• Tunnel and Underground Space
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• v.23 no.5
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• pp.362-371
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• 2013

We conducted hollow cylinder tensile strength tests and Brazilian tests in Seokmo granite to measure tensile strength necessary for estimating the magnitude of the maximum horizontal principal stress in hydraulic fracturing stress measurements. Two different pressurization rates were used in hollow cylinder tests. Tensile strengths were determined to be higher at higher pressurization rate, which suggests that tensile strength should be measurement at the same rate used in actual in situ hydraulic fracturing tests. Considering the effect of pressurization rate and specimen size on tensile strength, the hollow cylinder tests and Brazilian tests yield similar results each other. This demonstrates that Brazilian tests can be utilized to produce representative tensile strengths for interpretation of hydraulic fracturing test results.

• Tunnel and Underground Space
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• v.26 no.5
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• pp.363-374
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• 2016

All five independent elastic constants of a transversely isotropic rock sometimes need to be determined from a single specimen. Saint-Venant approximation has been widely used for a long time in the analysis of single specimen test. This paper has proven how this empirical equation can be mathematically transformed into a form of the apparent Young's modulus based on theory of elasticity. The transformed equation is a monotonous function on anisotropic angle and can be useful in the analysis of the in-situ stress measurement in an anisotropic rock mass. The estimations of data in literatures have shown that the measured values of $G^2$ are uniform on anisotropic angles and smaller than that of Saint-Venant's case. This decrement may be caused by sliding of the interface of strata and the decrement rate is inferred to relate well with the combination of bonding condition of strata and strength of rock material. Accumulation of these kinds of studies in the future enables to define the decrement and to determine elastic constants of a transversely isotropic rock from a single specimen from modifying Saint-Venant approximation.

• The Journal of Engineering Geology
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• v.19 no.1
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• pp.43-50
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• 2009

A 3-D numerical analysis was carried out to observe potential effects of orientation of inherent weak zones to tunnel behaviors and stress distributions during tunnel excavation. Weak zones used for the analysis were placed at the upper 1D part from crown, on the crown and on the center of face, using orientations derived from the 6th RMR parameter for assessment of joint orientation effect on tunnel. Mechanical properties of rock mass were derived through a in-situ displacement measurement-based back analysis. Finally, a classification chart for crown settlement with five ranks based on orientation and location of weak zones is suggested.

• Tunnel and Underground Space
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• v.30 no.2
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• pp.109-135
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• 2020

Globally, the deepening depth in the underground is a situation of the high interest for a purpose of the development of various facilities. The development of deep underground space should be based on the structural stability of rocks. Spalling is known to have an impact on the structural stability degradation in deep underground space. As an attempt to predict spalling, many researchers have proposed predicted conditions in accordance with stress states which occur around the tunnel, rock conditions, and types of rock. In addition, the analysis on spalling method has been verified by using computer modeling such as FLAC, EXAMINE, Insight 2D, UDEC and FRACOD, along with in-situ measurement results. In Canada URL (Underground Research Tunnel), CWFS model (Cohesion Weakening Frictional Strengthening) was used to precisely predict for the state of spalling, comparing spalling modeling. CWFS model has been identified as a reliable method for predicting such phenomena. This study aims to analyze several cases of spalling, and then make a comparison between the conditions for spalling occurrence and the predicted results of model CWFS. With this, it investigates the applicability of prediction of spalling, targeting pillar under deep depth condition.

• The Journal of Engineering Geology
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• v.30 no.1
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• pp.17-30
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• 2020

An optical televiewer is a geophysical logging device that produces continuous high-resolution full-azimuth images of a borehole wall using a light-emitting-diode and a complementary metal-oxide semiconductor image sensor to provide valuable information on subsurface discontinuities. Recently, borehole imaging logging has been applied in many fields, including ground subsidence monitoring, rock mass integrity evaluation, stress-induced fracture detection, and glacial annual-layer measurements in polar regions. Widely used commercial borehole imaging logging systems typically have limitations depending on equipment specifications, meaning that it is necessary to clearly verify the scope of applications while maintaining appropriate quality control for various borehole conditions. However, it is difficult to directly check the accuracy, implementation, and reliability for outcomes, as images derived from an optical televiewer constitute in situ data. In this study, we designed and constructed a modular fractured borehole model having similar conditions to a borehole environment to report unprecedented results regarding reliable data acquisition and processing. We investigate sonde magnetometer accuracy, color realization, and fracture resolution, and suggest data processing methods to obtain accurate aperture measurements. The experiment involving the fractured borehole model should enhance not only measurement quality but also interpretations of high-resolution and reliable optical imaging logs.