• Tunnel and Underground Space
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• v.30 no.6
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• pp.559-572
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• 2020

This paper performs laboratory experiments for borehole stability considering temperature and true triaxial stress condition, and observes a thermo-mechanical behavior of the rock under stress and temperature conditions of deep underground. China yellow sandstone and Hwangdeung granite specimens were used to perform a true triaxial compression test. Mechanical tests were carried out under nine confining pressure conditions, and thermo-mechanical tests using granite samples were carried out under six confining pressure conditions at 60-100℃. In the mechanical tests, maximum principal stress at borehole breakout was proportional to intermediate principal stress. In the thermo-mechanical tests, it was confirmed that thermal stress is added to the stress field of the borehole with the increase in temperature, resulting in additional breakout progress. To analyze the results of the laboratory experiment, Mogi-Coulomb failure criterion was used. The results of traditional triaxial compression test on cylindrical specimens and borehole breakout under true triaxial compressions matched well with Mogi-Coulomb failure criterion.

• Tunnel and Underground Space
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• v.17 no.2 s.67
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• pp.128-138
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• 2007

At low in-situ stress, the continuity and distribution of natural fractures in rock mass predominantly control the failure processes. However at high in-situ stress, the failure process are affected and eventually dominated by stress-induced fractures preferentially growing parallel to the excavation boundary. This fracturing is often observed in brittle type of failure such as slabbing or spatting. Recent studies on the stress- or excavation-induced damage of rock revealed its importance especially in a highly stressed regime. In order to evaluate the brittle failure around a deep underground opening, physical model experiments were carried out. For the experiments a new tue triaxial testing system was made. According to visual observation and acoustic emission detection, brittle failure grades were classified under three categories. The test results indicate that where higher horizontal stress, acting perpendicular $(S_{H2})$ and parallel $(S_{H1})$ to the axis of the tunnel respectively, were applied, the failure grade at a constant vertical stress level (Sy) was lowered. The failure initiation stress was also increased with the increasing $S_{H1}\;and\;S_{H2}$. From the multi-variable regression on failure initiation stress and true triaxial stress conditions, $f(S_v,\;S_{H1},\;S_{H2})$ was proposed.

• Tunnel and Underground Space
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• v.22 no.6
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• pp.462-470
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• 2012

Implementing the generalized Hoek-Brown failure criterion in the framework of the Mohr-Coulomb criterion requires the calculation of the equivalent friction angle and cohesion. In the conventional method based on the Balmer (1952)'s theory, the tangential instantaneous friction angle and cohesion are expressed in terms of the minimum principal stress ${\sigma}_3$, which does not provide the information about the dependency of the equivalent parameters on the hydrostatic pressure and the stress path. In this study, this defect of the conventional method has been overcome by representing the equivalent parameters in terms of stress invariants. Through the example implementation of the new method, the influence of the magnitude of the hydrostatic pressure and the Lode angle on the tangential instantaneous friction angle and cohesion is investigated. It turns out that the tangential instantaneous friction angle is maximum when the stress condition is triaxial extension, while the tangential cohesion is maximum when the stress condition is triaxial compression. The dependency of the equivalent Mohr-Coulomb strength parameters on the hydrostatic pressure and the Lode angle tends to be more substantial for the favorable rockmass of larger GSI value.

• Journal of the Korean Geotechnical Society
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• v.30 no.3
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• pp.73-85
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• 2014

The strength parameters used in rock mass design are mainly estimated by equations using Hoek-Brown failure criterion because the tests to obtain the values are limited and expensive. To estimate the strength parameters, the Hoek-Brown failure criterion should be transformed to the Mohr-Coulomb failure criterion. But the processes are more or less cumbersome due to the several stages including the computation and the analyzing steps. In this study, several rock states of various conditions were modeled and then the strength parameters were estimated using the Hoek-Brown failure criterion. Thereafter by analyzing the results, some charts and equations estimating the strength parameters through only one step or easily in the field using the values of RMC, the uniaxial compressive strength and the rock constant ($m_i$), were suggested. And then the suggested method was compared and discussed with the existing method.

• Proceedings of the Korean Society for Rock Mechanics Conference
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• 1995.03a
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• pp.181-185
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• 1995

현재 구미에서 터널의 설계에는 RMR, Q와 같은 암반분류 기법과 경계요소 해석법과 같은 간략한 탄성 프로그램에 경험적 파괴조건식을 적용하여 이완대를 계산하고 터널의 지보량을 추정하는 방식이 널리 적용되고 있다. RMR이나 Q와 같은 암반분류법은 지하공동의 안정성에 영향을 미치는 중요한 지질 요인들에 근거하여 암반을 몇가지 등급으로 분류하고 지보방법을 결정하는 분류 방식으로 가장 많이 사용되고 있으나 각 항목의 평가방식이 경험적인 판단을 요하게 되어 주관적인 오류에 빠질 가능성이 많고, 또 여러 가지 대체 수단이 있어 종합적인 판단을 얻기가 용이하지가 않다. (중략)

• Transactions of the Korean Society of Mechanical Engineers
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• v.13 no.1
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• pp.96-104
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• 1989

The behaviors of acoustic emission emitted in the tests of the fracture toughness and fracture stability are observed by using the specimens of aluminum 2024-T351 and 7039-T6 alloys. The empirical eqution of J-R curve is derived. It is demonstrated from the comparison of the fracture toughness obtained from J-R curve with that from ASTM standard E813-81 that the latter is larger than the former. The discontinuous point in the log-log graph of J-integral vs. total acoustic emission count is observed in between the two offset lines referred from ASTM standard E813-81, but it's physical meaning is uncretain. An empirical material tearing modulus is derived in terms of the total acoustic emission count and proved to be valid in fracture instability test.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.6
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• pp.206-215
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• 2017

An experimental program was carried out to investigate the mechanical properties of high-strength concrete. High-strength concrete with compressive strengths of 80 to 120 MPa was tested. Test results are presented regarding effect of water-binder ratio on compressive strength and compressive strength gain. In addition, the effect of curing methods on compressive strength, elastic modulus, splitting tensile strength, and modulus of rupture is investigated. Test results of elastic modulus, splitting tensile strength, and modulus of rupture are compared with predictions from the current design recommendations. Predictions of elastic modulus by using KCI recommendation has good agreement with test results. However, predictions of modulus of rupture by using KCI recommendation underestimate the test results. ACI 363R recommendations predict well test results of splitting tensile strength and modulus of rupture. ACI 363R recommendations for predicting splitting tensile strength and modulus of rupture can be used for high-strength concrete with compressive strengths up to 120 MPa.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.1
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• pp.58-65
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• 2007

Mixed mode interlaminar fracture behaviors of carbon fabric/epoxy composites were investigated through MMF (Mixed Mode Flexural) test by varying mixed mode ratio ranging from 20% to 90%. Mixed mode interlaminar fracture criteria based on NL point and 5% offset point were also suggested in order to predict mixed mode interlaminar fracture behaviors. Fracture surfaces and crack propagating behaviors were examined through a travelling scope and a scanning electron microscope. According to the results, mixed mode interlaminar fracture behaviors can be predicted by mixed mode interlaminar fracture criterion with m=1.5 and n=0.5 on the basis of NL point or mixed mode interlaminar fracture criterion with m=2 and n=3 on the basis of 5% offset point. Fracture surfaces and crack propagating behaviors are sensitive to mixed mode ratios. MMF test can be successfully applicable in evaluating mixed mode interlaminar fracture toughness of carbon fabric/epoxy composites.

• Tunnel and Underground Space
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• v.31 no.6
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• pp.480-493
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• 2021

Since the generalized Hoek-Brown criterion (GHB) provides an efficient way of identifying its strength parameter values with the consideration of in-situ rock mass condition via Geological Strength Index (GSI), this criterion is recognized as one of the standard rock mass failure criteria in rock mechanics community. However, the nonlinear form of the GHB criterion makes its mathematical treatment inconvenient and limits the scope of its application. As an effort to overcome this disadvantage of the GHB criterion, the explicit approximate analytical equations for the minimum principal stress, which is associated with the maximum principal stress at failure, are formulated based on the Taylor polynomial approximation of the original GHB criterion. The accuracy of the derived approximate formula for the minimum principal stress is verified by comparing the resulting approximate minimum principal stress with the numerically calculated exact values. To provide an application example of the approximate formulation, the equivalent friction angle and cohesion for the expected plastic zone around a circular tunnel in a GHB rock mass are calculated by incorporating the formula for the approximate minimum principal stress. It is found that the simultaneous consideration of the values of mi, GSI and far-field stress is important for the accurate calculation of equivalent Mohr-Coulomb parameter values of the plastic zone.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1992.04a
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• pp.285-290
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• 1992

기계 구조물의 과도한 진동은 기계의 성능 및 수명에 나쁜 영향을 미치며 기계및 부품의 파괴까지 유발하게 되는 원인이되므로 이를 제진하기위한 각종 방법이 오래전부터 연구되고 있다. 본 연구에서는 현재까지 진행된 충격식 댐퍼의 이론을 기초로 한 사이클당 두번의 충돌이일어난다는 가정하에, 파라메타 의 변화에따른 최적 감쇠특성을 얻기위한 최적간극과 최적 질량비를 쉽게구할 수 있는 식을 유도하였다. 그리고, 실험에 의하여 공진상태에 있어서 불의 충격력에 의한 감쇠특성을 조사하였고, 이론으로구한 식과 비교 검토함으로서, 실제 현장등에서 간편하게 근사식을 이용하여 최적조건을 구하고, 이 자료를 기초로하여 설계, 제작시 보다 간편하게 사용할 수 있도록 하였다.