failure initiation were varied with respect to the ratio of room and pillar widths. Therefore, it is necessary to simultaneously consider stability of both roof span and pillar for design of underground structure by the room-and-pillar method.

Evaluation of steel fiber reinforcement effect in segment lining by full scale bending test

An experimental research on the possibility of using fiber reinforced concrete precast tunnel segments instead of traditional reinforced concrete(RC) segment has been performed in europe. This solution allows removing the traditional reinforcement with several advantages in terms of quality and cost reduction. Full-scale bending tests were carried out in order to compare the behaviour of the segments under flexural actions on both rebar reinforced concrete and rebar-fiber reinforced elements. The test results showed that the fiber reinforced concrete can substitute the traditional reinforcement; in particular the segment performance is improved by the fiber presence, mainly in terms of crack.

An experimental study on bonding and bearing capacities of thin spray-on liner to evaluate its applicability as a tunnel support member

The use of Thin Spray-on Liner (TSL) as an alternative to shotcrete has drastically increased since 1990s when it was first developed and introduced to mines. In this study, tensile strength test, bond strength test, compression test with specimens coated by TSL, and two kinds of bending tests proposed by EFNARC (2008) were performed with two kinds of TSLs with different material compositions in order to evaluate their support capacities. As a result, both TSLs were shown to be satisfactory for the minimum performance requirements for a structural rock support suggested by EFNARC (2008) and tensile strength of a TSL was shown to increase as its content of polymer was higher. In contrast, its bond strength was shown to increase proportional to the content of a cementitious component especially at the early age.

Flexural performance evaluation of SFRC with design strength of 60 MPa

Based on Model Code 2010, flexural and residual strength, flexural toughness of SFRC with design strength of 60 MPa are evaluated. For comparisons, SFRC with design strength 40 MPa was tested. Distribution of steel fibers in crack surface of specimens was evaluated by visual inspection. The used steel fibers were hooked fibers with aspect ratio of 64, 67 and 80. In all specimens, mix ratio of steel fibers was 0.5% Vol. In results, only SFRC with the highest aspect ratio satisfied requirements specified in Model Code 2010. The results demonstrated that the use of high aspect ratio will provide enough flexural toughness for high strength concrete. Also, it is found that low slump of high strength concrete can help to enhance isotropic fiber distribution.